Aircraft Electrical Systems
Aircraft Electrical Systems
Other titles by the same author
Aircraft Instruments
Third Edition
EHJ Pallett
Copyright © 1987 by E H J Pallett
This edition is published by ,,rrangement with Pearson Education, Ltd and Dorling
Kindcrslcy l>ublishing Inc.
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ISBN 978-81-317-0389-2
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Preface to the Third Edition
Preface to the First Edition
1 Direct Current Power Supplies
2 Alternating Current Power Supplies
3 Power Conversion Equipment
4 External and Auxiliary Power Supplies
S Power Distribution
6 Circuit Controlling Devices
7 Circuit Protection Devices and Systems
8 Measuring Instruments and Warning Indication Systems
9 Power Utilization - Motors
10 Power Utilization - Systems
11 Electrical Dia.grams and Identification Schemes
l Electrical and Magnetic Quantities, Definitions and Units
2 Ohm's Law
3 Power in A.C. Circuits
4 Connection of Capacitors and Inductors
S Fundamental A,C, Circuits and Fonnulae
6 Conversion Factors
7 Power Generation System Applications
8 Electrical Diagram Symbols
9 Representative Aircraft Ice and Rain Protection Systems
10 Abbreviations and Acronyms associated with Electrical Systems
11 Logic Gates and Truth Tables
Solutions to Exercises
Preface to the
Third Edition
It is now almost eleven years since this book first made its appearance, and the continuing demand warranting
this, the third edition, has been most encouraging.
The original sequence of subject structuring has been retained since the reasons noted in I.he preface to the
first edition still apply. It has howeve r, been considered necessary to combine the contents of some chapters,
and in others the coverage has been expanded to lllustrate the application of principles to a greater number
of systems currently in use .
The application of signal processing by means of digital circuit techniques to aircraft systems has been norm
practice for a very long time. As far as what may be termed ''raw electrical systems'' are concerned, the impact
of these techniques has, in comparison to such systems as navigation, flight management and automatic flight
control, been somewhat Jess for~eboding. However, in relation to those aspects of power generation, distributior
and control, it is necessary to have a good understanding of the foregoing techniques, and in particular, the
use oflogic gates and interpretation of associated diagrams. This latter subject has, therefore, been included in
a new chapter to this edition of the book.
In preparing the revised material, the opportunity has been taken to clear up some anomalies that crt!pt
into the second edition and subsequent reprints, and I am indebted to those readers who submitted comments.
I am also indebted to others who made suggestions regarding the lnclusion of new material, and who supplied
information for reference purposes.
In conclusion, it is perhaps pertinent to note that tlus edition has been prepared during a transition from on,
publisher to another, and so I would like to thank the one under whose logo it now appears, not only for havin
undertaken their particular tasks, but also for establishing a new publisher/author association .
Preface to the
First Edition
Increases in size and speed, changes in shape and
functional requirements of aircraft have each been
possible by technlcal research and development and
the progress made not only applies to those visible
structural parts, but also to those unseen systems a.nd
services which enable it to function as an integrated
A system ranking very highly ,indeed in progression is the one concerned with electrical power
involving as it does various methods of generation,
distribution, control, protection and utilization. These
methods do, in fact, form a natural ''build-up" of an
aircraft's electrical system and their sequence sets a
convenient pattern on which a study of principles and
applications can be based. The material for book
therefore follows this pattern.
tn the early days of what is familiarly called "aircraft electrics", there was a certain distrust of the
equipment. Although there was acceptance of the fact
that electricity was necessary for operating the "wire·
less" equipment, a few lights and an engine ignition
system, many individuals were inclined to the view
that if other systems could not be operated either by
air, hydraulic oil, cables, numerous mechanical link·
ages or petrol, then they were quite unnecessary! A
majority of the individuals were mechanics, and the
ground engineers as they were then known, and un·
doubtedly, when "elcctrickery" began proving itself
as a system operating media, it came as a pleasant
relief to leave all relevant work to that odd character,
the electrician, who speaking in some strange jargon
and by means of diagrams containing numerous mystic
lines and symbols, seemed better able to cope with it
With the continued development of the various
types of aircraft, the sources of electrical power have
also varied from the simple battery and wind-driven
generator, through to the inost complex multiple a.c.
generating systems. Similarly, the application of power
sources have varied and In conjunction with developments in electronics, has spread into the areas of
other systems to the extent of performing not only a
controlling function but, as is now so often the case,
the entire operating function of a system. As a result,
the work of the electrician assumed greater importance
and has become highly specialized, while other maintenance specialists found , and continue to find it
increasingly necessary to broaden their knowledge of
the subject; indeed it is incumbent on them to do so
in order to carry out their important duties. This also
applies to pilots in order that they may meet the
technical knowledge requirements appropriate to
their duties and to the types of aircraft they fly.
Fundamental electrical principles are described in
many standard text books, and in preparing the
material for book it was in no way intended that
it should supplant their educational rolo, However, it
has been considered convenient to briefly review certain relevant principles in the chapters on generation
and conversion of power supplies, to "lead-in" to the
subject and, it is hoped, to convey more clearly how
they arc appUed to the systems described. In keeping
with the introductory nature of the book, and perhaps
more important, to keep within certain size limitations,
it obviously has not been possible to cover aU types
of aircraft systems. However, in drawing comparisons
it is found that applications do have quite a lot In
common, and so the examples finally chosen may be
considered sufficiently representative to provide a
useful foundation for further specialized study.
The details given embrace relevant sections of the
various syllabuses established for the technical examination of maintenance engineers and pilots by official
organizations, training schools and professional soc!·
eties. In this connection, therefore, it is also hoped
that the book will provide a useful source of reference.
A selection of questions are provided for each
chapter and the author is indebted to the Society of
licensed Aircraft Engineers and Technologists for
permission to reproduce questions selected from
examination papers.
Valuable assistance has been given by a number of
organizations in supplying technical data, and in
granting permission to reproduce many of the illustrations, grateful acknowledgement is hereby made to
the following Amphenol Ltd.
Auto Diesels Braby Lld,
Aviquipo <ilfBritain Ltd.
Belling & Lee Ltd .
British Aircraft Corporation (Operating) Ltd.
Britten-Norman Ltd.
Cannon Electric (G.B.) Ltd.
Dowty Electrics Ltd.
Graviner (Colnbrook) Ltd.
Hawker Siddeley Aviation Ud.
Honeywell Ltd.
international Rectifier Co. (G.B.) Ltd.
Lucas Aerospace Ltd.
Nrwton Brothers (Derby) Ltd.
Normalair-Garrett Ltd.
Plcssey Co., Ltd.
SAFT (United Kingdom) Ltd.
Sangamo Weston Ltd.
Shell Aviation News.
Smiths Industries Ltd.
Standard Telephones & Cables Ltd.
Thom Bendix.
Varley Dry Accumulators Ltd.
Finally, thanks are also due to the publishers for
having padently awaited the completion of sections of
manuscript and also for having accepted a number of
changes of subject.
Direct Current
Power Supplies
Depending on the type of aircraft, and the extent to
which electrical power is to be utilized for the
operation of its systems and components, the primary
supply of such power may either be direct current
(d.c.) or alternating current (:Le.). This chapter
deals with the first of these supplies and how it is
produced by both generators and batteries. Examples
of some typical aircraft systems are described in
Append.ix 7.
a reversal of the direction of induced voltage. As
rotation continues, the number of lines cut decreases
and the induced voltage reduces to zero as the loop
returns to position 1. Plotting of the induced voltage
tluoughout the full cycle produces the alternating or
sine curve shown.
Fundamental Principles of Generators
A generator is a machine that converts mechanical
energy into electrical energy by the process of electromagnetic induction. In both d.c. and a.c. types of
generator, the voltage Induced is alternating; the
major difference between them being in the method
by which the electrical energy is collected and applied
to the circuit externally connected to the generator.
Figure 1.l(a) illustrates a generator in its simplest
form , i.e. a single loop of wire "AB'' arranged to
rotate between the pole pieces of a magnet. The ends
of the wire are brought together to form a circuit
via slip ring;, brushes and the externally connected
load. When llie plane of the loop lies at right angles to
the magnetic field (position 1, Fig. I.I (b)) no voltage
is induced ln the loop. As the loop rotates through
90 degrees the wires cut the lines of force at right
angles until at position 2 the induced voltage is at a
maximum. As the loop approaches the vertical position
again the voltage decreases since the rate at which
lines of force are cut dimlnishes. At position 3 the
induced voltage Is zero. If rotation is continued, the
number of lines cut gradually increases, until at 270
degrees (position 4) it is once again maximum, but
as the cutting is in the opposite direction there is also
1, A:'.78 '-I
Fig I.I
{a) Simple form of generator
(b) Induced voltage
To convert the a.c. produced into unidirectional or
d.c., it is necessary to replace the slip rings by a
collecting devlct! referred to as a commutator. 1hls is
shown In Fig. 1.2 (a) and as will be noted it consists of
two segments insulated from each other and connected
to the ends of the loop. The brushes are set so that
each segment moves out of contact with one brush
and into contact with the other al the point where the
loop passes through the positions at which induced
voltage is minimum. In other words, a pulsating
current increasing to maximum in one direction only
is produced as shown by the curve in Fig. l .2(b ).
Fig 1.'.l
Efrec1 on output using soveral coils
(2) Separately-excited generators, in which electro·
magnets are excited by current obtained from a
separate source of d.c.
(3) Self-excited generators, in which elec_tro·
magnets are excited by current produced by the
machines themselves. These generators are furl her classified by the manner in which the fixed
windings, I.e. the electromagne1ic field and
armature windings, are interconnected.
In aircraft d.c. power supply systems, self-excited
shunt-wound generators are employed and the following details are therefore related only to this type.
Fixed Winding Arrangement
Fig 1.2
Conversion of a.c. to d.c.
(a) Use of commutator
(b) Current wave-form
In order to smooth out the pulsations and to pro·
duce a more constant output, additional wire loops
and commutator segments are provided. They are so
interconnected and spaced about the axis of rotation,
that several are always in a position of maximum
action, and the pulsating output is reduced to a ripple
as indicated in Fig. 1.3.
Figure 1.4 illustrates the arrangement of the fixed
windings of a basic four-pole machine suitable for use
as a self-excited generator. The fixed portion of the
armature circuit consists of the four brushes, the links
connecting together brushes of like polarity and the
Terminal box
Generator Classifications
Generators are classified according to the method by
which their magnetic circuits are energized, and the
following tluee classes are normally recognized (l) Permanent magnet generators.
Pig J.4
Fixed winding arrangements
cables connecting the linked brushes to the terminals
indicated A and A1 • The four field coils are of high
resistance and connected in series to form the field
winding. They are wound and connected in such a
way that they produce alternate North and South
polarities. The ends of the windin~ are brought out to
the terminals indicated as Zand Z 1 •
Generator Characteristics
The characteristics of a generator refer to the relationship between voltage and the current flowing in the
external c!rouit connected to a generator, i.e. the
load current, and there are two which may be closely
defined. These are: the external characteristic or
relationship between terminal voltage and load
current; and the internal charar,teristic or relationship
between the actual electromagnetic force (e.m.f.)
Operating Principle and Characteristic
When the armature is rotated the conduators cut the
weak magnetic field which Is due to residual magnetism in the electromagnet system. A small ~.m.f. Is
induced in the armature winding and is applied to the
field winding, causing current to flow through it and
so increasing the magnetic flux. This, in turn, causes a
progressive increase in the induced e.m.f. and field
current until the induced e.m.f. and terminal voltage
reach the steady open-circuit maximum.
The characteristic for this type of generator is shown
in Fig, 1.6 and it will be observed that the terminal
generated in the annature windings and load current.
These relationships are generally shown in the form of
graphs, with the graph drawn for one particular speed
of the generator.
..... , .,\
' \ \1
,,, I
Self-excited Shunt-wound Generators
Shunt-wound generators are one of three types in the
self.excited class of machine and as already noted are
used in aircraft d.c. power supply systems, The term
"shunt-wound" is derived from the fact that the
high-resistance field winding is connected across or
in parallel with the armature as shown in Fig. 1.5. The
Fig 1.6
Characteris1ic of sclf~xcited shunt-wound generator
voltage tends to fall with Increasing load current.
is due to the voltage drop (IR drop) in the armature
Fig 1.5
Connection of
shunt-field winding
armature current divides into two branches, one
forme'd by the field winding, the other by the
external circuit. Since the field winding is of high
resistance, the advantage is gained of having maximum
current flow through the external circuit and expenditure of unnecessary electrical energy within the generator is avoided.
winding and also to a weakening of the main fl~ by
armature reaction. The fall in terminal voltage reduces
the field current, the main flux is further weakened
and therefore a further fall in terminal voltage is produced.
If the process of increasing the load Is continued
after the full working load condition has been reached,
the termfoal voltage will fall at an increasing rate until
it can no longer sustain th~ load current and they both
fall to zero. With reduced excitation the external
characteristic of a shunt-wound generator falls much
more rapidly so that the point at which voltage collapse
occurs will be reached with a much smaller load current. In practice, field current is adjusted to maintain
constant voltage under all load conditions, by a voltage
regulator the operation of which will be described later.
Sometimes a generator will lose its residual magnetism or become incorrectly polarized because of heat,
shock, or a momentary current in the wrong direction. can be corrected by momentarily passing current
through the field from the positive terminal to the
negative terminal; a procedure known as "flashing the
Generator Construction
A typical self,exciled shunt-wound four-pole generator, whJch is employed in a current type of turbo-prop
civil transport aircraft, is illustrated in Fig. 1.7. lt is
designed to provide an output of 9 kilowatts at a continuous current of 300 amperes (A) over the speed
range of 4,500 to 8,500 rev/nun. In its basic form the
construction follows the pattern conventionally
adopted and consists of five principal assemblies:
namely, the yoke, armature, two end frames and
brush-gear assembly.
The yoke forms the main housing of the generator,
and is designed to carry the electromagnet system
made up of the four field windings and pole pieces. It
also provides for the attachment of the end frame
assemblies. The windings are pre-formed coils of the
required ampere-turns, wound and connected in
series in such a manner that when mounted on the
pole pieces, the polarity of the field produced at the
poles by the coil current is alternately North and
South {see Fig. 1.4). The field windings are suitably
insulated and are a close fit on the pole pieces which
are bolted to the yoke. The faces of the pole pieces
are subjected to varying magnetic fields caused by
rotation of the armature, giving rise to induced e.m.f.
which in tum produces eddy currents through the
pole pieces causing local heating and power wastage.
To minimize these effects the pole pieces are of
laminated construction; the thin soft iron laminations
being oxidized to insulate and to offer high electrical
resistance to the induced e.m.f.
During operation on load, the current flowing through
the armature winding of a generator creates a magnetic
field which is superimposed on the main field produced by field-winding curreht. Since lines of force
cannot intersect, the armature field distorts the main
field by an amount which varies with the load; such
distorting effect Is termed armatur11 reaction. If
uncorrected, armature reaction produces two addition.
undesirable effects: (i) it causes a shift of the Magnetic
Neutral Axis, i.e. the axis passing through two points
at which no e.m.f. is induced in a coil, setting up reactive sparking at the commutator, and (il) it weakens
the main field causing a reduction in generated e.m.f.
The position of the brushes can be altered to minimlze these effec~s under varying load conditions, but
a more effective method is to provide additional wjnd.
ings in the electromagnet system, such windings being
referred to as interpole and compensating windings.
lnterpole windings are wound on narrow-faced
auxiliary pole pieces located midway between the
main poles, and are connected in series with the
armature, The windings are such that an interpole has
the same polarity as the next main pole in the directio
of rotation, and as the fluxes are opposite in direction
to the armature flux, they can be equalized at all load!
by having the requisite number of turns.
ln order to provide true correction of armature
reaction, the effects produced by interpoles must be
supplemented, since alone they cannot entirely elim·
inate all distortion occurring at the main pole faces.
Compensating windings are therefore connected in
series with the lnterpole and armature windings, and
located In slots cut in the faces of the main pole shoes
The sides of the coils thus lie parallel with the sides of
the armature coils. The ampere.turns of the winding
are equal to those of the armature winding, while the
flux due to it is opposite in direction to the armature
flux . .
The effectiveness of interpoles in minimizing reactant
sparking Is limited by armature speed, and their appli1
ation as individual components of a field-winding
system is, therefore, restricted to generators operatlni
over a narrow speed range, e.g. the designed range of
the generator illustrated in Ffg. l. 7. ln the case of
generators designed for operation over a wide range,
e.g. 2850 rev/min up to I 0,000 rev/min, the use of
interpoles alone would produce a side effect resulting
in reactance sparking as the generator speed is reduce
from maximum to minimum. To counteract this, and
for a given load on the generator, it is necessary to
reduce the rnagnetomoUve force (m.m.f.) of the
interpoles. The desired effect may be obtalned by
winding auxiliary coils over the interpole coils
and connecting them in series with the generator shu1
field winding in such a way that each coil, when
energized by shunt field circuit current, produces an
Terrmnol <XWef
bol ibeonng
o amp
Shaft ond plate
Anti-drive eod
Beori ng suppoc1
End caver
R e t a i n i n g ~ I { ~~
Yoke ond field coils
Fig l.7
Sectioned view of a genera.tot
m,m.f. of opposite polarity to that produced by the
interpolc coil on the same pole shoe. An exact balance
between rcactance e.m.f. and commutation e.m.f. is
maintained over the full working range of generator
speed to assist in producing sparkless commutation.
The armature assembly comprises the main shaft
(which may be solid or hollow) core and main winding,
commutator and bearings; the whole assembly being
statically and dynamically balanced. Jn the generator
shown, the shaft is hollow and internally splined to
mate with splines of a drive shaft which passes through
the entire length of the armature shaft,
Armature windings are made up of a number of
individual identical coils which fit into slots at the
outer edges of steel laminations which form lhc core
of Lhe armature. The coils are made from copper
strip and as security against displacement by centri·
fugal force, steel wire (in some cases steel strip) is
bound round the circumference of the armature. The
ends of each coil are brought out to the commutator
and silver brazed to separate segments, the finish of
one coil being connected to the same segment as the
beginning of another coil. The complete winding thus
forms a closed circuit. The windings are invariably
vacuum-impragnated with silicone varnish to main·
tain insulation resistance under all conditions.
In common with most aircraft generators, the
commutator is of small diameter to ntinimize centri·
fugal stressing, and is built up of long, narrow copper
segments corresponding in number to that of the
field coils (a typical figure is 51 coils). The segment
surfaces are swept by brushes which are narrow and
mounted in pairs (usually four pairs) to maintain the
brush contact area per segment - an essential prerequisite for effective commutation.
The armatures of all aircraft generators arc
supported in !ugh efficiency ball or roller bearings, or
in combinations of these two types. Where combina·
tions are a single genera tor it will be found
that the ball bearing is invariably fitted at the drive
end of the armature shaft, and the roller bearing at
the commutator end. This arrangement perm.its
lateral expansion of the armature shaft, arising from
temperature increases in the generator, without expos·
ing the bearings to risk of damage. Bearings are lubricated either with a specified high-melting-point grease or
lubricating oil and may be of the sealed or non·
sealed types. Sealed grease-lubricated bearings are
pre-packed by the manufacturer and require no further
lubrication during the life of the bearing. Non-sealed
grease-lubricated bearings are assembled with suffic·
ient lubricant to last for the period of the generator
servicing cycle, In general the lubricant for oil.
lubricated bearings is introduced into the bearing
through the medium of oil-impregnated felt pads.
Seals are provided to prevent oil escaping into the
interior of the generator.
These assemblies are bolted one al each end of the
yoke and house the armature shaft bearings. The drive
end frame provides for Lhe attachment of the generator to the mounting pad of the engine or gear-box
drive (see also p. 8) and the commutator and
frame provides a mounting for the brush-gear assembly and, in.the majority of cases, also provides for
the attaclunent of a cooling air duct. Inspection and
replacement of brushes is accomplished by removing
a strap which normally covers apertures In the commutator end frame.
The brush·gear assembly is comprised of Lhe brushes
and tlte holding equipment necessary for retaining the
brushes in the correct position, and at the correct
angle with respect to the magnetic neutral axis.
Brushes used in aircraft generators are of the electro·
graphitic type made from artificial graphite, The
graphite is produced by taking several forms of
natural carbons, grinding them into fine powder,
blending them together and consolidating the mixture
into Lhc desired solid shape by mechanical pressure
followed by exposure to very high temperature in an
electric furnace. These brushes possess both the
robustness of carbon and Lhc lubricating properties of
graphite. In addition they are very resistant to burning by sparking, they cause little commutator wear
and their heat conductivity enables them to withstand overloads.
As stated earlier, an essential prerequisite for
effective conunutation is that brush contact area per
commutator segment should be maintained. is
accomplished by mounting several pairs of brushes in
brush holders; in the generator illustrated in Fig. I. 7
four pairs of brushes are employed. The holders take
the form of open-ended boxes whose inside surfaces
are machined to the size of a brush, plus a slight clearance enabling a brush to slide freely without tilting or
rocking. Contact between brushes and commutator is
maintained by the pressure exerted by the free ends
of adjustable springs anchored to posts on the brush
holders. Springs are adversely affected by current
passing through them; it is usual, the re fore, to nt an
insulating pad or roller at the end of the spring where
it bears on the top surface of the brush.
The brush holders are secured either by bolting
them to a support ring (usually called a brush rocker)
which is, in turn, bolted to the commutator end frame,
or as in the case of the generator illustrated, bolted
directly to the end fran1e. ln order to achieve the
best possible commutation a support ring, or end
frame, as appropriate, can be rotated through a few
degrees to alter the position of the brushes relative to
the magnetic neutral axis. Marks are provided on each
generator to indicate the normal operating position.
When four or more brush holders are provided, they
are located diametrically opposite and their brushes
arc alternately positive and negative, those of similar
polarity being connected together by bar and flexible
wire type links.
The brushes are fitted with short leads or "pigtails"
of flexible copper braid moulded into the brush during manufacture. The free ends of the pigtails termin·
ate in spade or plate type terminals which are con•
nected to the appropriate main terminals of the genera·
tor via the brush holders and connecting links.
The leads from brush-gear assernblies and field windings
are connected to terminal posts secured to a block
mounted on the commulator end frame or, in some
generators, on the yoke assembly (see Fig. 1.7). The
tenninals and block are enclosed in a box-like cover
also secured to the end frame. Entry for the output
sup.ply cables of the distribution system (refer to
Chapter 5) is through rubber clamps. The rotation of
a generator armatu re is specified in a direction, norm·
ally anti-clockwise, when viewed from the drive end
assembly. A movable link ls fitted between two of the
terminals which can be connected in an alternative
position should it be necessary for the genera tor to be
driven In the reverse direction.
Sparl<.ing at the brushes of a generator, no matter how
slight, results in the propagation of electromagnetic
waves which interfere with the reception of radio signals. The interference originating in generators may be
eliminated quHe effectively by screening and suppres·
sion, Screening involves the enclosure of a generator
in a continuous metallic casing and the sheathing of
output supply cables in continuous metallic tubing or
conduit to prevent direct radiation. To prevent inter·
ference being conducted along the distribution cable
system, the screened output supply cables are termin·
ated in filter or suppressor units. These units consist
of chokes and capacitors of suitable electrical rating
built into metal cases located as close to a generator
as possible. Independent suppressor units arc rather
cumbersome and quite heavy I and it is therefore the
practice in the design of current types of generator to
incorporate internal suppression systems, These
systems do not normally contain chokes, but consist
simply of suitably rated capacitors (see Fig, I. 7)
which are connected between generator casing (earth)
and terminals. The use of internal suppression systems
eliminates the necessity for screened output supply
cables and conduits thereby malting for a considerable
saving in the overall weight of a generator installation.
Rectified Power Supplies
In many of the smaller types of single-engined and
twin.engined aircraft, the primary d.c. power ls
supplied in a manner similar to that of automobiles,
i.e. It is a rectified output from a frequency.wild
alternating current generator. Its operating frequency
is about l 00 Hz at idling speed of the engine and
increases with speed to 1200 Hz or higher.
The generator or alternator as it is more generally
called, consists of a rotor, stator, slip ring and brush
assembly and end frames. ln addition six silicon
diodes are carried in an end frame and are connected
as a bridge rectifier (seep. 57) to provide the d.c.
for the aircraft's system. The principal constructional
features are illustrated in Fig. 1.8.
The rotor Is formed by two extruded steel pole
pieces which are press-fitted on to the rotor shaft to
Fig 1.8
Alternntor supplying a rectified output
sandwich a field coil and thus form the core of the
electromagnet, Each pole piece has six "fingers"
which in position, mesh but do not touch each
other. Excitation current is fed to a field coil on the
rotor via brushes, and slip-rings whlch are press-fitted
onto the rotor shaft.
The stator is made up of a number of steel stamp·
ings riveted together to form the core around which
the three star-connected phase coils are wound. One
end of each winding is connected to the bridge
rectifier assembly while the other ends are joined
to fonn what is termed the neutral point. The stator
assembly is clamped between the end frames.
Figure 1.9 illustrates the circuit diagram of the
alternator. Unlike a conventional d.c. generator; the
alternator has no residual magnetism and so its field
must be excited initially by d.c. from the aircraft's
battery or an external power supply. When d.c. is
switched on to the generator; the rotor field coil ls
energized and the pole piece "fingers" become
alternately north and south magnetic poles. As the
rotor rotates, the field induces a three-phase
altemating current within the stator which is fed
to the diodes for rectification, and then to the
aircraft's system. As will be noted from Fig. 1.9,
when the alternator is supplying the busbar, it will
also supply Its own field excitation current to
sustain the regulated output. The level of voltage is
regulated by a solid-state type of voltage regulator
(seep. 14).
Generator/Engine Coupling
Depending on the type and application, a generator
may be driven by an engine either from an accessories
gear box, or by a pulley and bell.
The generator already shown ln Fig. 1.7, ls an
example of one driven through gearing which forms
parl of an accessories gear-box. Depending on the
rated output of a generator and on the load requirements of the electrical system of a particular aircraft,
there is a specific gear drive ratio.
The drive from the gear-box is by means of a quill
shaft with either male or female serrations or splines
at one or both ends. The serrations or splines mate
with corresponding formations on the generator
armature shaft (see Fig. 1.7) to transmit the torque
delivered by the driving gear. One of the requirements
to be satisfied by a quill drive js that lt must effect·
lvely Interrupt transmission of the driving torque in
Fig 1.9
Circuit diagram of alternator
the event that the generator armature seizes up. This
is done by designing the drive shaft so that at one
section its diameter is smaller than the remaining
sections; thus providing a weak spot at which the
shaft will shear under the effect of an excessive
Quill drives are usually short and rigid, but in some
cases a long drive with one end mating with serrations
formed deep in a hollow armature shaft may be speci·
fled, This arrangement enables the drive to absorb
much of the mechanical vibration which is otherwise
passed to a generator from an accessories gear-box.
The method of securing a generator to an accessories gear-box varies, but in general it Is either one
utlllzing a mounting flange or one requiring a manacle
ring. In the mounting flange method, the end frame
at the drive end of a generator ls usually extended to
a larger diameter than the yoke, thus forming a projecting flange. Holes in the flange line up with and
accept studs which are located in the mounting pad
of the engine or gear-box, and the generator is finally
secured by nuts, locking washers, etc. An alternative
fonn of flange mounting is based on a generator end
frame having two diameters. The larger diameter is
no greater than that of the yoke and abuts on the
mounting pad while the reduced diameter provides a
channel or "gutter", between the yoke and the larger
diameter of the end frame, into which the mounting
studs project. Another variation of this form of
mounting is employed in the generator shown in
Fig. 1.7.
In the manacle-ring method of mounting the
generator drive end frame has an extension with a
recess in the mounting face of the driving unit. When
the generator extension is fully engaged with the
recess, a flange on the end frame abuts on a matching
flange formed on the driving unit mounting face. The
two flanges are then clamped together by a manacle
ring which, after being placed over them, is firmly
closed by a tensioning screw, A spigot arrangement is
usually incorporated to provide locat,ion of the
generator to the drive unit, and to absorb torque
reaction when the generator is operating.
The pulley and belt drive is commonly adopted
for driving altemators of the tYPe shown in Fig. 1.8,
and as may be seen from Fig. 1.10, it_is similttr in
many respects to the one adopted in automobiles.
The alternator ls secured to two mounting
brackets one of which is slotted, so that when the
corresponding securing bolt Is slackened, the alternator may be positioned about the ot] bolt for
the purpose of adjusting belt tension. The required
drive ratio is, of course, detennined by the diameters
of the engine and altomator pulleys.
Fig l.10
Pulley and belt drive
Cooling of Generators
The maximum output of a generator, assuming no
limit to input mechanical power, Is largely detennined
by the ease with which heat (arising from hysteresis,
thermal effect of current in windings, etc.) can be
dissipated. With large-bulk generators of relatively low
output the natural processes of heat radiation from the
extensive surfaces of the machine carcase may well
provide sufficient cooling, but such "natural" cooling
is inadequate for the smaller high-output generators
used for the supply of elec.trical power to aircraft, and
must, therefore, be supplemented by forced cooling.
The most commonly accepted method of cooling
is that which utilizes the n1m or blast effect resulting
from either the slipstream of a propeller or the airstream due to the aircraft's movemenL A typical cooling system is shown in a basic form In Fig. 1.11 . The
air is forced at high speed into an intake and is led
through light-alloy ducts to a collector at the commu·
tator end of the generator. The air discharges over the
brush-gear and commutator to cool this natural area
of high temperature, and then passes through the
length of the machine to exhaust through apertures,
surrounded by a perforated strap, at the drive end. In
order to assist in ram-air cooling and also to pro".ide
some cooling when the aircraft ls on the ground,
many types of generator have a fan fitted at the drive
end of the armature shaft.
Eng,ne nacelle
Romo,, flow
end of gene1otor
Fig. l.11
1·ypical cooling ~ystem
Cooling of the alternator shown in Fig. 1.8 Is
provided by a fan at the drMng end and by air
passing through slotted vents in the slip ring end
frame, Heat at the slllcon diodes Is dissipated by
mounting them on steel plates known as "heat sinks".
Brush Wear
The carbon from which electro-graphllic brushes are
made is extremely porous and some of the pores are
so very fine that carbon has an exceptional ability to
absorb other substances into its structure, and to
retain them. Moisture is one of these substances and it
pjays an important part in the functioning of a brush
contact by affording a substantial degree of lubrication.
The moisture is trapped under the inevitable irregular,
itles of the contact faces of the brushes and forms an
outside fi..lm on the commutator and it is with
mm that the brushes make contact. °Just how vital a
part moisture docs play was, however, not fully realized
until aircraft began operating at high altitudes and the
problem arose of brushes wearing out very rapidly
under these conditions. Investigations inl o the problem
showed that the fundamental difficulty was the
extreme dryness of the atmosphere, this, in its tum ,
producing th ree secondary effects: (I) friction between
brushes and commutator because the lubricating film
cannot form, (ii) contact resistance becomes negligible
giving rise to heavy reactive sparking and accelerated
brush erosion and (iii) static electrical charges due to
friction, producing molecular broakdown of the
These effects have been largely eliminated by usin1
brushes which have a chemical additive as a means of
replacing the function which atmospheric moisture
plays in surface skin formation. Two distinct categor·
ies are in general use: brushes of one category form a
constant-resistance semi-lubricating film on the corn·
mutator, while those in the other category are, in
effect, self-lubricating brushes which do not form a
The composition of the film-forming brushes
includes chemicals (e.g. barium fluoride) to build
Up progressively a constant-resistance semi-)ubricatin
film on the commutator surfaces. Brushes of this
category do not wear abnormally at altitudes up to
60,000 feet providing that generators to which they
are fitted have been previously ''bench run" for sorm
hours to allow the formation of the protective film.
This film. once formed, is very dark in colour and rn:
often give the impression of a dirty commutator.
Brushes of the non-film-forming category contain
lubricating ingredient such as molybdenum disulphid
which is often packed in cores running longitudinally
through the brushes. Since the brush is se!f-lubricall11
it is unnecessary for generators fitted with this type
to be run for hours prior to entering service. Howeve
they do have the clisadvantage of appreciably shorter
life, due to somewhat more rapid wear, when compared with film.forming brushes.
Voltage Regulation
The efficient operation of aircraft electrical equipm,
requiring d.c. depends on the fundamental rcquirern
that the generator voltage at the distribution busbar
system be maintained constant under all conditions
load and at varying speeds, within the limits·of a pn
scribed range. It is necessary , the refore, to provide a
device that will regulate the output vullage of a gen,
ator at the designed value and within a specified to!,
There are a number of factors which , either sepa
or in combination, affect the output voltage of a d.,
generator, and of these the one whlch ca.n most con
veniently be controlled is the field circuit current,
which in its turn controls the nux density. This con
trol can be effected by incorporating a variable resi:
in series with the field winding as shown in Fig. J. l
Adjustments to this re'sistor would vary the resistan
of the field winding, and the field current and outp1
voltage would also vary and be brought to the requi
controlling value. The application of the resistor in
manner indicated is, however, limited since it is essen·
tial to incorporate a regulating device which will automatically respond to changes of load and speed, and
also, automatically make the necessary adjustments to
the generator field current. Three of the regulation
methods conunonly adopted are: the vibrating
contact method; the one based on the pressure/
resfatance characteristics of carbon, namely, the
carbon pile method, and the one based on solidstate circuit principles.
Fig 1.12
Control of f1eld circuit cutrent
Vibrating Contact Regulator Vibrating contact regu·
Iators are used in several types of small aircraft employ
Ing comparatively low d.c. output generators and a
typical circuit for the regulation of both voltage and
current of a single generator system is shown in basic
form in Fig. 1.13. Although the coil windings of each
regulator arc interconnected, the circuit arrangement
is such that either the voltage regulator only or the
current regulator only can operate at any one time. A
third unit, called a reverse current cut-out relay, also
forms part of some types of regulator, and since tl1e
relay has a circuit protection function, a description
of its construction and operation will be given in
Chapter 7.
Voltage Regulator unit consists of two windings
assembled on a common core . The shunt winding consists of many turns of fine gauge wire and is connected
in series with the current regulator winding and in
parallel with the generator. The series winding, on the
other hand, consists of a few turns of heavy gauge wire
and is connected in series with the generator shuntfield winding when the contacts of both regulators are
closed, i.e. under static condition of the generator
system. The contact assembly is comprised of a f'ixed
contact and a movable contact secured to a flex.ibly-
To (11;lrib~tion
. - - - - -- - - - - - - - - - - - - - -- - - - . - - - - - - 1 - - -system
Fig l.l3
Vibrating contact regulator principle
hinged armature. Movement of the armature and,
therefore, the point at which contact opening and
closing takes place is controlled by a spring whlch is
pre-adjusted to the required voltage setting,
When the generator starts operating, the contacts
of both regulators remain closed so that a positive
supply can flow through the generator shunt·field
winding to provide the necessary excitation for .raising
the generator output. At the same time current passes
through the shunt winding of the voltage regulator
and, in conjunction with the series winding, it
Increases the regulator's electromagnetic field. As soon
as the generator output voltage reaches the pre,adjusted
regulator setting, the electromagnetic field becomes
strong enough to oppose the tension of the am1ature
spring thereby opening the contacts. In this equilibrium
position, the circuit to the series winding Is opened
causing its field to collapse. At the same time, the
supply to the generator field winding passes through a
resistance (R) which reduces the excitation current
and, therefore, the generator output voltage. The
reduced output in turn reduces the magnetic strength
of the regulator shunt wincling so that spring tension
closes the contacts again to restore the generator out·
put voltage to its regulated value and to cause the
foregoing operating cycle to be repeated. The frequency
of operation depends on the electrical load carried by
the generator; a typical range is between SO to 200
times a second.
In regulators designed for use with twin·generator
systems, a third coil is also wound on the electro·
magnet core for paralleling purposes (seep. 16) and
is connected to sepatate paralleling relays,
Cun-ent Regulator This unit linuts generator current
output In exactly the same way as the voltage regulator
controls voltage output, i.e. by controlling generator
field~xcitation current. Its construction differs only
by virtue of having a single winding of a few turns of
heavy wire.
When electrical load demands are heavy, the voltage
output value of the generator may not increase suffic.
iently to cause the voltage regulator to open its con·
tacts. Conseqi•P.ntly, the output will continue to
increase until It reaches rated maximum current, this
being the value for which the current regulator is set.
At this setting, the current flowing through the regu·
lator winding establishes a strong enough electromagnetic field to attract the armature and so open the
contacts. Thus, it is the current regulator which now
inserts resistance R in the generator shunt-field circuit
to reduce generator output. As soon as there is sufficien
drop in output the field produced by the regulator
winding is overcome by spring tension, the contacts
close and the cycle again repeated at a frequency
similar to that of the voltage regulator,
Carbon Pi_/e Regulator Carbon has a granular surface
and the contact resistance between two carbon faces
that are held together depends not only on the actual
area of contact, but also on the pressure with which
the two faces are held together. if. therefore, a number
of carbon discs or washers are arranged in the form
of a pile and connected in series with the shunt field
of a generator (see Fig. 1.14) the field circuit resistance
can be varied by increasing or decreasing the pressure
applied to the ends of the pile and changes in generator output voltage therefore counteracted. Since this
method eliminates the use of vibrating con tacts, it is
applied to generators c-apablc of high current output,
and requiring higher field excitation current. The
necessary variation of pile pressure or compression
under varying conditions of generator speed and load,
is made through the medium of an electromagnet and
spring-controlled armature which operate in a similar
manner to those of a vibrating contact regulator.
Under static conditions of the generator system,
the carbon pile is fuUy compressed and since there is
no magnetic ''puU" on the armature, the resistance in
the generator shunt.field circuit is minimum ai1d the
air gap between the regulator armature and electro·
magnet core is maximum. As the generator starts
operating, the progressively increasing output voltage
is applied to the regulator coil and the resulting field
establishes an increasing "puU" on the armature.
During the initial "run-up" stages, the combination of
low voltage applied to the regulator coil, and the
maximum air gap between arma ture and core, results
in a very weak force of attraction being exerted on the
armature. This force is far smaller than that of the
spring control, hence the armature maintains its original
position and continues to hold the carbon pile in the
fully compressed condition; the shunt-field circuit
resistance is thus maintained at minimum value during
run-up to allow generator output voltage to build up
as rapidly as possible. This condition continues unaltered until the voltage has risen to the regulated
value, and at which equilibrium is established between
magnetic force and spring-control force. The armature
ls free to move towards the electromagnet core if the
force of magnetic attraction is increased as a result
of any increase in generator speed within the effective
Arma re
Spnng oonlrol
Fig 1.14
Carbon pile voltage regulation
speed range. Ln these circumstances pile compression is
further reduced so that there is more between
discs to increase resistance and so check a rise in
generator output voltage; it also increases the spring
loading that holds the armature away from the core.
Thus, a condition of equilibrium is re-established with
the armature in some new position, but with the
output voltage still at the required regulated value.
Any reduction of generator speed, within the
effective speed range, produces 11 reduction in generator output voltage thus disturbing regulator armature
equilibrium in such a manner that the spring-control
force predominates and the armature moves away
from the electromagnet core. The carbon pile is re·
compressed by this movement to reduce the generator
shunt-field circuit resistance and thereby increase
generator output voltage, until the regulated output
ls again brought fo a state of equilibrium. When
progressive reduction of generator speed results In a
conclitfon of maximum pile compression, control of
generator output vijltage Is lost; any further reduction
of generator speed, below tho lower limit of the
effective range, resulting in proportional decrease in
output voltage .
When a generator has been run up and connected
to its distribution busbar system, the switching on of
various requisite consumer services, will Impose loads
which disturb the equilibrium of the regulator
armature. The effect is, in fact, the same as if the
generator speed had been reduced, and the regulator
automatically takes the appropriate corrective action
until the output voltage is stabilized at the critical
value, Conversely, a perceptible decrease in load,
assuming generator speed to be constant and the
regulator armature to be in equilibrium, results in the
regulator taking the same action as in the case of an
increase in generator speed.
Construction The pile unit is housed within a ceramic
tube which, in turn, is enclosed in a solid casing, or
more generally, a finned casing for dissipating the
heat generated by the pile. The number, diameter, and
thickness of the washers which make up the
pile, varies according to the specific role of the
regulator. Contact at each end of the pile is made by
carbon inserts, or in some types of regulator by silver
contacts within carbon inserts. The initial pressure of
the pile is set by a compression screw acting through
the pile on the armature and plate-type control spring
whlch is supported on a bi-metal washer. The washer
compensates for temperature effects on voltage coil
resistance and on any expansion characteristics of the
regulator, thus maintaining constant pile compression.
The electromagnet assembly comprises a cylindrical
yoke in which is housed the voltage coil, a detachable
end-plate and an adjustable soft-iron core. A locking
device, usually in the form of screws, is provided to
retain the core in a pre-set position.
Depending on the design of generating system,
voltage regulators may be of the single-unit type,
shown in Fig. 1.1 5, which operates in conjunction
with separate reverse current cut-outs, voltage
differential sensing relays _and paralleling relays, or
integrated with these components to fonn special
control units or panels.
Fig 1,15
Typlc::111 slng!e,unll type regulator
I . Armature stop screw
2. Magnet cuse
3. Ilea\ dlssipator
4. Tcrrninal blocks
5. Chassis
Fig. 1.16 shows the circuit afrangement of a typica
solid-state voltage regulator as employed with the
type of alternator shown in Fig, 1,8. Before going
into its operation,.however, it will be helpful at this
stage, to briefly review the primary function and
fundamental characteristics of the device known as
the transistor.
The primary function of a transistor is to " transfe r
resistance" within itself and depending on its connection within a circuit it can turn current "on'' and
"off' and can increase output signal c.onditions; In
other words, it can act as an automatic switching
device or as an amplifier. It has llO moving parts and is
made up of three regions of a certain material, usually
germanium, known as a semiconductor (see also p. 53;
and arranged to be In contact with each other in some
definite conducting sequence. Some typical transistor
contact arrangements are shown In Fig. 1.17 together
with the symbols used. The letters "p'' and "n"
refer to the conductivity characteristic of the
germanium and signify positive-type and negative-type
respectively. A transistor has three external connec.
lions corresponding to the thiee regions or elements
known as the emitter whlch injects the current carrien
at one end, the collector which collects the current at
the other end, and the base which controls the amoun
of current flow. The three elements are axranged to
contact each other in sandwich form and in the
sequence of either n•p·n or p-n-p. When connected in
a circuit the emitter is always forward-biased in order
to propel the charged current carriers toward.s the
collector, which is always reverse-biased in order to
collect the carriers. Thus, the emitter of an n·p·n transistor has a negative voltage applied to it (with respect
to base) so as to repel negative electrons in the forwar,
direction, while a positive voltage is applied to the
emitter of a p-n-p transistor so as to repel positively
charged "holes" in a forward direction.
Since reverse bias is always applied to collectors
then the collector of an n·p·n tJansistor is made
positive with respect to the emitter In order to attract
negative electrons. Similarly, the collector of a p-n-p
tramlstor Is made negative with respect to the emitter
so as to attract positively charged "holes".
The conventional current flow is, of cou.rse, opposi
to the electron flow and passes through a transistor
and tho circuit external to it, from emitter to collecto1
and through the base. Th1s is inclicated on the symbofa
adopted for both transistor arrangements, by arrows
on the emitter (see Fig. 1.17). Any input voltage that
increases the forward bias of the emitter, with respect
To services
.------ Alternolor
!l l l
Volloge regulotOI
+ --
+ --
Reverse currant
tn this
section blockeel by
'z' unl1l breakdown
Fig 1.16
Solld~iaie voltage regulator
-• -
Elee1,on llow G,'i101es'
- --currenl llow
Ern,tter b1os Colleelor b,os
Em111cr b•OS Colleclor b,os
Pig 1.17
Transistor contact arrangoments
- -+ Boitery current
~ectified curre!lt
Reverse current
to the base, increases the emitter•to-collector current
flow, and conversely, the current flow is decreased
when an input voltage decreases forward bias. The
characteristics of transistors are such that small
changes-in the emitter-base circuit current result in
relatively large changes in collector current thereby
making transistors efficient amplifying devices. By
alternately connecting and disconnecting the base
circuit to and from a forward biasing voltage, or
similarly, by alternately applying a forward and
reverse voltage, base current and th~ collector
current, can be caused to flow and to cease Oowing.
In this manner, a transistor can thereby also function
as a switching device.
ln the regulator circuit shown in Fig. 1.16, the
three transistors (TR 1 , TR1 and TR 3 ) are connected
in the n-p-n arra11gement. When the system control
switch is "on", excitation current flows initially from
the battery to the base of TR2 and through a voltage
dividing network made µa of resistances R1 , R, and
RV 1 . The purpose of this network In conjunction with
the Zener diode ''Z" (see also p. 55) is to establish
the system-operating voltage. With power applied lo
the base ofTR1 , the transistor is switched on and
battery current flows to the collector and emitter
junction. The amplified output in the emitter circuit
nows lo the base of TR3 thereby switching it on so
that the battery current supplied to the field winding
can be conducted to ground via the collector-emitter
junction of TR), When the generator is running, the
rotating magnetic field induces an alternating current
in the stator and this is rectified and supplied to the
d.c. power system of the aircraft.
When the alternator output voltage reaches the pro.
set operating value, the current flowing in the reverse
direction through the Zener diode causes it to break
down and to al.low the current to flow to the base of
TR 1 thus switching it on. The collector-emitter junc·
lion ofTR 1 now conducb, thereby diverting current
away from the base of TRz and switching it off. This
action, in turn, switches offTR 3 and so excitation
current to the alternator field winding is cut off. The
rectifie.r across the field winding (D 1) provides a path
so that field current can fall at a slower rate and thus
prevent generation of a high voltage at TR) each time
it is switched off,
When the alternator output voltage falls to a value
which permits the Zener diode to cease conduction,
TR 1 will again conduct to restore excitation current
to the Oeld winding, This sequence of operation is
repeated and the alternator output voltage is thereby
maintained al the preset operating value.
on their negative sides, via a series "load-sharing" or
''equalizing" Joop containing equalizing coils (Ce) ea
coil forming part of the Individual voltage regulator
Equolizinq current r0
Fig 1.18
l'rindplc of load·sharing
=>- Voltage cool curr
_, Eovolizir,g currc
VOlloq;i ccal turrcn1
EquQI if'ln(] c;urr
Lin13 i;,QOIDCIOf ">
Paralleling and Load-Sharing
In multi-engined aircraft, it is generally desirable that
the generato rs driven by each engine should operate
in parallel the reby ensuring that in the event of an
engino or generator failure, there is no interruption of
primary power supply. Parallel operation requires
that generators carry equal shares of the system load,
and so their output voltages must be as near equal as
possible under a11 operating conditions. As we have
already learned, generators are provided with a voltage
regulator which exercises independent control over
voltage output, but as variations in output and
electrical loads can occur, it is essential to provide
additional voltage regulation circuits having the
function of maintaining balanced outputs and load
sharing, The method most commonly adopted for
this purpose Is that which employs a ''load -equalizing
circ1,1it" "to control generator output via the voltage
regulators, The principle as applied to a twinge nerator system is illustrated in much-simplified
form by Fig. 1.18. The generators arc interconnected
!111rq ,aJoys
(b l
Fig 1.19
Load sharing (c31hon pile regulators)
The resistances R1 and R1 represe nt the resistances
of the negative seclions (lnterpolc windings) of the
generators, and under balanced load-sharing condilic
the volts drop across each section will he the same,
No 1 Voltage regulator
· - - -- - - - - ·- - - - -
No. 2 Voltage regulator
I I i:t
,,~t.!Jr r
- - - - --+--+---
Paralleling relay unit
1 - - - - - - - -- -- ,
Fig l.20
Load sharing {vibrating co.nlact regulators}
i.e. V 1 .. 11 R1 and Y 2 - 12 R1 • Thus, the net volts
drop will be zero and so no current will now through
the equalizing coils.
Let us now assume that generator No. I lends to
take a somewhat larger share of the total load than
generator No. 2. In this condition the volts drop V1
will now be greater than Yi and so the negative
section of generator No. I will be at a lower potential.
As a re-sull, a current le will flow through the equalizing coils which are connected in such a manner tlrnt
the effect of 10 is to raise the outpu t voltage of generator No. 2 and reduce that of No . I, thereby effectively reducing the unbalance in load sharing.
Figure 1.19 illustrates the principle as applied to an
equalizing circuit which approximates to that of a
practical generating system utilizing carbon pile voltage regulators. The equalizing coils are wound on the
same magnetic cores as the voltage coils of the regulators, thus , assuming the same unbalanced conditions
as before, the current le flows in a direction opposite
to that flowing through the No. 2 generator voltage
regulator coil, but in the same direction as the voltage
coil current in No. I regulator. The magnetic effect of
the No. 2 regulator voltage coil wiU therefore be
weakened resulting in a decrease in carbon pile resistance and an increase in the output ·o r No. 2 generator
(see also p. 12), enabling it to take more of the load.
The magnetic effect of the No. I regulator voltage coil
on the other hand, is strengthened, thereby increasing
carbon pile resistance and causing No . .1 generator to
decrease its output and t'o shed some of its load. The
variations in output of each generator continues until
the balanced load.sharing condition is once again
restored, whereby the equalizing-circuit loop ceases
to carry current.
The principle of paralleling as applied to a twin
d.c. generator system utilizing vibrating contact
regulato rs is shown in rig. 1.20. In this case, the
equalizing or paralleling circuit comprises an
additional coil "Eq'' in the voltage regulation sections
"A" of each regulator, and a paralleling relay unit.
When both generators are in operation and supplying the requisite regulated voltage, the contacts in the
voltage and current (''B") regulation sections of each
regulator are closed. The contacts of lhe reverse
current relays "C" are also closed thereby connecting
both generators to the .busbar. The ovtputs from each
generator are also supplied to the coils of the
paralleling relay unit and so the contacts of its relays
are closed. Thus, together with each of the coils
''Eq", the equalizing or paralleling circuit is fom1ed
between the generator outputs. Under load-sharing
conditions, the current flowing through the coils
"Ecf' is in the same direction as that through the
voltage coils of the voltage regulating sections of each
regulator, but in equal and opposite directions at the
contacts of the paralleling relay unit.
If the voltage output of one or other generator,
e.g. number l. should rise, there will be a greater
voltage input to the voltage regulatLng section of
the number I voltage regulator compared to the
input at the corresponding section of the nu11lber 2
regulator. There will therefore, be an unbalanced
Oow of current through the equalizing circuit such
that the increase in current through the coll ''Ee(
of the number I voltage regulator will now assist the
magnetic effect of the voltage coil "D" causin~ the
relay contacts to open. The resistance thereby
inserted in the {lcld circuit of number J generator
reduces its excitation current and its voltage output.
Because of the unbalanced condition, the Increased
current in the equalizing circuit will also flow across
the paralleling relay unit contacts to the coil "Eq''
in the number 2 voltage regulator so that it opposes
the magnetic effect of its associated coil "D" .
In paralleled alternator systems using solid-state
voltage regulators, any unbalanced condition is
detected and adjusted by interconnecting the
regulators via two additional paraUeling transistors,
one in each regulator.
In almost all aircraft electrical systems a battery has
the following principal functions (i) To help maintain the d.c. system voltage under
transient conditions. The starting of la.1ge d.c. motordriven accessories, such as inverters and pumps, requires
high input current which would lower the busbar volt.
age momentarily unless the battery was available to
assume a share of the load. A similar condition exists
should a short circuit develop in a circuit protected by
a heavy duty circuit breaker or current limiter. This
function possibly applies to a lesser degree on aircraft
where the electrical system is predominantly a.c., but
the baste principle 'still holds true.
(ii) To ,upply power for short term heavy loads
when generator or ground power is not available,
e.g. internal starting of an engine.
(ill) Under emergency conditions, a battery is
intended to supply limited amounts of power. Under
these conditions the battery could be the sole remaining source of power to operate essential flight instru-
ments, radio communication equipment, etc., for as
the spaces of the plates are packed with pastes of
long as the capacity of the battery allows.
acUve lead materials. The two plate groups are interA battery Is a device which converts chemical
leaved so that both sides of every positive plate face a
energy into electrtcal energy and is made up of a numnegative surface. The plates prevented from cornber of cells which, depending on battery utilization,
ing into contact with one another by means of
may be of the primary type or secondary type. Both
separators (not shown) made from materials having
types of cell operate on the same fundamental principle, high insulating qualities and ability to permit uni.e. the exchange of electrons due to the chemical
obstructed circulation of the electrolyte at the plate
action of an electrolyte and electrode materials. The
surfaces. Each group of positive plates and negative
essential differences between the two lies in the action
plates is connected through a strap to a terminal post
that occurs during dJschargc. In the primary cell this
at Lhe top and on opposite sides of the cell, TI1e interaction destroys the active materials of the cell, thus
nal resistance of a cell varies immensely with the dislimiting its effective life to a single discharge operation,
tance between the positive and negative electrode sur·
whereas in the secondary cell the discharge action confaces; therefore, to obtain the lowest possible resistance
verts the active material into other forms, from which
the gap between the plates of each group is made as
they can subsequently be electrically reconverted,
small. as is practicable. A cell contains an odd number
into the original materials, Thus, a secondary cell can
of plates, the outermost ones belonging to the negative
have a life of numerous discharge actions. followed by
plate group. The reason for this arrangement is that
the action of re-conversion more commonly known
unlike a positive plate a negative plate will not distort
as charging. The batteries selected for use in aircraft
when the electromechanical action is restricted to one
therefore employ secondary cells and are either of the
side only. The plate assemblies of a cell are supported
lead-acid or nickel-cadmium type.
in an acid-proof container.
Lead-Acid Secondary Cell
The basic construction of a typical cell is shown in
Fig. 1.21. lt consists essentially of a posillve electrode
and a negative electrode, each of wh_ich is, in turn ,
made up of a group of lead-antimony alloy grid plates;
~!~?ri; :~~~~ ~~~
Each positive plate of a fully-charged coll consists of
the lead-an timony alloy grid into which lead peroxide
paste (PbO~) has been forced under pressure. The
negative plates are of similar basic structure, but with
pure spongy lead (Pb) forced in to Lhe grid. The electro·
Jyte consists of two constituents, sulphuric acid
(HiS0 4 ) and water, which are mixed in such proportions that the relative density is generally about
1-25 to l ·27.
During discharge of the cell, that is, when an
external circuit is completed between the positive and
negative plates, electrons are transferred through the
circuit from lead to lead peroxide and the net result
of the chemica) reaction is that lead sulphate (PbS04)
forms on both plates. At lhe same time molecules of
water are formed, thus weakening the electrolyte. For
all practical purposes, the cell is considered to be discharged when both plates are covered with lead sulphate and the electrolyte has become quite we:lk.
The cell may be recharged by connecting the
positive and negative plates, respectively, the
positive and negative terminals of a d.c. source of
slightly higher voltage than the cell. All the foregoing reactions are then reversed ; the lead sulphate on
the positive plate being restored to lead peroxide, the
negative plate restored to spongy lead, and the electrolyte restored to its original relative density.
negative plot! qr0up1
1nterloc~ed. Plgte
;cporators ore nohhown
Po61tlve plalo
Fig 1.21
Typical lead-acid secondary cell
t,/e9otiw pJgte
Two types of lead-acid battery may be found in
general uso; in one the electrolyte is a free liquid while
in the other it is completely absorbed into the plates
and separators. An example of the former type of
battery is illustrated in Fig. 1.22. The unit has a 24·
volts output and consists of two 12-volt cell blocks
moulded in high-impact plastic material and housed in
iUl acid-proofed aluminium container. The links interconnecting the cells and cell blocks are sealed and
suitably insulated to prevent contact with the container.
A plastic tray is fitted on to the top edges of the container and is sealed around the cell vent plugs by
rubber pads,and plastic sealing rings. Tho tray forms
the base of a chamber for tho ventilation of acid
vapours. A plastic lid combined with an acid-proofed
V8nt pl,.,q Qnd wD"'10r
aluminium alloy hold-down frame completely
encloses the chamber, Connections are provided at
each end of the chamber for coupling the pipes from
the aircraft's battery compartment ventilation system
(seep. 24).
The battery illustrated in Fig. 1.23 utilizes a more
specialized form of cell constru~llon than that just
described. The plates, aclive materials and separators
are assembled together and arc compressed to form a
solid block. The active material is an infusorlal earth,
known as kieselguhr, and is very porous and absorbent. Thus, when the electrolyte is added, instead of
remaining free as in the conventional types of battery,
it Is completely absorbed by the active material. This
has a number of advantages; notably improved electromechanical activity, no disintegration or shedding of
active material, thus preventing internal short-circuits
caused by "sludge", low internal resistance and a
higher capacity/weight ratio than a conventional
battery of comparable capacity.
The cells are assembled as two 12-volt unit~ in
monobloc containers made of shock-resistanl polystyrene and these are, in turn, housed in a polyester·
bonded fibreglass outer container whlch also supports
the main terminal box. A cover of the same material
as the case is secured by four bolts on the end flanges
of the case.
Nickel-Cadmium Secondary Cell
Fig 1.22
Lead-acid baltery (free liquid type)
ln this type of cell the positive plates are composed of
nickel hydroxide, Ni(OHh, the negative plates of
cadmium hydroxide Cd(OHh and the electrolyte is a
solution of distilled water and potassium hydroxide
(KOH) with a relative densily of from l •24 to l ·30.
Batteries made up of tht:se cells have a number of
advantages over the lead-acid type, the most notable
being their ability to maintain a relatively steady voltage when being discharged at high cu(rents such as
during engine starting.
The plates are generally made up by a sintering
process and the aclive materials are impregnated into
the plates by chemical deposition. This type of con.
struction a1Jows the maximum amount of active
-material to be employed in the electrochemical action
After impregnation with the active materials, the
plates are stamped out to the requisite size and are
built up Into positive and negative plate groups, inter·
leaved and connected to terminal posts in a manner
somewhat similar to the lead-acid type of cell.
Conne<:tor bdr
Connon .,.
[email protected]
rm1nol 1nsuloto,
COM1111 roeeotocle/
i,ner'-",nq 2 eonroct ~ioove1l
Fig 1,23
Lca.d11cid battery (11bsorbed liquid type)
Insulation is done by means of a fabric-base separator
in the form of a continuous strip wound between the
plates, The complete plate group is mounted in a
sealed plastic container.
During charging, the negative plates lose oxygen and
become metallic cadmium. The positive plates Bie
brought to a higher state of oxidation by the charging
current until both materials are completely converted;
i.e. all the oxygen is driven out of the negative plates
and only cadmium remains, the positive plates pick
up the oxygen Lo form nickel oxides. The cell emits
gas towards the end of the charging process, and during
overcharging; the gas being caused by decomposition of
the waler component of the electrolyte into hydrogen
at the negative plates and oxygen at the positive plates.
A slight amount of gassing is necessary to completely
charge the cell and so it therefore loses a certain
amount of water.
The reverse chemical action takes place during discharging, the negative plates. gradually gaJ.ning back
the oxygen as the positive platos lose it. Due to this
interchange there is no gassing on a normal llischarge.
In this way, the chemical energy of the plates is con.
verted into electrical energy, and the electrolyte is
absorbed by the plates to a point where it is not visible
from the top of the cell. The electrolyte docs not play
an active part in the chemical reaction; it Is used only
to provide a path for current flow.
The chemical reaction of a nickel-cadmium cell is
summarized in Table l .l and may be compared with
that taking place in a lead-acid battery cell,
The construction of a typical battery currently in use
is shown in Fig. 1.24. All the cells are linked and contained as a rigid assembly in the case, A space above
the cells provides a ventilation chamber whjch is
completely enclosed by a lid held in position b.y a
pair of bolts anchored to the aircraft battery com·
partment. Acid vapours are drawn out from the
chamber via the vents in the battery case and the
interconnecting pipes of the wrcraft's battery com·
partment ventilation system.
Capacity of Batteries
The capacity of a battery, or the total amount of
energy available, depends upon the size and number of
plates. More strictly it Is related to the amount of
material available for chemical action.
The capacity rating is measured in ampere-hours
and is based on the·maxirnum current, in amps, which
it will deliver for a krtown time period, until it is dis-
Corr yino ho~dle
VPnl n•po
Mo•n bot lery
' 1;.onnet tor
Fig. 1,:2.4
Nickel-cadmium type battery
Table 1.1
Chemical Reactions of Batteries
Batt11n' Type
State of
Positive Plate
Ncga tivo Pia te
(Lead Dioxide)
(Lead Sulphate)
(Lead Sulphate)
Ni,O, 11nd Nl,0 1
{Nickol Hydrol<idc)
(Nickel Ox1des)
Coni;entrnted Sulphuxic Ad
PbS0 4
(Cadmium Hydtoxlde)
Weak Sulphuric Acid
KOH (PotaS!lum hydroxl<
unaffected by state of cha
charged lo a permissible minimum voltage of each
cell. The lime taken to discharge is called the discharge
rate and the rated capacity of the battery is the product of this rate and the duration of discharge (in
hours). Thus, a battery which discharges 7 A for S
hours is rated at 35 ampere-ht>urs capacity. Some
typical discharge rates of lead-acid and nickel·
cadmium batteries are shown in Fig. 1.25.
Ampere· hour5
Fig 1.25
Typical d.ischarge rates of lcad•acid
and nickel-cadmium
All ballcries display certain Indications of their state
of charge, and these are of practical help in maintaining operating conditions.
When a lead-acid battery is in the fully.charged
condition each cell displays three distinct Indications:
the terminal voltage reaches its maximum value and
remains steady; the relative density of lhe electrolyte
ceases to rise and remains constant; the plates gas
freely. The relative density is the sole reliable guide to
the electrical condition of the cell of a battery which
is neither fully charged nor yet completely discharged.
If the relative density is midway between the normal
maximum and minimum values then a cell is approximately half discharged.
Checks on the relative densily of batteries which
do not contain free electrolyte cannot be made; the
state of charge being assessed only from voltage
As we have already learned (see p. 21), the
electrolyte in the cells of a nickel-cadmium battery
does not chemically react with the plates as the
electrolyte does In a lead-acid battery. Consequently,
the plates do not deteriorate, nor docs the relative
density of the electrolyte appreciably chan~e. For this
reason, it is not possible to detennine the stale of
charge by checking the relative density. Neither can
the charge be determined by a voltage test because of
the inherent characteristic that the voltage remains
constant over a major part of the discharge cycle, The
only possible check that a battery Is f\Jlly charged is
the battery voltage when "on-chargeu: additionally,
the electrolyte should be at maximum level under
these conditions.
Fom1alion of white crystals of potassium carbonate
on a properly serviced nickel-cadmium battery
installed in an aircraft may indicate that the battery
is being overcharged. The crystals form as a result of
the reaction of expelled electrolyte vapour with carbo11
Batteries are capable of performing to their rated
capacities when the temperature conditions and charging rates are within the values specified. In the event
that these are exceeded "thermal runaway" can occur,
condition which causes violent gassing, boiling of the
electrolyte and finally melting of the plates and casing,
with consequent danger to the aircraft structure and
jeopardy of the electrical system,
Since batteries have low thexmal capacity heat can
be dissipated and this results in lowering of the effecl·
ive internal resistance. Thus, when associated wi th
constant voltage charging, a battery will draw a higher
charging current and thereby set up the "runaway"
condition of ever-increasing charging currents and
tempera tu res.
ln some ai rcraft, parlicularly those employing
nickel-cadmium batteries, temperature-sensing
devices are located within the batteries to provide a
warning of high battery tempe ratures and to prevent
overcharging by disconnecting the batteries from
the charging source at a predetermined temperature
(see also p. 29).
Depending on the size of aircraft and on the power
requirements for the operation of essential services
under emergency conditjons, a single battery or
several batteries may be provided. When several
batteries are employed they are, most often, connected in parallel although in some types of aircraft a
series connection is used, o.g. two 14-volt batteries in
series, while in others a switching arrangement is
Fig. 1,26
Typical battery installations
incorporated fo r changing from one method of con·
nection to the other.
Batteries are installed in individual compartments
specially designed and located to provide adequate
heat dissipation, ventilation of gases and protection
of airframe structure against corrosive elements. At
the same time batteries should be located es near
to the main and battery busbars as physically possible
in order to avoid the use of Long leads and consequent
high resistance. Batteries are normally mounted on,
and clamped to, a tray secured to the aircraft structure.
The tray forms a catchment for any acid which may
escape from the battery. Trays may be of any material
which is acid-proof, non-absorbent and resistant to
1easonable impacts. Many reinforced plastics are suitable but metal trays are, on the whole, undesirable.
Where metal trays are unavoidable they are treated
with an anti-corrosive paint or, in some cases, sprayed
or coated with p.v.c. The structure under and around
the battery area is also treated to avoid corrosive
attack by acid fumes and spray. Batteries are securely
clamped and anchored to their structure to prevent
their being torn loose in the event of a crash landing,
thus minimizing the risk of fire. Two typical battery
installations are illustrated In Fig. 1.26.
Venting of batteries and battery compartments may
take various forms since il depends largely on the
installation required for a particular type of aircraft.
Rubber or other non•corrosive pipes are usually
employed as vent lines which terminate at ports in the
fuselage skin so that the airflow over it draws air
th.rough the pipes by a venturi action. In some cases,
acid traps, in the form of polythene bottles, are
inserted in the lines to prevent acid spray being ejected
on to the outer-skin of the aircraft.
In the installation shown in Fig. 1.26(b) fumes and
gases generated by the battery are extracted by the
difference of pressure existing across the aircraft.
burlng normal light air tapped from the cabin
pressurization system enters the battery ventilation
chamber and continues through to the outside of the
aircraft . On the ground, when no pressure differential
exists, a non-return valve fitted in the air inlet prevents
fumes and gases from escaping into the aircraft. These
typical venting arrangements are illustrated schematic·
ally in Fig. 1.27 .
Fig l .Z7
Battery venting arrangements
The method of connecting batteries to their respective
busbars or power distribution points, depends largely
on the type of battery employed, and on the aircraft's
electrical system. In some cases, usually on the smaller
types of aircraft, the connecting leads are provided
With forked lugs which fit on to the appropriate
battery terminals. However, the method most
conunonly employed is the plug and socket type
connector shown in Fig. 1.28. It provides better connection and, furthermore, shields the battery terminals
and cable terminations.
s 1,,01etywu; ~
~olu 0.12~ d 111
B 8 5 1 1ofry
Fig. 1.28
Battery plug connector
The socket comprises a plastic housing, incorporated
as an integral part of the battery, two shrouded plug
pins and the female threaded portion of a quick-start
duead lead-screw, The plug consists of a plastic
housing incorporating two sruouded spring.loaded
sockets and terminals for the connection of battery
leads, and the male half of the mating lead-screw
operated by a handwheel. The two halves, on being
engaged, are pulled into position by the lead-screw
which thereafter acts as a lock. Reverse rotation of
the handwheel separates the connector smoothly with
very little effort. In fhis way high contact pressures
and low resistance connections are possible and are
consistently maintained.
Figure 1.29 shows the circuit arrangement for a
battery system which is employed in a current type of
turboprop airliner ; the circuit serves as a general guide
to the methods adopted. Four batteries, in parallel are
directly connected to a battery busbar which, in the
event of an emergency, supplies power for a limited
period to essential consumer services, i.e. radio, firewarning and extinguishing systems, a compass system,
etc. Direct connections are made to ensure that
battery power is avallable at the busbar at all times.
The batteries also require to be connected to
ensure that they are maintained in a charged condition.
In the example illustrated this is accomplished by
connecting the batteries to the main d.c. busbar via a
battery relay, power selector switch and a reverse
current circuit breaker.
Under normal operating conditions of the d.c.
supply system, the power selector switch is set to the
"battery" position (In some aircraft this may be
termed the "flight" position) and, as will be noted,
current flows from the batteries through the coil of
the battery relay, the switch, and then to ground via
the reverse cunent circuit breaker contacts. The
current flow through the relay coil energizes H,
causing the contacts to close thereby connecting the
batteries to the main busbar via the coil and second
set of contacts of the reverse current circuit breaker.
The d.c. services connected to the mai.n busbar are
supplied by the generators and so the batteries will
also be supplied with charging current from this
Under emergency conditions, e.g. a failure of the.
generator supply or main busbar occurs, the batteries
must be isolated from the main busbar since their
total capacity is not sufficient to keep all services in
operation. The power selector switch must therefore
be put to the "ofr' position, thus de-energizing the
battery relay. The batteries then supply the essential
services for the lime period pre-calculated on the
basis of battery capacity and current consumption of
the essential services.
The reverse current circuit breaker in the system
shown is of the electromagnetic type and its purpose
is to protect the batteries against heavy current flow
from the main busbar. Should this happen the current
reverses the magnetic field causing the normally closed
contacts to open and thereby interrupt the circuit
between the batteries and main busbar, and the
battery relay coil circuit.
To oe11ert1tor ,y,lem,
o'>doll dc.>trvicH
_.__Battery bulOP•
·-~~.::- ~ ..
\o'ollmeter selfctor
Revene current C/B
, .,.~
To l..(1etnol ----0
- - - -; I
power Cireuil
and ground
po...1r p1u9
L--- -_,
r- _.. "'\
---- Wfrtnt flo,.,r ftQm boller1e~
• • ·-
,- - - ,
: . :l l : :
C"1:0t9•rtQ c;:ummt flow from
: l :
r- -- ,
' T ' ' -r-1 1 ;
1 :
~-l.; ~-1~~--r-.J
- ,
L ..
...l- :
J -.. A
Fig 1.29 battery system circuit
The battery system in some types of turboprop
powered aircraft is so designed that the batteries
may be switched from a parallel configuration to a
series configuration for the purpose of starting an
engine from the batteries. The circuit arrangement
of one such system using two 24-volt nickel-cadmium
batteries is shown in simplified form in Fig. 1".30.
Under normal parallel operating conditions,
battery l is connected to the battery busbar via its
own battery relay , and also contacts 1a-1b of a
battery switching relay. Battery 2 is directly
connected to the busbar via its relay.
When it is necessary to use the batteries for
starting an engine, i.e. to make an ''internal" start,
both batteries are first connected ~o the battery
busbar in the normal way, and the 24-volt supply
is fed to the starter circuit switch from the busbar.
Closing of the starter switch energizes the corresponding starter relay, and at the same time the 24-volt
supply is fed via th~ starting circuit, to the coil of
the battery switching relay thereby energizing it.
Contacts 1a-1 b of the relay are now opened to
interrupt the direct connection between battery 1
and the busbar. Contacts 3a·3b are also opened to
interrupt the grounded side of battery 2. However,
since contacts 2a-2b of the switching relay are
simultaneously moved to the closed position, they
connect both batteries in series so that 48 volts
is supplied to the busbar and to the starter motor.
After the engine has started and reached self.
sustaining speed, the starter relay automatically
de-energizes and the battery switching relay coil
circuil is interrupted to return the batteries to their
nonnal parallel circuit configuration.
The power selector switches are left in the
"battery" position so that when the engine-driven
generator is switched onto the busbar, charging
current can flow to the batteries.
Banerv busbar
11 -- -I Bauery
eanel\l I
Relev 1 j
- -
of charging it. The purpose of the fuse in the closing
circuit is to interrupt the charge in the event of a
"shorted" battery.
When the battery is being charged in this manner,
the voltage and current output from the external
power unit must be properly regulated.
Relay 2
-- Closing circuit
Power selector
E~lernal o
Power aoioclor
o b°1ornal
o-r-- - - - -.....-- -- -+
I Bollery
I : , ~1
, ....i...
rI •T-1
: I
I relay
- - --- 1
, ,.
L .
I Dotwnwltc~ln~
·-.1 ___ -.J
I I master
To ~iiolna
SUitting Sy!;tQm
Pig 1.30
Parnllel/series connection of batteries
Battery Chargin g from External Power
[n some single-engined aircraft systems, the battery
may be charged when an external power unit is
plugged into the aircraft. This is achieved by a
battery relay closing circuit connected across the
main contacts of the relay as shown in Fig. 1.31 ,
With the external power connected and switched
:in, power is ;ivailable to the battery relay output
tenninal via the closed contacts of the external
power relay. At the same time, power is applied to
the battery relay closing circuit via ils diode and
resistor which reduces the voltage to the input side
)fthe battery relay's main contacts and coil.
When the battery master swftch Is selected to
·'on", sufficient current Oows through the coil of
:he battery relay to energize it. The closed contacts
~f the relay then allow full voltage from the external
JOwer unit to 11ow to the battery for the p.urpose
i +-11,
! I
\ ._,/
'- --.
Battery charging from external power
"On -board" Battery Charger Units
In most types of turbojet transport aircraft currently
in service, the battery system incorporates a separate
unit for maintaining the batteries in a stage of charge.
Temperature-sensing elements are also normally
provided in order to automatically isolate the charging
D.C. lrom mo,n T.RU.-----<>1,
1 ~'
Emerg. po,,,-e< swit<i'1
();rec I from IX>tti!<ies
!;ensirig relays
3,p Oulpul
3~!~ {--:---11
airren1 CIB
1a,: ~ ~
~ -><>--'---=--'-------- - - - - - -- - - - - ,
Charging ame~t
) . - - - - - - - - 1 ?JWl!r
I e, :
Temperoture 1-------------'
0 .C.R~oy
Cho«~er I IX>' t
I I ~---- -11
Fig. 1. 32
ln-5itu battery charging system
Tobo11 bus
" ery
circuit whenever there is a tendency for battery over.
heating to occur. The circuits of "on.board" charger
units as they are generally termed, vary between
aircraft types, and space lim.its description of all of
them. We may however, consider two examples
which highlight some of the variations to be found.
The much simplified circuit shown in Fig. 1.32
is based on the system adopted for the McDonpeU
Douglas DC· IO. In this particular application, the
required output of 28 volts is achieved by connecting
two 14.yolt batteries in series. Furthermore, and
unlike the system shown in Fig. 1.29, the batteries
are only connected to the battery busbar whenever
the normal d,c, supply (in this case from transformer/
rectifier units) is not available. Connection to the
busbar and to the charger unit Is done automatically
by means of a "charger/battery" relay and by
sensing relays.
When power is available from the main generating
system, d,c, is supplied to the battery busbar from a
transformer/rectifier unit and, at Ute same time, to
the coils of the sensing relays. With the relays energized, lhe circuit through contacts A2·A3 is interropted while the circuits through contacts Bl -B2 are
made. The battery switch, whlch controls the operation
of the charger/battery relay, is closed to the "batt"
position wlien the main electrical power is available,
and the emergency power switch is closed in the " off'
The charger/battery relay is of the dual type, one
relay being a.c, operated and the other d.c. operated,
The a.c. relay coil is supplied with power from one
phase of the main three-phase supply te> the battery
charger, and as will be noted from the diagram, the
relay is energized by current passing to ground via the
contacts Bl .B2 of the sensing relays, the battery switch
and the emergency switch. Energizing of the relay
closes the upper set of contacts (Al-A2) to connect
the d.c, positive output from the battery charger to
the batteries, thereby supplying them with charging
In the event of main power failure, the battery
charger will become inoperative, the a.c. charger
relay will de-energize to the centre off position, and
the two sensing relays will also de-energize, thereby
opening the contacts Bl ·82 and closing the contacts
A2·A3. The closing of contacts A2·A3 now permits a
positive supply to flow direct from the battery to the
coil of the d.c. battery relay, which on being energized
also actuates the a.c. relay, thereby closing contacts
B1-B2 which connect the batteries direct to the battery
busbar. The function of the battery relay contacts is
to connect a supply from the battery busbar to the
relays of an emergency warning light circuit. The charg·
ing unit converts the main three.phase supply of 115/
200 volts a.c. into a controlled d.c. output at constant
current and voltage, via a transformer and a full-wave
rectifying bridge circuit made up of silicon rectifiers
and silicon controlled rectifiers (see also p. 57). The
charging current is limited to approximately 65 A, and
in order to monitor this and the output voltage as a
function of battery temperature and voltage, temperature-sensing elements within the batteries are
connected to the S.C.R. "gates" via a temperature and
reference voltage control circuit, and a logic circuit.
Thus, any tendency for overcharging and overheating
to occur is checked by such a value of gate circuit
current as will cause the S.C.R. to switch off the
charging current supply.
The second example shown in Figs. 1.33 and
1.34, is based on that used in the Boeing 737. The
charger operates on 115 volt 3-phase a.c . power
supplied from a ''ground service" busbar, which in
turn, is normally powered from the number I
generator busbar, and/or from an external power
source (see page 27). Thus, the aircraft's battery is
maintained in a state of charge both in flight and
on the ground.
In fUght the a.c. supply is routed to the charger
through the relaxed contacts of a battery charger
transfer relay and an APU start Interlock relay.
The d.c. supply for battery charging is obtained
from a transfom1er-rectil1er unit within the charger,
and it maintai11S cell voltage levels in two modes of
operation: high and low. Under normal operating
conditions of the aircraft's power generation system,
the charging level is in the high mode since as will
be noted from the diagram, the mode control relay
within the charger is energi:zed by a rectified output
tluough the battery thermal switch, and the relaxed
contacts of both the battery bus relay and the
external power select relay, Above 16 amps the
charger acts as an unregulated transformer-rectifier
unit, and when the battery has sufficient charge
that the current tends to go below 16 amps, the
charging current is abruptly reduced to zero. The
current remains at zero until the battery voltage
drops below the charge voltage, at which time the
charger provides the battery with a pulsed charge
and the process is repeated. The pulsing continues
until the control circuits within the charger change
the operation to the low mode, approximately two
, 1 l o· _o.. i·!- --- ,- - -
- -- - - -,
Meterin9 shunt
i----u........u l
Battery busbar
1- --
'Hot' battery busbar
Battery busbar
From main
Fis 1,33
Battory charger control circuit
External a.c. busbar
No 1 Generator busbar
No 2 Main bvsbar
6_- 6 .
, - - - - -- - - 1 - - 1 - - -- - - - - - - '
<>-+-- -------
28-V d.c.
(No 2 Generator
control unit)
No 1 Transfer busbar
See Fig. 1.
I L ,
::B l
APU start
lnterlock j
_ __ _
minutes after pulse charging commences.
In the event that the number one generator
supply fails there will be a loss of a.c. power to
the ground service busbar, and therefore, to the
battery charger. However, with number two
generator still on line, a transfer signal from its
control unit is automatically supplied to the coil
of the battery charger transfer relay, and as may be
seen from Fig. 1.34, its contacts change over to
connect the charger to the a.c. supply from number
two generator, and so charger operation is not
The APU start interlock relay is connected in
parallel with a relay in the starting circuit of the
APU, and is only energized during the initial stage
of starting the APU engine , This prevents the starter
motor from drawing part of its heavy starting
current through the battery charger. The interlock
relay releases automatically when the APU engine
reaches 35% rev/min.
In addition to the control relay within the battery
charger, there are three other ways in whlch the
charging mode can be controlled, each of them
fulfilling a protective role by Interrupting the ground
circuit to the mode control relay and so establishing
a low mode of charge. They are - (i) opening of the
battery thermal switch in the event of the battery
temperature exceeding 46 °C (I J5 °F); (ii) loss of
d.c. power from the designated transformer-rectifier
unit causing the battery transfer relay to relax and
the battery bus relay to energize ; and (iii) energizing
of the fuelling panel power select relay when external
a.c. power is connected to the aircraft . The latter is
of importance since if the charger was left to operate
in the high mode, then any fault ln the regulation of
the external power supply could result in damage to
the aircraft battery.
Alternating Current
Power Supplies
Before studying the operation of some typical genera·
ting systems currently in use It will be of value to
recapitulate certain of the fundamentals of alternating
current behaviour, and of terminology commonly used.
and Frequency
The voltage and current produced by the generator of
an a.c. system build up from zero to a maximum of
one polarity, then decay to zero, build up to a maximum of opposite polarity, and again decay to zero.
This sequence of build up and reversal follows a sine
wave form and is called a cyde and the number of
cycles in unit time (usually one second) is called the
frequency (see Fig. 2 .J ). The unit of frequency
measurement is the Hertz (Hz).
In a conventional generator, the frequency ls
lnstgnl cnsoy~ vglue
tlmpli lude or peok volue
(1m0 1 , 1n8)
- - Cyelc - --
Fig 2.1
Cycle and frequency
dependent upon the speed of rotor rotation within its
stator and the number of poles. Two poles of a rotor
must pass a given point on the stator every cycle;
) _ rev/min x pairs of poles
Frequency c.p.s. 60
For example, with a 6-pole generator operating at
8000 r.p.m.,
8000 X 3
"'400 or 400 Hz
Frequency= ·
For aircraft constant frequency systems (see p. 46)
400 Hz has been adopted as the standard.
Instantaneous and Amplitude Values
Al any given instant of time the actual value of an
alternating quantity may be anything from zero to
a maximum in either a positive or negative direction;
such a value is called an Instantaneous Value. The
Amplitude or Peak Value is the maximum instant·
aneous value of an alternating quantity in the positive
and negative directions.
The wave form of an alternating e.m.f. induced in
a single-turn coil, rotated at a constant velocity in a
uniform magnetic field , is such that at any given
point in the cycle the instantaneous value of e.m.f.
bears a deflnHc mathematical relationship to the
amplitude value. Thus, when one side of the coil
turns through 6° from the ze ro e.m.f. position and in
the positive direction , the instantaneous value of e.m.f.
ls the product of the amplitude (EmaJ and the sine of
(J or, in symbols:
Einst .. Emaxsin (J
Similarly, the instantaneous value of current is
I ilist "'I max sin 0.
Root Mean Square Value
The calculation of power, energy, etc., in an a.c. cir·
cuit is not so perfectly straightforward as it is in a
d.c. circuit because the values of current and voltage
are changing throughout the cycle. For this reason,
therefore, an arbitrary "effective'' value is essential.
This value is generally termed the Root Mean Square
(r.m.s.) value(see Fig. 2.2). It is obtained by taking a
Fig 2,2
R.M.S. value of alternating current
number of instantaneous values of voltage or current,
whichever is required, during a half cycle, squaring
the values and taking their mean value and then taking
the square root. Thus, if six values of current "I" are
taken, the mean square value is:
I 11 + h 1 + I31 + Ii+ 1~2 + 162
and the r.m,s. value is:
+h2 +ll:l42 +1sl+J6l
The r.m.s. value of an alternating current is related
to the amplitude or peak value according to the wave
form of the current. For a sine wave the relationship
is given by:
r.m.s. - ..; - 0,707 Peak
"'V2 r.m.s. :;, 1-414 r.m.s.
Phasing and Phase Relationships
In connection with a.c. generating systems and
associated circuits, the term "pha&e" is used to indicate
the number of alternating currents being produced
and/or carried simultaneously by the same circuit.
Furthermore, it is used in designating the type of
generating system and/or circuit, e.g. a ''single-phase"
system or one producing single-phase current, ilnd a
polyphase'' system or one producing several single
alternating currents differing in phase. Aircraft poly.
phase systems and circuits ate normally three-phase,
the three currents differing in phase from each other
by 120 electrical degree~.
The current and voltage in an a.c. circuit have the
same frequency, and the wave form of the alternating
quantities is similar, i.e. if the voltage is sinusoidal
then the current Is a1so sinusoidal. In some circuits the
flow of current is affected solely by the applied voltage so that both voltage and current pass through zero
and attain their peaks in the same di1ection simultaneously; under these conditions they are said to be
"in phase". In many circuits, however, the currertt flow
is influenced by magnetic and electrostatic effects set
up in and around the circuit, and although at the same
frequency, ·voltage and current do not pass through
zero at the same instant. In these circumstances the
voltage and current are said to be "out of phase", the
difference between corresponding points on the wave.
fonns being known as the phase difference. TI1e term
"phase angle" is quite often used, and is synonymous
with phase difference when expressed in angular
measure, The phase relationships for the three basic
forms of a.c. circuits, namely, pure resistive, induct·
ive and capacitive, are illustrated in Fig. 2 .3.
In a pure resistive circuit (Flg. 2.3(a)) the resistance
is constant, therefore magnetic and electrostatic effects
are absent, and the applied voltage is the only factor
affecting current flow. Thus, voltage and C\lrrent are
"in phase" in a resistive circuit.
In a pure inductive circuit (normally some resistance
is always present) volt:ige and current are always out
of phase. This is due to the fact that a magnetic field
surrounds the conductors, and since it too continually
changes in magnitude and direction with the altemat·
ing current, a self-induced or "reactance" e.m.f. ls
set up in the circuit, td oppose the change of current
in the circuit. As a result the rise and fall of the
current is delayed and as may be seen from Fig. 2.3(b)
the current "lags" the voltage by 90 degrees.
Capacitance in an a.c, circuit also opposes the
current flow and causes a phase difference between
applied voltage and current but, as may be noted from
Fig. 2.3(c) , the effect is the reverse to that of indu'ct•
ance, i.e. the current ''leads" the voltage by 90 degrees.
Where the applied voltage and current are out of
phase by 90 degrees they are said to be in quadrature.
A three-phase circuit is one in which three voltages
are produced by a generator with three coils so spaced
within the stator, that the thre(.l voltages generated
are equal but reach their amplitude values at different
times. For example, Jn each phase of a 400 Hz, three·
phase generator, a cycle is generated every 1/400
second, In its rotation, a magnetic pole of the rotor
passes one coil and generates a maximum voltage; onethird of a cycle (I/ 1200 second) later, this same pole
passes another coil and generates a maximum voltage
in it. Thus, the amplitude values generated in the th ree
coils are always one-lhird of a cycle (120 electrical
degrees; 1/1200 second) apart.
The interconnection of the coils to form the three
phases of a basic generator, and the phase sequence, ii
shown in fig. 2.4. The output terminals of generators
are marked to show the phase sequence, and these
terminals are connected to busbars which are idenlifie
Imo• •
_ lo I
Pure rcs,sfive -
Emo, ·
r:ig 2.4
Three,phase system
Interconnection of Phases
.' ',,.
' '- ~ ~
Pure inductive -
l loqs behind E
Emaa -
l mo , • - -
(e l
Pure eopoc,tive -
I leOd$ f:'.
Fig 2.3
A.C. circuits ph~sc relationship
Each phase of a three-phase generator may be brough
out to separate terminals and used to supply separate
groups of consumer services. i his, however, is an
arrangement rarely encountered in practice since
pairs of "line" wires would be required for each
phase and would involve uneconomic use of cable.
The phases are, therefore, interconrwctcd normally
by either of the two methods shown in Fig. 2.5 .
The "Star" connection ((a)) is i.:onimonly used in
genera tors. One end of each phase winding is con,
nected !o a common poinl known as the 11eu Ira/ poit
while the opposite ends of lhe windings arc connec:lf
to three separate lines. Thus, two-phase windings are
connected between each pajr of lines. Since similar
ends of the windings are joined, the two phase e.m.f.
are in opposition and out of phase and the voltage
between lines (E1) is the phase voltage (Eph) multi·
plied by y3. For example, if Cph is 120 vults, then
ELequals 120 x J. 732, or 208 volts approx. As rar a
line and phase currents are concerned, these are equ,
to each other in this type of circuit connection.
If necessary, consumer services requiring only a
single-phase supply can be tapped into a three-phase
star-connected system with a choice of two different
voltage levels. Thus, by connecting from one phase
to neutral or ground, we obtain a single-phase 120
volts supply while connecting across any pair of lines
we can obtain a single·pl1ase 208 volts supply.
p F ,,, Effective Power (kW)
' · Apparent Power (kV A)
"' cosine phase angle ..p
~ - 12_o _v _ ___,__ __
--,,-1. 1
I ol
circuit which the generator is to supply and the phase
relationships of voltage and current, and is expressed
as a ratio termed the power factor (P.F.). This may
be written:
- --r~
3 _•...:..
Pig. 2.5
If the voltage and current are in phase (as in a
resistive circuit) the power factor is 100 per cent or
unity, because the effective power and apparent power
are equal; thus, a generator rated at 100 kV A in a circuit with a P.F, of unity will have an output l 00 per
cent efficient and exactly equal to 100 kW.
When a circuit conteins inductance or capacitance,
then as we have already seen (p. 33) current and
voltage arc not in phase so thai the P.F. Is less than
unity. The vector diagram for a current I lagging a
voltage Eby an angle ip is shown in Fig. 2.6. The
current is resolved into two components at right
angles, one in phase with E and given by I cos I(), and
the other in quadrature and given by l sin'{), The
in,phase component ts called the active, wattful or
working component (kW) and the quadrature com,
ponent is the idle, wattless or reactive component
(kVAR), The importance of these components will
be more apparent when, later in this chapte r, methods
Interconnection of phases
(a) "Star" connection
(b) "Delta "
Figure 2.5(b) illustrates the "Delta'' method of
connection, the windings being connected in series to
form a closed "mesh" and the lines being connected
to the junction points. As only one phase winding is
connected between each pair of lines then, in the
delta method, line voltage (EL) is always equal to
phase voltage (Eph,), The Urie curren t, however, is the
difference between the phase cunents connected to
the line and is equal to the phase current (Iph,)
multiplied by .../3.
Generator Power Ratings
The power ratings of a.c. generators are generally
given in kilovolt-amperes (kVA) rather than kilowatts
(kW) as in the case of d,c, machines. The primary
reason for this is due to the fact that in calculaUng
the power, account must be taken of the difference
between the true or effective power, and the apparent
power. Such a difference arises from the type of
Reacl!vo comPQnent I sm .p
Fig 2.6
Components of current due to phase dlfferen~
of load sharing between generators are discussed.
Most a.c. generators are designed to take a pro·
portion of the reactive component of current through
their windings a· d some indication of thls may be
obtained from the information given on the generator
data plate. For example, the output rating may be
specified as 40 kV A at 0,8 P.F. This means that the
maximum output in kW is 0·8 x 40 or 32 kW, but
that the product of volts and amperes under all
conditions of P.F. must not exceed 40 kV A
A frequency.wild system is one in which the frequency
of its generator voltage output is permitted to vary
with the rotational speed of the generator. Although
such frequency variations are not suitable for the
direct operation of all types ofa.c. consumer equip·
ment, the output can (after constant voltage regula·
tion) be applied directly to resistive load circuits such
as electrical de-icing systems, for the reason that resist•
ance to alternating current remains substantially con.
stant, and is independent of frequency.
Generator Construction
The construction of a typical frequency-wild
generator utilized for the supply of heating current
to a turbo-propeller engine de-icing system is illus.
trated In Fig. 2.7. It has a three-phase output of
22 kV A at 208 volts and it supplies full load at
this voltage through a frequency range of 280 to
400 Hz. Below 280 Hz the field current is limited
and the output relatively reduced, The generator
consists of two major assemblies: a fixed stator
assembly in wltich the current is induced, and a
rotating assembly referred to as the rotor. The
stator assembly is made up of high permeability laminations and is clamped in a main housing by an end
frame having an integral flange for mounting the
generator at the corresponding drive. outlet of an
engine-driven accessory gear-box. The stator winding
is star connected, the star or neutral point being made
by linkJng three ends of the winding and connecting
it to ground (see also p. 35). The other three ends
of the winding are brought out to a three-way output
terminal box mounted on the end frame of the
generator. Three small current transformers are fitted
into the terminal box and form part of a protection
system known as a Merz-Pdce system (seep. 122).
Oulpt,1l l;rm1n1;1I
trgn~ fgrmer
f 1C11oho~
p10 1e
cove r gnd
b. lf ouUei !!POUi
f:ig 2,7
Freq uency•wUd generator
The rotor assembly has six salient poles oflamina·
ted construction; their series·connected field windings
terminate at two slip rings secured at one end of the
rotor shaft. Three spring-loaded brushes are equispaced
on each slip Ting and are contained within a brush-gear
housing which also forms a bearing support for the
rotor. The brushes are electrically connected to d.c.
input terminals housed in an excitation terminal box
mounted above the brush-gear housing. The terminal
box also houses capacitors which are connected
between the terminals and fn1me to suppress inter·
fcrence in the reception of radio signals. At the drive
end, the rotor shaft Is serrated and an oil seal, housed
in a carrier plate bolted to the main housing, is fitted
over the shaft to prevent the entry of oil from the
driving source into the main housing.
The generator is cooled by ram air (see also
Chapter l, p. 9) passing inlo the main housing via an
inlet spout at the slip ring end, the air escaping from
the main housing through ventilation slots at the drive.
end. An air-collector ring encloses the slots and is
connected to a vent through which the cooling air is
finally discharged, Provision is made for the installation of a lherrnal(y.operated switch to cater for an
overheat warning requirement.
In the development of electrical power supply systems,
notably for large aircraft, the idea was conceived of
an "all a.c." system, i.e. a primary generating system
to meet all a.c. supply requirements, in particular
those of numerous consumer services dependent on
constant-frequency, to allow for paralleled generator
operation, and to meet d.c. supply requirements-via
transformer and rectifier systems,
A constant frequency is inherent in an a.c. system
only if the generator is driven at a constant speed,
The engines cannot be relied upon to do this directly
and, as we have already learned, if a generator is
connected directly to the accessory drive of an engine
the output frequency will vary with engine speed.
Some form of conversion equipment is therefore
required and the type most widely adopted utilizes
a transmission device interposed between the engine
and generator, and which incorporates a variableratio drive mechanism. Such a mechanism is referred
to as a constant-speed drive (CSD) unit and an
example is shown in Fig. 2.8.
The unit employs a hydromechanical variable.
ratio drive which in its basic form (see Pig. 2.9)
Gcrni ml or mo.in1,nq
ono couphnq
Fig 2,8
Constant speed drive unit
consists of a variable displacement hydraulic unit,
a fixed displacement hydraulic unit and a differential
gear. The power used to drive the generator is con.
trolled and transmitted through the combined effects
of the three units, the internal arrangement of which
are shown in Fig. 2.10. Oil for system operation Is
supplied from a reservoir via charge pumps within
the unit, and a governor.
Charge--------- - - -- ---,
Input r---i==l
and governor
Ba.sic arrangement of a CSD unit
Plana! otars
To pumps
Fig 2,10
Underdtive phase
The variable displacement unil consists of a
cylinder block, reciprocating pistons and a variable
angle wobble or swash plate, the latter being
connected to the piston of a control cylinder. Oil
to this cylinder is supplied from the governor. The
unit is driven directly by the input gear and the
differential planet gear carrier shaft, so that its
cylinder block always rotates (relative to the port
plate and wobble plate) at a speed proportional to
input gear speed and always in the same direction.
When the control cylinder moves the wobble plate
to some angular position, the pistons within the
cylinder block are moved in and out as the block
rotates, and so the charge oil Is compressed to a
high pressure and then ''ported'' to the fixed displacement unit. Thus, under these conditions the
variable displacement unit functions as a hydraulic
The supply of charge oil to the unit's control
valve Is controlled by a governor valve which is
spring biased, flyweight operated and driven by
the output gear driving the generator, It therefore
responds to changes in transmission output speed.
The fixed displacement unit is similar to the
variable displacement unit, except that its wobble
plate which has an inclined face, is fixed and has
no connection with the control cylinder. When oil
is pumped to the fixed displacement unit by the
variable one, it functions as a hydraulic motor and
its direction of rotation and speed is detennined
by the volume of oil pumped to it. It can also
function as a pump ·and therefore supply t.he
variable displacement hydraulic unit.
The differential gear consists of a carrier shaft
carrying two meshing (1 :1 ratio) planet gears, and
a gear at each end; one meshing with the input gear
and the other with the gear which continuously
drives the variable displacement unit cylinder block.
The carrier shaft always rotates in the same direction
and at a speed which, via the input gear, varies with
engine speed. Surrounding the carrier shaft are two
sep:uate "housings'', and since they have internal
ring gears meshing with the planet gears, then they
can be rotated differentially. Each housing also has
an external ring gear; one (input ring gear) meshing
with the fixed displacement unit gear, and the other
(output ring gear) meshing with the output gear drive
to the generator, Thus, with the CSD in operation,
the output ring gear "housing" serves as the continuous drive transmission link between engine and
generator. Since the input ring gear "housing" is
geared to the fJXed displacement hydraulic unit,
then depending on whether this unit is acting as a
motor or a pump, the "housing'' can rotate in the
same direction as, or opposite to, that of the carrier
shaft and the output ring gear "housing". In this way,
speed is added to, or subtracted from, the engine
speed, and through the gear ratio (2 : 1) between the
ring gears and the carrier shaft planet gears, the
output ring gear "housing" rotational speed will be
appropriately adju~ted to maintain constant governor
When the input speed, via the input gear, is
sufficient to produce the required output speed,
the drive to the generator is transmitted straight
through the differential and output ring gear. The
variable displacement hydraulic unit cylinder block
Is continuously rotating, but the position of its
wobble plate Is such that no charge oil is pumped
to the fixed displacement unit. The cylinder block of
this unit and the output ring gear "housing" do not,
therefore, rotate during straight th.rough drive.
1f the input speed supplied to the transmission
exceeds that needed to produce the required output
speed, the governor in sensing the speed difference
will cause oil to flow away from the control valve.
In this condition, the transmission is said to be
operating in the underdri11e phase and is shown in
Fig. 2.10. The control valve changes the angula.r
position of the variable displacement unit's wobble
plate so that the volume of oil for accommodating
the oil in the bores of the cylinder block Is increased,
allowing oil to be pumped at high pressure from
the fixed displacement unit. The pressure of the
pistons against the inclined face of the unit now
causes its cylinder block to rotate in the same
direction as that of the variable displacement unit,
This rotation is transmitted to the input ring gear
''housing" of the differential unit, so that it will
rotate in the same dlrection as the output ring
gear "housing", and the carrier shaft, Because the
input ring gear ''housing" is now rotating in the
same direction as the carrier shaft then the speed
of the freely rotating planet geaI meshing with the
housing will be reduced. The speed of the second
planet gear will also be reduced in direct ratio
thereby reducing the speed of the output ring
gear "housing''. This hydromechan1cal process of
speed subtraction continues until the required
generator drive speed is attained at which the trans.
mission will revert to straight.through drive
operation .
When the input speed SU_Pplied to the transmission is lower than that needed to produce the
required output speed, the governor causes charge
oil to be supplied to the control valve. In this con.
dition, the transmission is said to be operating in
the overdrive phase and this is shown in Fig. 2.11.
As will be noted, the change in angular position of
the variable displacement unit's wobble plate now
causes it to pump high pressure oil to tho fixed
displacement unit. The cylinder block and input ring
gear "housing" therefore rotate in the opposite
direction to that of the underdrlve phase, and so
it increases the rotational speed of the planet gears
and output ring gear "housing". Thus, speed is
added to restore the required generator drive speed.
In multi-CSD generator systems the control of
the drives Is important in order that real electrical
load (see p. 48) will be evenly distributed
Pt1nt! gea11
To oovarno, ,;,"..
ring geu
To P'!mps
Fig 2.11
Overdrive phase
between the generatou. Any unbalance in real load
is automatically sensed by control units and load
controllers in the generator systems and, since
correction must be made at the generator drive ,
signals resulting from an unbalance are fed to an
electromagnetic coil within the basic governor of
each CSD (see page 39). The electromagnetic
field interacts with additional pennanent magnet
flyweights driven by the governor, to produce a
torque which. in conjunction with centrifogal force
provides a "fine" adjustment or trimming of the
governor control valve, and of the output speed to
the generators.
A typical CSD/generator installation is shown
in Fig. 2.12.
The disconnection ofa C.S.D. transmjssion system
following a malfunction, may be accomplished
mechanically by levers located in the flight crew com·
partment, electro-pneumatica.lly, or as is more common,
by an electro-mechanical system. In this system (see
1. CSD
2. Gonorator
Fig 2.12
CSD/gencrator inst;illatio11
3, CSD oil scrvico port 5. CSD oil cooler
4. CSD oil filter
6, Wet spline cavity service port
Doo tooth clurc~
1eparatlon point
Fig. 2.13) the drive from the engine is transmitted
to the c.s.n. via a dog-tooth clutch, and disconnect
i.s initially activated by a solenoid controlled from
the flight crew compartment.
When the solenoid is energized, a spring-loaded
pawl moves in to con tact with threads on the input
shaft and then serves as a screw causing the input shaft
to move away from the input spline shaft (driven by
the engine) thereby separating the driving dogs of the
clutch. In some mechanisms a magnetically.operated
indicator button is provided in the reset handle, which
lies flush with the handle under normal operating
conditions of the drive. When a disconnect has ta.ken
place, the indicator button is released from magnetic
attraction and protrudes from the reset handle to provide a visual inclica tion of the disconnect.
ResetUng of disconnect mechanisms can only be
accomplished on the ground following shutdown of
the appropriate engine. In the system illustrated,
Shown dlseonnocred
Fig 2.13
disconnect mechani1
reset Ung is accomplished by pulling out the reset
handle to withdraw the threaded pawl from the input
shaft, and allowing the rese t spring on the shaft to
re-engage the clutch. Al the same time, and with the
solenoid de-energized, the solenoid nose pin snaps
into position in the slot of the pawl.
Generator Construction
A sectioned view of a typical constant frequency
generator is illustrated in Fig. 2.14, It consists of
three principal components : a.c. exciter which
generates the power for the main generator field;
rotating rectifier assembly mounted on, and
rotating with, the rotor shaft to convert the exciter
output to d.c.; and the main generator. All three
components are contained within a cast aluminium
casing made up of an end bell section and a stator
frame section; both sections are secured externally by
screws. A mounting flange, which is an integral part
of the stator frame, carries twelve slots reinforced by
steel inserts, and key-hole shaped to facilitate attach·
ment of the generator to the mounting studs of the
constant-speed drive unit.
The exciter, which is located in the end beU section
of the generator casing, comprises a stator and a threephase star-wound rotor or exciter armature. The excite
armature is mounted on the same shaft as the main
generator rotor and the output from its three-phase
windings is fed to the rotating rectifier assembly.
The rotating rectifier assembly supplies excitation
current to the main generator rotor field coils, and
since together with tlte a.c. exciter they replace the
conventional brushes and slip rings, they thereby elimil
ate the problems associated with them. The assembly
is contained within a tubular insulator located in the
hollow shaft on which the exciter and main generator
Output leminol Oulpul term,nal
Exciter shunt field and
s labillz ing w1nd1ngs
board and cover
Thermaslalic switch
El(Citer ma,n poles
Exciter ormalure
Slotted rolor
~ ~ ~ : \ - - - - - ~ Capacitor
~ ~ ~~ ---Exciter stator
Fig 2.14
Constant frequency generator
rotors are mounled; located in this manner they are
close to the axis of rotation and arc not, the refore,
subjected to excessive centrifugal forces. A suppression
capacitor is also connected in the rectifier circuit and
is mounted at one end of the rotor shaft. Its purpose
is to suppress voltage ''spikes" created within the
diodes under certain operating conditions.
The main generator consists of a tluee·phase star·
wound stator, and an eight-pole rotor and its associ:i.ted field windings which are connected lo the output
of the rotating rectifier. The leads from the three
stator phases arc brought directly to the upper surface
of an output terminal board, thus permitting the aircraft wiring to be clamped directly against the phase
leads without current passing through the terminal
studs. In addition to the field coils, damper (amort•
isseur) windings are fitted to the rotor and are located
in longitudinal slots in the pole faces. Large copper
bands, under steel bands at each end of the rotor
stack, provide the electrical squirrel-cage circuit. The
purpose of the damper windings is to provide an
induction motor effect on the generator whenever
sudden changes in load or driving torque tend to cause
the rotor speed to vary above or below the normal or
synchronous system frequency. In isolated generator
operation, the windings serve to reduce excessively
high transient voltages caused by line-to.line system
faults, and to decrease voltage unbalance, during
unbalanced load conditions. In parallel operation (see
p. 47), the windings also reduce transient voltages
and assist in pulling in, and holding, a genera.tor in
The drive end of the main rotor shaft consists of a
splined outer adaptor which fits over a stub shaft
secured to the main generator rotor. The stub shaft; in
turn, fits over a drive spindle fixed by a centrally
located screw to the hollow section of the shaft con·
taining the rotating rectifier assembly. The complete
shaft is supported at each end by pro-greased sealed
The generator is cooled by ram air which enters
through the end bell section of the casing and passes
through the windings and also through the rotor shaft
to provide cooling of the rectifier assembly. The air
is exhausted through a pcrfora tcd screen around the
periphery of the casing and at a point adjacent to the
main generator stator. A thermally-operated overheat
detector switch is screwed directly through the stator
frame section into the stator of the main generator,
and is connected to an overheat warning light on the
relevant system control panel.
Further information on the circuit arrangement
of the generator ls given on page 45.
Pcrrnanent magnet
ge!11cralor stator
Fig 2.15
Integrated drive generator
Integrated Drive Generators
straightforward manner by residual magnetism in the
electromagnet system and by the build up of current
tluough the field windings. The field current, as it is
called, Is controlled by a voltage regulator system.
The excitation of a.c. generators, on the other hand,
involves the use of somewhat more complex circuits,
the arrangements of which are essentially varied to
suit the particular type of gener.ator and its control·
Ung system, However, they all have one common
feature, i.e. the supply of direct current lo the field
windings to maintain the desired a.c. output.
As will be noted from Fig. 2.15, an integrated drive
generator is one in which the CSD and generator
are mounted side by side to foon a single compact
unit. This configuration reduces weight, requires
Jess space, and in comparison with the "end-to.end"
configuration it reduces vibration. The fundamental
construction and operation of both the generator
and drive units follow that described in the preceding paragraphs. The essential difference relates
to the method of cooling the generator. lnstead of
air being utilized as the cooling medium , oil is
pumped through the generator; the oil itself is in
turn cooled by means of a heat exchanger system.
Frequency -Wild Generators
Figure 2.16 ls a schematic illustration of the method
adopted for the generator illustrated in Fig. 2.7. In
this case, excitation of the rotor field Is provided by
d.c. fronr the aircraft's main busbar and by rectified
a.c. The principal components and socUons of the
control system associated with excitation are: the
control switch, voltage regulation section, field
excitation rectifier and current compounding section
The production of a desired output by any type of
generator requires a magnetic field to provide excita·
lion of the windings (or starting and for the subsequent operational running period. In other words,
a completely self-starting, self-exciting sequence is
required; In d.c. generators, this is achieved in a fairly
__. D.C. from busbot
- _ _., Rectified o.c .
28· V d.c .
208-V o.c.
rec t1 fier
Exc ita tion
Fig 2.16
Frequency-wild generator excitation
consisting of a tluee-phase current transformer and
The primary windings of the compounding trans•
fonner are in series with the three phases of the
generator and the secondary windings in series with
the compounding rectifier.
When the control switch is in the "start'' position,
d.c. from the main busbar is supplied to the illp rings
and windings of the generator rotor; thus, with the
generator running, a rotating magnetic field is set up
to induce an alternating output in the stator, The
output is tapped to feed a magnetic amplifier type of
voltage regulator which supplies a sensing current
signal to the excitation rectifier (seep. 45). When
this signal reaches a pre-determined off-load value, the
rectified a.c. through the rotor winding is sufficient
for the generator to become self-excited and indepen.
dent of the main busbar supply which is then disconnected,
The maximum excitation current for wide,speedrange high-output generators of the type shown in
Fig. 2.7 is quite high, and the variation in excitation
current necessary to control the output under varying
"load" conditions is such that the action of ilie voltage
regulator must be supplemented by some other medium
of variable excitation c1ment. This is provided by the
compounding transformer and rectifier, and by connecting them in the manner already described, direct
current proportional to load current is supplied to the
rotor field windings.
Constant-Frequency Generators
The exciter stator of the generator described on
page 41 is made up of two shunt field windings,
a stabilizing winding and also six pennanent magnets ;
the latter provide a residual magnetic field for initial
excitation. A thennistor is located in series with one
of the parallel shunt field windings and serves as a
temperature compensator. At low or normal ambient
temperatures, the high resistance of the thermistor
blocks current now in its winding circuit so that it
causes the overall shunt field resistance to be about
that of the remaining winding circuit. At the higher
temperature resulting from nom1aJ operation, the
resistance of each single circuit increases to approximately double. At the same time, however, the
thermistor resistance drops to a negligible value
permitting approximately equal current to f1ow in
each winding circuit.
The stabilizing winding is wound directly over the
shunt field windings, and with the permanent magnet
poles as a common magnetic core, a transformer type
of coupling between the two windings is thereby
provided. The rectifier assembly consists of 1ix silicon
diodes separated by insulating spacers and connected
as a three-phase full-wave bridge.
The excitation circuit arrangement for the genera to
is shown schematically in Fig. 2.17. When the
generator starts running, the Oux from the permanent
magnets of the a.c. exciter provides the initial flow of
current in its rotor windings. As a result of the initial
current Dow, armature reaction is set up and owing to ti
position of the permanent magnetic poles, the reactior
polarizes the main poles of the exciter stator in the
proper direction to assist the voltage regulator in taking over excitation control.
The three-phase voltage produced in the windings ii
supplied to the rectifier assembly, the d.c. output of
which is, In turn, fed to the field coils of the main
generator rotor as the required ex:cilation current. A
rotating magnetic field is thus produced which induce1
a three-phase voltage output in the main stator windings. The output is tapped and is fed back to the shun1
field windings of the exciter, through the voltage
regulator system, in order to produce a field supple·
mentary to that of the permanent magnets. ln this
manner the exciter output is increased and the main
generator is enabled to build up its output at a faster
rate. When the main output reaches the rated value,
the supplementary electromagnetic field controls the
excitatfon and the effect of the permanent magnets is
almost eliminated by the opposing armature reaction.
During the initial stages of generator operation,
the current flow to the exciter only passes through
one of the two shunt field windings, due to the inverse
temperature/resistance characteristics of the thermistc
As the temperature of the winding increases, the
thermistor resistance decreases to allow approximate!)
equal current to flow in both windings, thus maintain·
ing a constant effect of the shunt windings.
In the event that excitation current should suddenl
increase or decrease as a result of voltage fluctuations
due, for example, to switching of loads, a current will
be induced in lhe stabilizing winding since it ac ts as a
transformer secondary winding. This current is fed int
the voltage regulator as a feedback signal to so adjust
the excitation current that voltage fluctuations resulti.
from any cause are opposed and held to a minimum.
Permanent magnet~
Current tron,formor
- ·-J
Out ul
Power llonsto,me,
To mo,n o.c. busbor!
Voltoqt rC<Julotor
Excite, outpul
Ree1i f1 cd o. c. ~•c11011on
Ma1n o.c. oulp1,11
Re9u1orea e~c11a11on current
E, c1loft0n (.urrcnt under
ioull cond,lian
Stobi,z,nq tcedbocl< "Qnol
ond load
mognelic amplifitr
F'ig 2.17
Circuit diagram of constant frequency generator
The control of the output voltages of a.c. generators is
also an essential requirement, and from the foregoing
description of excitation methods, it will be recognized
that the voltage regulation principles adopted for d.c.
generator~ can also be applied, i.e. automatic adjustment of excitation current to meet changing conditions
of load and/or speed. Voltage regulators nom1ally form
part of generator system control and protection units.
Frequency-Wild Generators
Figure 2.18 ls a block functio'hal diagram of the
method used for the voltage regulation of the
generator illustrated in Fig. 2.7. Regulation is accomplished by a network of magnetic amplifiers or
transducers, transfom1ers and bridge rectifiers
interconnected as shown. In addition to the control
of load current delivered by the generator, a further
factor which will affect control of field excitation
is the error between the line voltage desired and the
actual voltage obtained. As already explained on
page 44, the compounding transfonner and rectifier
provide., excitation current proportional to load
current, therefore the sensing of error voltages and
necessary re-adjustment of excitation current must
be provided by the voltage regulation network.
It will be noted from the diagram that the threephase output of the generator is tapped at two points;
at one by a three-phase transformer and at the other
by a three-phase magnetic amplifier. The secondary
winding of one phase of the transformer Is connected
to the a.c. windings of a single•phase "error sensing"
magnetic amplifier and the three primary windings
are connected to a bridge "signal" rectifier. The d.c.
output from the rectifier is then fed through a voltage.
sensing circuit made up of two resistance anns, one
(ann ''A") containing a device known as a barretter
the characteristics of which maintain a substantially
constant current through the arm, the other (arm
"B") of such resistance that the current flowing through
it varies linearly with the line voltage. The two current
signals, which are normally equal at the desired line
voltage, are fed in opposite directions over the a.c.
output windings in the error magnetic amplifier. When
there· is a change in the voltage level, the resulting
variation in current Oowing through ann "B" unbalances the sensing circuit and, as circuil has the
same function as a d.c. control winding, it changes the
reactance of the error magnetic amplifier a.c. output
windings and an amplified error signal current ls pro·
duced. After rectification, the signal is then fed as d.c,
control current to the three-phase magnetic amplifier t
thus causing its reactance and a.c. output to change
208•V o.c,
1>----EN or Q(Tlpt, f ,er
~,gnol (d.r-l
E,c,101 ,on
l:'.rror sens,nQ s,gnol
Fig 2.18
Voltage rcguh1tlon
also. This results in an increase or decrease , as appropriate, of the excitation current now to the generator
rotor field winding, continuing until the line voltage
produces balanced signal conditions once more in the
error sensing circuit.
Constant-Frequency Systems
The regulation of the output of a constant-frequency
system is also based on the principle of controlling
field excitation, and some qf the teclmiques thus far
described are in many instances applied. In installa·
tlons requiring a multi-arrangement of constant·fre·
quency generators, additional circuitry is required to
cont1ol output under load-sharing or parallel operating
conditions and as thls control also Involves field excita·
tion, the overall regulation circuit arrangement is of
an integrated, and sometimes complex, form. At this
stage, however, we are only concerned with the fonda·
mental method of regulation ond for this purpose
we may consider the relevant sections or stages of the
circuit shown schematically in Fig. 2.19.
The circuit is comprised of three main sections; a
voltage error detector, pre-amplifier and a power
amplifier. The function of the voltage error detector
is to monitor the generator output voltage, compare
it with a fixed reference voltage and to transmit any
error to the pre-amplifier, It is made up of a threephase bridge rectifier connected to the generator output, and a bridge circuit of which two arms cont.tin
gas-filled regulator Lubes and two contai n resistances.
The inherent characteristics of the tubes are such thal
they maintain an essentially constant voltage drop
across their connections for a wide range of current
through them and for this reason they establish the
reference voltage against which output voltage is con·
tinuously compared. The output side of the bridge is
connected to an "error" control winding of tl1e preamplifier and then from this amplifier to a "signal"
control winding of a second stage or power amplifier.
Both stages a:re three-phase magnetic amplifiers. The
final amplified signal is then supplied to the shunt
windings of the generator a.c. exciter stator (see also
Fig. 2.17).
The output of the bridge rectifier In the error
detector is a d.c. voltage slightly lower than the average of the three a.c. line voltages ; it may be adjusted
by means of a variable resistor{RV 1) to bring the
regulator system to a balanced condition for any
nominal value of line voltage, A balanced condition <
the bridge circuit concerned ls obtained when the
voltage applied across the bridge (points '' A" and ,"B
i.s exactly twice that of the voltage drop across the
two tubes. Since under this condition 1 the voltage dr
across resistors R1 and R2 will equal the drop across
each tube, then no current will flow in the output ci.J
cuit to the error control winding of the pre-amplifier
If the a.c. line voltage should go above or below t!
fixed value, the voltage drops across R 1 and R1 will
,---- I
L.........._.. _ _ __
Power omplif,er
General or
_ _ __
~r,or delec tor
Fig 2.19
Const an t•frcqu,mcy system voltage regulation
differ causing an unbalance of the bridge circuit and a
flow of currenl lo the "error" control winding of the
pre-amplifier. The direction and ma~i~ude of curre~t
flow will depend on whether the va.nation, or error m
line voltage, is above (positive error signal) or below
(negative error signal) the balanced nominal value, and
on the magnitude of the variations.
When current flows through the "error" control
winding the magnetic flux sot up alters the ~ot_al flux
in the cores of the amplifier, thereby establishing a
proportional change in the amplifier output which is
applied to the signal winding of U1e power amplifier.
· If the error signal is negatfve it will cause an increase
in core flux, thereby Increasing the power amplifier
output current to the generator exciter field wintling.
Pora positive error signal the core flux and excitation
current will be reduced, Thus, the generator output is
controlled to the presel value which on being attained
restores the error detector bridge circuit to the
balanced condition.
ReguJators nonnally incorporate torque-limiting
circuitry which limits the torque at mechanical
linkages to a safe value by limiting the exciter field
Frequency-Wild Systems
In systems of this type, the a.c. output is supplied
to independent consumer equipment and since the
frequency is allowed to go uncontrolled, then
paralleling or.sharing of the a.c. load is not possible.
In most applications this is by design: for example,
in electrical de-icing equipment utilizing resistance
type heaters, a variable frequency has no effect on
system operation ; therefore reliance is placed more
on generator dependability and on the simplicity of
the generating system . Ln rectified a.c. systems fre.
quency is also uncontrolled, but as most o~ the output
is utilized for supplying d.c. consumer equipment,
load sharing is more easily accomplished by paralleling
the rectified output through equalizing circuits in a
similar manner to that adopted for d.c. generating
systems (see p. 16).
Constant-Frequency Systems
These systems are designed for operation under load·
sharing or paralleling conditions and In this connection
regulation of the two para.meters, real load and
reacti11e load, is required. Real load is the actual
working load output in kilowatts (kW) available for
supplying the various electrical services, and the
reactive load is the so-called ''wattless load'' which
is in fact the vector sum of the inductive and
capacitive currents and voltage in the system
expressed in kilovolt-amperes reactive (kV AR) . (See
Fig. 2.6 once again.)
Since the real load is directly related to the input
power from the prime mover, i.e. the aircraft engine,
real load-sharing control must be on the engine. There
are, however, certain practical difficulties involved, but
as it is possible to reference back any real load unbalance to the constant-speed drive unit between
engine and generator, real load-sharing control is
effected at this unit by adjusting torque at the output
drive shaft.
Reactive load unbalances are corrected by control·
ling the exciter field current delivered by the voltage
regulators to their respective generators, In accordance
with signals from a reactive load-sharing circuit.
Real Load-Shoring The sharing of real load between
paralleled generators is determined by the real relative
rotational speeds of the generators which in turn
inOuence the voltage phase relationships.
As we learned earlier (seep. 38) the speed of a
generator is determined by the initial setting of the
governor on its associated constant speed drive. It is
not possible, however, to obtain exactly identical
governor settings on all constant speed drives employed
in any one installation, and so automatic control of
the governors becomes necessary,
A.C. generators synchronous machines. Therefore when two or more operate in parallel they lock
t9gether with respect to frequency and the system
frequency established is that of the generator whose
output is at the highest level_. Since this is controlled
by speed-governing settings then it means that the
generator associated with a rugher setting will carry
more than its share of the load and will supply energy
which tends to motor the other machines in parallel
with it. Thus, sharing of the total real load is unbalanced, and equal amounts of energy in the form of
torque on the· generator rotors must be supplied.
Fundamentally, a control ~ystem is comp.rised of
two principal sections: one u1 which the unbalance is
determined by means of current transformers, and
the other (load controlling section) in which torques
are established and applied. A circuit diagram of the
system as applied to a four-generator installation is
shown. schematically in Fig. 2.20.
The current transformers sense the real load dis.
tributlon at phase "C'' of the supply from each generator, and are connected in series and together they
form a load sharing loop. Each load controller is
made up of a two-stage magnetic amplifier controlled
by an error sensing element in parallel with each cur·
tent transformer. The output side of each load con·
troller is, in turn, connected to a solenoid in the speed
governor of each constant speed unit,
When current flows through phase "C" of each
generator, a voltage proportional to the current is
induced in each of the current transformers and as
they are connected In series, then current will flow In
the load sharing loop. This current is equal to the
average of the current produced by all four trans·
Let us assume that at one period of system opera·
tion, balanced load sharing conditions are obtained
under which the current output from each transformer
is equal to five amps, then the average flowing in the
load sharing loop will be five amps, and no current cir•
culates through the error sensing elements. If now a
generator, say No. 1, runs at a higher speed governor
setting than the other three generators, it will carry
more load and will increase the output of its associ·
ated current transformer.
The share of the load being carried by the other
generators falls proportionately, thereby reducing the
output of their current transformers, and the average
current flowing in the load sharing loop remains the
same, i.e. five amps. lf, for example, it is assumed that
the output of No, 1 generator current transformer is
Lncreased to eight amps a difference of three amps
will flow through the error sensing clement of its
relevant load controller, The three amps difference
divides equally between the other gene rators and so
the output of each corresponding current transformer
is reduced by one amp, a difference which flows
tluough the error sensing elements of the load controllers. The error signals are then applied as d.c.
control signals to the two-stage magnetic amplifiers
and are fed to electromagnetic coils which are mounte
adjacent to permanent magnet flyweights and form
part of the governor in each constant speed drive unit
(see page 39). The current and magrtetic field
simulate the effects of centrifugal forces on the fly.
weights and are of such direction and magnitude as
to cause the flyweights lo be attracted or repelled.
Thus, in the unbalanced condition we have assume<
- --
--=.:- -
To speed
To speed
To ~peed
To speed
Fig 2.20
RcaJ load-sharing
i.e. No. 1 generator at a higher governor setting, the current and field resulting from the error
signal applied to the corresponding load controller
flows In the opposite sense and repels the flyweights,
thereby simulating a decrease of centrifugal force.
The movement of the flyweights causes to flow to
underdrlve and the output speed of the constant speed
unit drive decreases, thereby correcting the governor
setting to decrease the load being taken by No. 1
generator. The direction of the current and field in
the load controller sensing elements of the remaining
generators is such that the governor flyweights in
their constant speed drive units are attracted, allowing
oil to flow to overdrive, thereby increasing the load
being taken by each generator.
Reactive Load.Sharing The sharing of reactive load
between paralleled generators depends on the relative
magnitudes of their output voltages which vary, and
as with all generato r systems are dependent on the
settings of relevant voltage regulators and field
excitation current (see also p. 43). If, for example,
the voltage regulator of one generator is set slightly
above the mean value of the whole parallel system,
the regulator wiU sense an under-voltage condition
and it will accordingly increase its excitation current
in an attempt to raise the whole system voltage to
its setting. However, this results in a reactive component of current flowing from the ''over-excited"
generator which flows in opposition to the reactive
loads of the other generators. Thus, its load is
increased while the-loads of the other generators
are reduced and unbalance in reactive load sharing
exists. I l is·therefore necessary to provide a circuit to
correct this condition.
In·principle, the method of operation of the reactive
load-sharing circuit is similar to that adopted in the
real load-sharing circuit described eailier. A difference
in the nature of the circuitry should however be noted
at this point. Whereas in the real load-sharing circuit
the current transformers are connected directly to
the error detcctlng elements in load controlling units,
in a reactive load-sharing circuit (see Fig. 2.21) they
are connected to the primary windings of devices
called mutual reactors. These are, in fact, transfom1ers
which have (i) a power source connected to lheiI
secondary winding$ in addition to their primaries; in
this instance, phase "C" of the generator output, and
(il) an a1r gap in the iron core to produce a phase
displacement of approximately 90 degrees between
the primary current and secondary voltage, They
serve the purpose of delivering signals to the voltage
regulator which is proportional to the generator'$
reactive load only.
When a reactive load unbalance occurs, the current
transformers detect this in a similar manner to those
associated with the real load-sharing circuit and they
cause differential currents to flow in the primary
wind!n~ of their associated mutual reactors. Voltages
proportional to the magnitude of the differential
currents are induced in the secondary windings and
will either lead or lag generator current by 90 degrees,
When the voltage induced in a particular reactor
secondary winding leads the associated generator
current it indicates lhat a reactive load exists on the
generator; in other words, that it is taking more than
its share of the total load. In this condition, the
voltage will add to the voltage sensed by the secondary winc!.ing at phase "C". If, on the other hand. the
voltage lags the generator current then the generator
is absorbing a reactive load, I.e. it is taking less share
of the total load and the voltage will subtract from
that sensed at phase "C".
The secondary winding of each mutual reactor Is
connected in series with an error detector in each
voltage regulator, the deteclor functioning in the
same manner as those used for voltage regulation
and real load-sharing (see pp. 48 and 49).
Let us assume that No. I generator takes the
greater share of the load, i.e. it has become over·
excited. The voltage induced in the secondary wind·
ing of the corresponding mutual reactor will be
additive and so the error detector will sense this as
an overvoltage. The resulting d.c. error signal is
applied to the pre-ampl1fler and then to the power
amplifier the output of which is adjusted to reduce
the amoun t of exciter current being delivered to the
No. I generator. In the case of the other three
generators they will have been carrying less than
their of the reactive load and, therefore , the
Mutual rcoetot$
To pre -ornpli!ier
and genero tor
shunt f ield
To pre •ornplif 1er
end generator
shunt f ield
To pre-ampltl1er
ond genera tor
shunt f ield
Fig 2.21
Re~ctive load-sharing
To pre •ompltl!or
and generolor
$hvnl field
voltages induced in their mutual reactors will have
lagged behind the currents from the generators,
resulting in opposition to the voltages sensed by
the secondary windings. Thus, the output of each
power amplifier will be adjusted to increase the
amount of exciter current being delivered to their
associated generators until equal reactive load-sharing
is restored between generators within the prescribed
Synchronizing Lights
In some power-generating systems a method of
indlcatlng synchronization between generator outputs fomu part of the paralleling system, and consists
of lights and frequency adjustment controls.
A schematic diagram of the method based on that
adopted for the triple generator system of the Boeing
727 is given in Fig. 2.22.
The lights are connected into phases ''A" and
"C" of each generator between the generator breakers
and a synchronizing busbar, via a selector switch. The
switch is also used for connecting a voltmeter and a
frequency meter to each generator output phase "B".
The frequency adjustment controls are connected
into the circuit of the load controllers (see also
page 49).
The generators are connected to their respective
load busbars and the synchronizing busbar,
generator breakers and bus-tie breakers respectively :
each breaker being closed or tripped by manual
operation of switches on a panel at the Flight
. Engineer's station. The breakers also trip automatically in the event of faults detected by the generator
control and protection system. The field relays are
similarly operated. Indicator lights are located
adjacent to all switches to indicate either of the
closed or tripped conditions.
Synchronising busbar
Bus-tie _ _
No 2 a.c. bus
[email protected]}:::j _
- -
-- ~
~ J
Fig 2,2:Z
Prior to engine starting, the bus-tie breakers and
field Felays are clos~d (indicator lights out) and the
generator breakers are tripped (indicator lights on).
As the first engine is started, the meter selector
switch is positioned ·at GEN 1 to connect phases
"A" and "C" of this generator to the synchronizing
busbar via the synchronizing lights. Phase "B" is
connected to both the voltmeter and frequency meter
the readings of which are then checked. Since at
this moment, only the number 1 generator is in
oporatfon, then with respect to the other two, it
will of course, produce maximum voltage and phase
difference and both synchronidng lights will flash
at a Jugh frequency as a result of the current flow
through them , The frequency control knob fo r
the generator is then adjusted until its load controller has trinuned the CSP/generator speed to
produce a "master'' frequency of abbut 403 Hz,
and simultaneous Oashing of both synchronizing
When the second engine is started, the meter
selector switch is positioned at GEN 2 to connect
the synchronizing lights and meters to the appropriate phases of number 2 generator, and its
frequency is also adjusted in the mrumer just
described. The number 1 generator is then connected to its load busbar by closing its generator
breaker, This action also connects the generator
to the synchronizing busbar, and since the synchronizing lights are now sensing the output of the
second on-coming generator, their flashing frequency
will be very much less as a result of less voltage and
wiase difference between the two generator outputs.
The frequency of the second generator ls then
adjusted to obtain the greatest time interval between
flashes of the synchronizing lights, and while the
lights are out (indicating both sources of power are
in phase) the numbe r 2 generator is connected to
its load busbar by closing its breaker.
As the third engine is started, the meter selector
switch is positioned at GEN 3, and,by following the
same procedure just outlined, number 3 generator
is connected to its load 'busbar. With all three
generators thus connected their subsequent operation
Is taken care of automatically by the load.sharing
sensing circuits of the associated control and protection unit.
It is important to note that a generator must
never be connected to its load busbar when tho
synchronizing lights are on. Such aclion would
impose heavy loads on the generator or CSD and
possibly cause damage to them, If, at any time the
synchronizing lights flash alternately, a phase
reversal is indicated and the appropriate generator
should not be used.
The application of generators dependent upon an
airst{eam as the prime mover is by no means a
new one and, having been adopted in many early
types of aircraft for the generation of electrical
power, the idea of repeating the practice for to-day's
advanced electrical systems would, therefore, seem
to be retrogressive, However, an alr·drive can serve
as a very useful stand-by in the event of failure of a
complete main a.c, generating system and it is in
this emergency role that it is applied to some types
of aircraft.
The drive consists of a two-bladed fan or air
turbine as it is sometimes called, and a step-up ratio
gear train which connects the fan to a single a.c.
generator. The generator is of a similar type to the
main generator (see also p. 41) but has a lower
output rating since it is only required to supply the
consumer equipment essential under emergency conditions. The complete unit is stowed on a special
mounting in the aircraft fuselage, and when required
is deployed by a mechanically linked release handle
in the llight compartment. When deployed al air·
speeds of between 120 to 430 knots, lhe fan and
generator- are driven up to their appropriate speeds bl
the airstream, and electrical power is delivered via a
regulator at the rated values. A typical nominal fan
speed is 4,800 rev/min and is self-governed by varyin1
the blade pitch angles. The gearbox develops a gener·
ator shaft speed of 12,000 rev/min. After deploymen
of the complete unit, it can only be res towed when
the aircraft is on the ground.
Power Conversion Equipment
In aircraft electrical installations a number of different
types of consumer equipment are used which require
power supplies different from those standard supplies
provided by the main generator. For example, in an
aircraft having a 28 volts d.c. primary power supply,
certain instruments and electronic equipment are
employed which require 26 volts and 115 volts a.c.
supplies for their operation, and as we have already
seen, d.c. cannot be entirely eliminated even in aircraft
which are primarily a.c. In concept. Furthermore, we
may also note that even within the items of consumer
equipment themselves, certain sections of their cir•
cuits require clifferent types of power supply and/or
different levels of the same kind of supply. It therefore becomes necessary to employ not only equipment
which will convert electrical power from one form to
ano ther, but also equipment which will convert one
form of supply to a higher or lower value.
The equipment required for the conversion of main
power supplies can be broadly divided into two main
types, static and rotating, and the fundamentals of
construction and operation of typical devices and
machines are described under these headings.
Static Converting Equipment
The principal items which may be grouped under this
heading are rectifiers and transformers, some applic·
ations of which have already been discussed in
Chapter 2, and static d.c./a.c. converters.
The latter llerns are transistorized equivalents of
rotary inverters and a description of their construction
and operating fundamentals will be given at the end
of chapter.
The process of converting an a.c. supply Into a d.c.
supply is known as rectification and any static appar·
atus used for this purpose is known as a rectifier.
The rectifying action is based on the principle
that when a voltage Is applied to certain combinations
of metallic and non-metallic elements in contact with
each other, an exchange of electrons and positive
current carriers (k_nown as "holes") takes place at
the contact surfaces. As a result of this exchange, a
barrier layer is formed which exhibits different
resistance and conductivity characteristics and allows
current to now through the element combination
more easily in one direction than in the opposite
direction. Thus, when the applied voltage is an
alternating quantity the barrier layer converts the
current into a undfrectionaJ flow and provides a
rectified output.
One of the elements used in combination is
referred to as a "semi-conductor" which by definition
denotes that it possesses a resistivity which lies
between that of a good conductor and a good insulator. Semi-conductors are also further defined by
the number of carriers, I.e. electrons and positive
"holes", provided by the "crystal lattice'' form of the
element's atomic structure. Thus, an element having
a majority of electron carriers is termed ' 1n-type"
while a semi-conductor having a majority of "holes" is
termed "p-type".
If a p-type semi.conductor is in contact with a
metal plate as shown in Fig. 3.1, electrons migrate from
the metal to fill the positive holes In the semi-conductor,
and this process continues until the transference of
charge has established a p.d. ·Sufficient to stop it. By
this means a very thin layer of the semi-conductor is
cl~ared of positive holes and thus becomes an effective
insulator, or barrier layer. When a voltage is applied
such that the semi-conductor is positive with respect
to the metal, positive holes mi~ate from the body of
the semi-conductor into the barrier layer, thereby
reducing its ''forward" resistance and restoring con•
ductivity. If, on the other hand, the semi-conductor
Is made negative to the metal, further electrons are
drawn from the metal to fill more positive holes and
the "reverse'' resistance of the barrier layer is thus
increased. Toe greater the difference in the resistance
to current flow in the two directions the better is the
rectlfylng effect.
YQr nt~h
flecl,fy,ng Junclton
Hole IQr
mounting boll
Fig 3.2
Cross-section of a selenium rectifier clement
e l;Jectrons
(±) Fbs1hvc ·holes'
8Electron OCcilp!Orotom~
Fig 3,l
SemJ-conductor/met3,1 junction
A similar rectifying effect is obtained when an n·
type semi-conductor is in contact with metal and a
difference of potential is established between them,
but in this case the direction of ''easy" current flow
is reversed. l.n practice, a small current does flow
through a rectifier in lhe reverse direction because
p-type material contains a small proportion of free
electrons and n-type a small number of positive holes.
In the rectification of main a.c. power supplies,
rectifiers are now invariably of the type employing
the p-type non-metallic semi-conductors, selenium
and silicon. Rectifiers employing germanium (a
metallic element) are also available but as their
operating temperature is limited and protection
against short duration overloads is difficult, they
are not adopted in main power systems.
The selenium rectifier is formed on an aluminium
sheet which serves both as a base for the rectifying
junction and as a surface for the dissipation of heat.
Across-section of an element is shown dlagrarnrnatic·
ally in Fig. 3.2 and from this it will be noted that
the rectifying junction covers one side of the base with
the exception of a narrow strip at the edges and a
small area around the fixing hole which is sprayed
with a layer of insulating varnish. A thin layer of a lowmelting point alloy, referred to as the counter
electrode, is sprayed over the selenium coating and
insulating varnish. Contact with the two elements of
the rectifying junction, or barrier layer, is made
through the base on one side and the counter elec·
trode on the other.
Mechanical pressure on the rectifying junction
lends to lower the resistance in the reverse direction
and this is prevented in the region of the mounting
studs by the layer of varnish.
In practice a number of rectifying elements may
be connected in series or parallel to form what is
generally referred to as a rectifier stack. Two
typical stacks are shown in Fig. 3.3.
Fig 3.3
Typical rectifier stacks
When connected in series the elements increase
the voltage handling ability of a rectifier and when
connected In parallel the ampere capacity is increased.
Silicon rectifiers, or silicon junction diodes as they
are commonly known, do not depend on such a large
barrier layer as selenium rectifiers, and as a result they
differ radically in both appearance and size . This will
be apparent from Fig. 3 .4 which illustrates a junction
diode of a type similar to that used in the constant·
frequency generator described in Chapter 2.
H!lrmet,colly • sealed
cav, ty
Rec t ,fy,ng
- junct,on
5,licon wafer
Threaded mounl irq
slud ond cothOde
Pig 3.4
Silicon junction diode
n,e silicon is in the form of an extremely small
slice cut from a single crystal and on one face it has a
fused aluminium alloy contact to wh;ch Is soldered an
anode and lead. The other face is soldered to a base,
usually copper, which forms the cathode and at the
same time serves as a heat sink and dissipator. The
barrier layer is formed at the aluminium-silicon
To protect the junction from water vapour and
other deleterious materials, which can seriously
impair its performance , it is mounted in a hermeticallysealed case.
The limiting factors in the operation of a rectifier
are: (i) the maximum temperature permissible and
(ii) the minimum voltage , Le. the reverse voltage,
required to break down the barrier layer. In selenium
rectifiers the maximum temperature Is of the order
of 70°C. For germanium the temperature is about
S0°C, while for silicon up to l 50°C may be reached
without destroying the rectifier. It should be noted
that these figures represent lhe actual temperature at
the rectifying junction and therefore the rectifier, as
a complete unit, must be at a much lower temperature.
Proper cooling under all conditions is, therefore . an
essential requirement and is normally taken care
of by blower motors or other forced air methods such
as the one adopted in the constant-frequency
generator referred to earlier.
Voltage ratings are detormined by the ability of a
rectifier to withstand reverse voltage without passing
excessive reverse current. and the characteristics are
such that reverse current does not increase propor·
tionately to the applied voltage. This is because once
all the current carriers have been brought into action
there is nothing to carry any further current. However,
at a sufficiently high voltage the resistance in the
reverse direction breaks down completely and reverse
current increases very sharply. The voltage at which
breakdown occurs is called the Zener voltage, and as
it depends on the impurity content of the material
used, a constant value can be chosen by design and
during manufacture of a rectifier. For power rectifica•
tion, rectifiers must have a high Zener voltage value
and each type must operate at a reverse voltage below
its designed breakdown value . Some rectifiers, however, are designed to break down at a selected value
within a low voltage range (between 2 and 40 volts is
typical) and to operate safely and continuously at
that value. These rectifiers are called Zener Diodes
and since the Zener voltage Is a constant and can there·
fore serve as a reference voltage. they are utllized
mostly in certain low voltage circuits and systems for
voltage level sensing and regulation (see aJso p. 15).
An S.C.R., or thyristor as it is sometimes called, is a
development of the silicon diode and It has some of
the characteristics of a thyratron tube. It Is a threeterminal device, two terminals corresponding to those
of an ordinary silicon diode and the third, called the
"gate" and corresponding to the thyratron grid. The
construction and operating characteristics of the
device are shown In Fig. 3 .S. The silicon wafer which
is of the "n-type" has three more layers formed with·
in it in the sequence indicated.
When reverse voltage is applied an S.C.R. behaves
In the same manner as a normal silicon diode, but
when forward voltage is applied current flow is practically zero until a forward er! tlcal " breakover" voltage is reached. The voltage at which breakover takes
place can be varied by applying small current signals
between the gate and the cathode, a method known
as "firing". Once conduction has been initiated it can
be stopped only by reducing the voltage to a very low
value. The mean value of rectified voltage can be
controlled by adjusting the phasing of the gate signal
with respect to the applied voltage. Thus, an S.C.R.
not only performs the function of power rectification,
but also the function of an on-off switch, and a vari-
Gale Co1h0de
reokover vol tage
Ocv,ee eon be l,red ot any
<1es,red volue ol fo,word
volloge by applying ga le
Colhoda conne,:tor
Gote connector
Fig 3.5
Silkon controlled rectifier
able power output device. A typical application of
S.C.R switching is in the battery charger unit already
referred to on p. 29. Fig. 3.6 shows how an S.C.R.
produces a variable d.c. voltage which, for example,
would be required In a variable speed motor circuit ,
!IS gate signal currents or "firing point" is varied,
C.,rfenl flow
!r--- -- ~-- -- - --.
AC v,
Fig 3,6
Variable d.c. 011tput from-a silicon controlled rectifier
_ __
1• 1 holt·cyc;le
.,,,,,,. Cur,ont flow m 2"' ho ll · GtCI!
.-.,. Currqnt flow
Rectifiers are used in single-phase and three-phase
supply systems and, depending on the conversion
requirements of a circuit or system, they may be
arranged to give either half-wave or full-wave rectification. In the former arrangement the d.c. output is
available only during alternate half-cycles o(an a.c.
input, while in the latter a d.c. output is available
throughout a cycle.
The single-phase half-wave circuit shown in Fig.
3.7(a) is the simplest possible circuit for a rectifier
and summarizes, in a practical manner, the operating
principles already described. The output from the
single rectifier is a series of positive pulses the number
of which is equal to the frequency of the input voltage.
For a single-phase a.c. Input throughout a full cycle, a
bridge connection ofrectiflers is used (Fig. 3.7(b)).
For half-wave rectification of a three-phase a.c.
input the circuit is made up of three rectifiers in the
manner shown in Fig. 3.8. This arrangement is com·
parable to three single-phase rectification circuits, but
since the positive half-cycles of the input are occurring at time intervals of one third of a cycle (120
degrees) the number of d.c. pulses or tho tipple fre·
quency is increased to three times that of the supply
and a smoother output waveform is obtained.
Figure 3.9 shows the circuit arrangement for the
full-wave rectification of a three-phase a.c. input; it
Outpu t
Fig. 3.7
Single-phase rectification
(a) Half·Wllvi::
(b) Full-wave
is of the bridge type and is most commonly used for
power rectification in aircraft. Examples of threephase bridge rectifier applications have already been
shown in Chapter 2 but we may now study the circuit
operations in a little more detail.
o ut ,
Curren, fl:lw phQW I
Fig 3,8
Toretrphase hal!-wave re~tification
0.C. QUlpu l
- · - Pho,e I
- - - !'hose 2
...... Phase!
- ii
D.C.ourpu t
L,ne volt~ between Phgse, I aod 2
t,,ir"IQ vQltl. bel ween Pho~es I Oi'IO
Be lween pho~e:!. 2 Ol"IO I, ~l t- onq R( c°'°ucl
8elweo n pho~e·. 3 Clnd I, f13 • and R 1- CQl'ldu<;.I
Between phoses 3, onO 2, R, .. and
R-t COI\Ouc1
L,ne volh between Pnoses 2 ono
Fig 3,9
Operation of a full-wave bridge rectifier
In this type of circuit only two rectifiers are con·
ducting at any instant; one on the positive side and the
other on the negative side. Also the voltage applied to
the bridge network Is that between two of the phases,
i.e. the line voltage . Let us consider the points "A"
and "B" on the three phase voltage curves. These
points represent the line voltage between phases 1
and 2 of the supply and from the circuit diagram we
note that rectifiers R 1 + and R1 - only will conduct.
From "B" to "C" the line voltage corresponds to that
between phases l and 3 and R1+ now conducts in
conjuncjion with Rs- , Between the points "C" and
"D" the line voltage corresponds to that between
phases 2 and 3 so that rectrner R1 + now takes over
and conducts in conjunction with R3-. This process
continues through the remaining tluee conducting
paths, the sequence of the relevant phases and the
rectifiers which conduct being as tabulated in Fig. 3.S
The output voltage, which is determined by the
distance between the positive and negative crests, con
slsts of the peaks of the various line voltages for phase
angles of 30 degrees on eithe r side of their maxima.
Since the negative half-cycles art1 Included, then the
ripple frequency of a bridge rectifier output Is six
times that of the a.c. input and an even smoother
waveform is obtained.
form it is used for the transformation of single-phase
a.c. The second, known as the shell type , can be used
for either single-phase or three-phase transformation
and is one in which half the laminations are U-shaped
and the remainder are T·$haped, all of them being
assembled to give a magnetic circuit with two paths.
In both forms of construction the joints are staggered
in order to minimize the magnetic leakage at the joints.
The laminations are held together by core clamps.
In some designs the cores are formed of strips
which are wound rather like a clock spring and
bonded together. The cores are then cut into two Cshaped parts to allow the pre-wound coils to be fitted.
The mating surfaces of the two parts are often ground
to give a very small effective gap which helps to mini·
mize the excitation current. After assembly of the
windings the core parts are clamped together by a
steel band around the outside of the core.
Transformer windings are of enamelled copper
wire or strip, and are normally wound on the core
one upon the other, to obtain maximum mutual
inductive effect, and are well insulated from each
other. An exception to this normal arrangement is
in a variant known as an auto-transformer, in which
the windings are in series and on a core made up of
L-shaped laminations , Part of both primary and
secondary windings are wound on each side of the
core. On a shell.type transformer both windings are
wound on the centre limb for single-phase operation,
and for three·ph.asc operation they are wound on
each limb. Alternative tappings are generaUy provided on both windings of a transformer for different input and output voltages, while in some types
a number of dlfferent secondary windings provide
simultaneous outputs at different voltages.
Circuit Connections. Voltage transformers are
connected so that the primary windingS are in paral·
lel with the supply voltage; the primary windings of
current transformers are connected in series. A singlephase transformer as the name suggests is for the
transformation of voltage from a single-phase supply
or from any one phase of a three-phase supply. Transformation of three•phase a.c. can be carried out by
means of three separate single-phase transformers, or
by a single three-phase transformer. Transformers for
three-phase circuits can be connected in one of several
combinations of the star and delta connections (see
also Chapter 2), depending on the requirements for
the transformer. The arrangements are· illustrated in
Fig. 3.11.
When the star connection is used in three-phase
transformers for the operation of three-phase consumer
equipment, the transformer may be connected as a
three-phase system (Fig. 3.11 (a)). If single-phase loads
have to be powered from a three-phase supply_ii is
sometimes difficult to keep them balanced, it is therefore essential to provide a fourth or neutral wire so
that connections of the loads may be made between
this wire and any one of the three-phase lines (Fig.
3,1 l(b)).
- -~
-- --
Fig 3.ll
Circuit connections for three-phase tJBnsformcr~
(a) Star connection thr,c-wiro
(b) Star connection four-wire
(c) Star and Delta connection
The interconnection of neutral points of two star
windings is sometimes undesirable because this provides an external path for the flow of certain harmonic
currents which can lead to interference with radio
communications equipment. This is normally over·
come by connecting one of the two transformer wind·
ings in delta, for example, if the transformer supplies
A transformer is a device for converting a.c. at one
frequency and voltage to a.c. at the same frequency
but at another voltage. It consists of three main parts:
(i) an iron core which provides a circuit of low
reluctance for an alternating magnetic field created
by, (ii) a primary winding which Is connected to the
main power source and (iii) a secondary winding
which receives electrical energy by mutual induction
from the primary winding and delivers it to the
secondary circuit. There are two classes of transformers, voltage or power transformers and current
Principle. The three main parts are shown schematically in Fig. 3.10. When an alternating voltage is
applied to the primary winding an alternating
current will flow and by self-induction'will establish
Lominoted cote
wound one on the other) a voltage is established in
the secondary winding.
When a load is connected to the secondary winding
terminals, the secondary voltage causes current to
flow through the winding and a magnetic flux is pro·
duced which tends to neutralize the magnetic flux
produced by the primary current. This, in turn,
reduces the self-induced, or opposition, voltage in
the primary winding, and allows more current to flow
in it to restore the core flux to a value which is only
very slightly less than the no-load value.
The primary current increases as the secondary
load current increases, and decreases as the secondary
load current decreases. When the load is disconnected,
the primary winding current is again reduced to the
small excitation current sufficient only to magnetize
the core.
To accomplish the function of changing voltage
from one value to another, one winding is wound with
more turns than the other. For example, if the prim•
ary winding has 200 turns and the secondary 1000
turns, the voltage available at the secondary terminals
1 0
will be 2°o 0 or 5 times as great as the voltage applied
to the primary winding. This ratio of turns (N,) in lhc
secondary to the number of turns (N 1) in the primazy
is called the turns or transformation ratio (r) and it
is expressed by the equation.
: : ~-- ______ : ____J : :
". -
, __ __
- - - - - - - - - - .. . , , I
-- -- ----
,,.T _,,
Ni ,,,§.
N1 E1
where E1 and E1 are the respective voltages of the two
When the transformation ratio is such that the
transformer delivers a higher secondary voltage !ha.n
the primary voltage it is said to be of the "step·up"
type. Conversely, a "step-down" transformer is one
which lowers the secondary voltage. The circuit arrangements for both types are also shown in Fig. 3.10.
Step-down ,otiO
Fig 3.10
Transformer pri11ciple
a voltage in the primary winding which is opposite
and almost equal to the applied voltage, The difference between these two voltages will allow just enough
current (excitation current) to flow in the primary
winding to set up an alternating magnetic flux in the
core. The flux cuts across the secondary winding and
by mutual induction (in practice both windings are
Construction of Voltage Transformers.
The core of
a voltage transformer is 'laminated and conventionally
is built up of suitably shaped thin stampings; about
0-0i 2 in. thick on average, of silicon-iron or nickel.
iron. These materials have the characteristics of fairly
high resistivity and low hysteresis; therefore, in the
laminated form, the effects of both eddy currents and
hysteresis are reduced to a minimum. Two different
forms of construction are in common use.
In one the laminations are L-shaped and are
assembled to provide a-single magnetic circuit; ln this
an unbalanced load, the primary winding is in star
and the secondary Is In delta as shown in Fig. 3.1 l(c).
Current transformers are used in many a.c. generator
regulation and protection systems and also in con·
junction with a.c. ammeters. These transformers have
an input/output current relationship which is i~versely
proportional to the turns ratio of the primary and
secondary windings. A typical unit is shown in Fig.
3.12. It is designed with only a secondary winding on
a toroidal strip-wound core of silicon-iron. The
assembly together with the metal base Is encapsulated
in a resin compound moulding. The polarity of the
transformer is indicated by the markings Hl on the
side facing the generator and H2 on the side facing the
The prim,!ry winding is constituted by passing a
main cable of the power system, through the core
aperture. The cable ls wound with a single tum if it
carries high currents, and with two or three turns if it
carries low currents. Tho operating principle is the
same as that of a conventional transformer.
1n some aircraft generating systems, a number of
current transformers are combined into single package
assemblies to provide a means of centralizing equip·
ment location. One such assembly Is illustrated in
Fig. 3.13'. It consists of seven transformers which are
supplied with primary voltage via the three feeder
terminals and by insulated busbars passing through
the cores of the transformers which are arranged In
three sets. The busbars terminate in the flexible
insulated straps. Secondary leads from the various
transformers are brought out through a common
Contrary to the practice adopted for voltage trans·
formers, whenever the secondary windings of current
transformers are disconnected from their load circuits,
terminals must be short-circuited together. If this is
not done, a dangerous voltage may develop which
may be harmful to anyone accidentally touching the
terminals, or may even cause an electrical breakdown
between the windings.
In circuit applications normally requiring only a small
ste p·U p or step-down of voltage, a special variant of
transformer design is employed and this is known as
an auto-transformer. Its circuit arrangement ls shown
in Fig. 3.14 and from this it will be no ted that its
most notable feature is that it consists of a single
winding tapped to form primary and secondary parts.
ln the example illustrated the tappings provide a
stepped-up voltage output, since the number of
primary turns is less than that of the secondary turns.
When a voltage is applied to the primary terminals
current wUl flow through the portion of the winding
spanned by thesr. terminals. The magnetic flux due
to this current will flow through the core and will
therefore, link with the whole of the winding. Those
turns between the primary terminals act In the same
way as the primary winding of a conventional transformer, and so they produce a self-induction voltage
In opposition to the applied voltage. The voltage
induced in the remaining turns of the winding will
be additive, thereby giving a secondary output voltage
Termina l XI
' Term,nol X2
', ldon1d,cotion ploto
Fig 3.12
Current tran~former
S,d~ Hl - lowordslJSide H2· towords
Metol base , , /
Current -
'O" corryino c;;;uctor
eo ost transformer,
Ftoxlblo $'!rap,
secondary o;itpvt
Mttorino tron,formo,
Fig 3.13
C\men t transformer package
greater than the applied voltage. When a load circuit
is connected to the secondary terminals, a cummt due
to the induced voltage will flow tluough the whole
winding and will be in opposition to the primary
current from the input terminals . Since the turns
between the primary terminals are common to input
and output circuits alike they carry the difference
O\l lpul
l"ig 3,14
Circuit ~rrangemenl of un auto-transformer
between the induced current and primary current.
and they may therefore be wound with smaller gauge
wire than the remainder of the winding.
Auto-transformers may also be designed for use in
consumer circuits requiring tluee·phase voltage at
varying levels. The circuit arrangement of a typical
step,up transformer applied to a windshield antiicing circuit is shown in Fig. 3 .1 5. The three windings
are star-connected and are supplied with the "primary
voltage of 208 volts from the alternator system. The
secondary tappings are so arranged that up to four
output voltage levels may be utilized.
Transformers are usually rated in volt-amperes or
k.ilovolt·amperes. The difference between the output
terminal voltages al full-load and no-load , with a con·
stant Input voltage, is called the regulation of the
transformer. As in the case of an a.c. generator,
regulation Is expressed as a pcrcen tage of the full-load
voltage, and depends not only on actual losses (e.g.
hysteresis, eddy current and magnetic leakage) but
also on the power factor of the load. Thus, an
inductive load, Le. one having a lagging power factor,
will give rise to a high percentage regulation, wh.ile
with a capacitive load , I.e. one having a leading power
r - - - --
210 - v
11 10-v 0u1p.,1
210 - v Pr>osc'a '
a,o- v
{0--0 - - -- - - --
Phase 'A'
Input 208 · V
O~tput 480 -V
{ Ovtpul ~20 -V
- - - -- ....J
{o--0-------- - - - -- ----'
270 • v Poose 'c ·
4 JO - v output
< > -1- --
- - - -- - - -- ~
Fig 3.15
Tappings of a typical three-phase auto•traJJsformer
factor, the regulation may be a negative quality giving
a higher output voltage on full-load than on no-load.
Changes in power supply frequency, or the connection of a transformer to a supply whose frequency
differs from that for which the transformer was
designed, has a noticeable effect on its operation. This
is due to the fact that the resistance of primary windings are so low that they may be considere d to be a
purely inductive circuit. If, for example, the frequency
is reduced at a constant value of voltage, then the
current will rise. The increased current will, In turn,
bring the transformer core nearer to magnetic saturation and this decreases the effective value of inductance
leading to still larger current. Thus, if a transformer
Is used at a frequency lower than that for which it
was designed : there is a risk of excessive heat generation in the primary wincling and subsequent burn out.
On the other hand, a transformer designed for low
frequency can be used with higher frequencies, since
in this case the primary current will be reduced,
Transformer-rectifier units (T.RU.'s) are combinations of static transformers and rectifiers , and are
utilized in some a.c. systems as secondary supply
units, and also as the main conversion units in aircraft
having rectified a.c. power systems.
Fig. 3.16 illustrates a T.R.U. designed to operate
on a regulated three.phase input of 200 volts at a
frequency of 400 Hz and to provide a continuous
d.c. output of 110 A at approximately 26 volts. The
circuit is shown schematically in Fig. 3 .17. The unit
consists of a transformer and two three-phase bridge
rectifier assemblies mounted in separate sections of
the casing. The transformer has a conventional star.
wound primary winding and secondary windings
wound in star and delta. Each secondary winding is
connected to Individual bridge rectifier assemblies
made up of six silicon diodes, and connected in
parallel. An anunetcr shunt (dropping 50 mV at
100 A) is connected in the output side of the reel·
ifiers to enable current taken from the main d.c.
output terminals lo be measured at ammeter auxiliary
terminals. These terminals, together with all others
associated with input and output circuits, are grouped
on a panel at one end of the unit. Cooling of the unit
is by natural convection through gauze-covered ventilation panels and in order to give warning of overheating conditions, thermal switches are provided at
the transformer and rectifier assemblies, and are
connected to independent warning lights. TI1e switches
are supplied with cl.c. from an external source
(normally one of the busbars) and their contacts
close when temperature conditions at their reSJ:lective
locations rise to approxima tely l 50°C and 200°C.
Tran!lormo ' ,oetion
Rocti lier suction
itstum tuure _wamln9
Fig 3.16•rectl!ier
- - - - - - --,
'---- - --+- -
+ }. Main
Pig 3.17
Schematic circuit of a transformct·rectifler unit
2B•volts d.c.
To 11S-V e.c.
_ __ _ _
_ _ _
Rotary inverter operation
_ _ _
Rotary Converting Equipment
The most widely-known device under this heading is
the inverter designed to produce either 26 volts or
115 volts 400 Hz a,c, depending on the secondary
a.c. power requirements of an aircraft's electrical
system. Although now largely superseded by inverters
of the solid-state circuit or static type, rotary inverters
are still utilized in a number of the smaller types of
A rotary inverter consists of a d.c. motor driving
an a.c. generator, and since many of the systems
which are to be operated from it are dependent on
constant voltage and frequency, the a.c. supply must
be regulated accordingly. The methods of regulation
may vary, but we may consider the commonly
adopted method shown in Fig. 3.18.
When the inverter is switched on, d.c. is supplied
to the motor armature and shunt field winding,
and also to the excitation field winding of the
generator. Thus, the motor squts driving the generator
which will produce a three.phase a.c. output at
115 volts. In order to control the voltage at thls
level, the d.c. supply is passed through a resistor in
series with the generator field , This resistor is preset
to give the required excitation current at the regulated
d.c. system voltage level. Since the frequency of the
generator output is dependent on speed, then a
preset resistor is also connected in series with the
motor shunt field to provide sufficient excitation
current to flltl the motor and generator at the speed
necessary to produce a 400 Hz output.
Figure 3.19 illustrates a sectional view and circuit
arrangement of another type of rotary irtverter, and
F1old coil
zav tiv,tior
DC bNS!'«Qr
2BV d c supply
c::,. Rectified o.c
Shvnl {
Fig 3.19
Rotary Inverter (carbon•pile regulation)
(voltage a nd trcquoncycontrotl
although it Is only to be found on some older types
of aircraft, it is an interesting example of variation
in application of principles.
The motor and generator share a common armature
and field system, and control of voltage and frequency
is based on the carbon pile regulator principle (see
page 12).
The d.c. section of the machine is of the four-pole
compound-wound type, the d.c. being supplied'to the
annaturc winding, series and shunt-field windings.
The a.c. section corresponds to a star-wound genera.
tor, the winding being located in the slots of the
armature and beneath the d.c. winding. The a.c.
winding is connected to a triple slip ring and bn1shgear assembly at the opposite end to the commu·
tator. Thus, when the inverter is in operation, a threephase output is induced in a· rotating winding and
not a fixed stator winding as in the case of a con·
ventional a.c. generator.
The a,c. output is rectified and supplied to the
voltage coil of the regulator which varies the pile
resistance in the usual manner, this, in tum, varying
the current flow through the common field system
to keep both the voltage and frequency of the a.c.
output within limits.
-v _ __
These inverters perform the same conversion function
as the rota.ry machines described eadier, but by means
of solid-state or static circuit principles. They are
employed in a number of types of aircraft in some
cases as a normal source of a.c. power, but more
usually to provide only emergency a.c. power to
certain essential systems when a failure of the normal
l l 5-volts source has occurred. The function of an
inverter used for the conversion of battery supply to
single•phase 115-volts ·a.c. Is shown in the block
diagram of Fig. 3.20.
The d.c. is supplied to transistorized circuits of a
filter network, a pulse shaper, a constant current
generator, power driver stage and the output stage.
After any variations in the Input have been filtered
or smoothed out, d.c. is supplied to a square-wave
generator which provides first•stagc conversion of the
d.c. into square·wave form a.c. and also establishes the
required operating frequency of 400 Hz. This output is
then supplied to a pulse shaper circuit which controls
the pulse width of the signal and changes its wave
form before it is passed on to the power driver stage.
It will be noted from the diagram that the d.c. required
for pulse shaper operation is supplied via a turn-on
wove generator
_l!_ Notch
Turn on
'me ,--.......--,
Fig 3.20
S1ntic inverter principle
~ 115-V
horrnon,c r---- - 400~
delay circuit. The reason for this is to cause the pulse
shaper to delay its output to the power driver stage
until the voltage has stabilized. The power driver
supplies a pulse-width modulated symmetrical output
to control the output stage, the signal having a squarewave form. The power driver also shorts itself out each
time the voltage falls to zero, i.e. during "notch time".
The output stage also produces a square-wave output but of variable pulse width. This output is fmally
fed to a filter circuit which reduces the total odd
harmonics to produce a sine wave output at the volt·
age and frequency required for operating the systems
connected to the inverter.
As In the case of other types of generators, the
output of a static inverter must also be maintained
within certain limits. In the example illustrated, this
is done by means of a voltage sensor and a current
sensor, both of which produce a rectified a.c. feed·
back signal which controls the "notch time'' of the
pulse shaper output tluough the medium of a regulator circuit and a notch control circuit.
External and Auxiliary
Power Supplies
Electrical power is required for the starting of engines,
operation of certain services during "tum-round"
servicing periods at airports, e.g. lighting, and for the
testing of electrical systems during routine maintenance checks. The batteries of an aircraft are, of
course, a means of supplying the necessaty power, and
although capable of effecting engine starts their
capacity does not permit widescale use on the ground
and as we have already learned from Chapter 1, they
are restricted to the supply of power under emergency
conditions. It is necessary, therefore, to incorporate a
separate circuit through which power from an external
ground power unit (see Fig. 4.1) may be connected to
the aircraft's distribution busbar system. Jn its sim•
plest form, an external power supply system consists
of a connector located in the aircraft at a con.
veniently accessible point (at the side of a fuselage
for example) and a switch for completing the circuit
between the ground power unit and the busbar
In addition to the external power supply system,
some types of aircraft. carry separate batteries which
·<:an supply the ground services In the event that a
ground power unit is not available in order to conserve
the main batteries for engine starting.
In the majority of large public transport aircraft,
complete Independence of ground power units is
obtained by special auxiliary power units installed
within the aircraft.
D.C. Systems
A basic system for the supply of d.c. is shown in Fig.
4.2, and from this it will also be noted how, in addi·
tion to the external power supply, the battery may be
connected to the main busbar by selecting the
"flight" position of the switch. AJ the name suggests
Moon d,c. busbor
Bollery busbor
r--- - , Power
... I
- I - -· - · ---J
·-o:t'FloQhl '
Gro..ond pcwer
I I ·I
Pig 4,1
Ground power unit
Flg. 4.2
Buie oxt~mal power supply system
this is the position to which the switch is selected when
the aircraft is in flight since under this condition the
generator system supplies the main busbar and the
battery is constantly supplied with charging current.
The external power connector symbol mown in
the diagram represents a twin-socket type of unit
which although of an obsolete type is worth noting
because it established certain aspects which are basic
in the design of connectors or receptacles
as they are also called, namely the dimensioning of
pins and sockets, and the method of protecting them.
The pins were of different diameters to prevent a
reverse polarity condition, and the cover of the unit
had to be rotated to expose the sockets.
An example of a current type of unit ls shown In
Fig. 4.3. It consists of two positive pins and one
negative pin; one of the posit! ve pins is shorter and
of smaller diameter than the remaining pins. The
pins are enclosed by a protective shroud, and the
complete unit is normally fitted in a recessed housing
located at the appropriate part of the airframe
structure. Access to the plug from outside the air·
craft, is via a hinged flap provided with quick-release
~-p,n pluQ
E•I H nol supply
Fig 4.3
Ex ternal power supply connection
The circuit of a th ree-pin receptacle system is
illustrated in Fig, 4.4, and from this it will be noted
that the short positive pin ts connected in the coil
circuit of the external power relay . The reason for
this is that in the event of the external supply socket
being withdrawn with the circuit ''live'', the external
power relay will de-energize before tho main pins are
disen~aged from the socket. Tl1is ensures that breaking of the supply takes place at the heavy-duty
contacts of the relay thus preventing arcing at the
main pins.
Main d..(. bu~bar
[email protected] c;1,1rr9n
circv11 t,r1oke1
' - - -- .. E-xT-t<>. ' •
t,; ;:,
; Pow,,
I;clcc tor
bo tttry
Externa l power
Fig 4.4
Three-pin receptacle system
In some aircraft d.c. power is distributed from a
multiple busbar system and it is necessary for certain
services connected to each of the busbars to be oper·
ated when the aircraft is on the ground. This require!
a more sophisticated arrangement of the external
power supply system and the circuit of one such
arrangement is shown Lri Fig. 4.5. l n addition to
the extemal supply relay or contactor, contactors
for "tying" busbars together are provided, together
with magnetic indicators to indicate that all connections are made.
When the extemal ground power unit is connecte
to the aircraft and the master switch is selected "on'
it energizes the extemal supply cont actor, thus closi
its auxiliary and main sets of contacts. One set of
auxiliary contacts complete a circuit to a magnetic
indicator which then indicates that the external sup1
is connected and on (''C" in Fig. 4 .5), a second set
complete circuits to the coils of No . 1 and No . 3 bus
tic contactors while a third and main heavy-duty set
connect the supply direct to the "vital" and No. 2.
busbars. When both bus·tie contactors are energized
their main contacts connect the supply from the
external supply contactor to their respective busbllJ's
Indication that both busbars are also "tied" to the
ground power supply is provided by magnetic indica
tors ''A'' and "B" which are energized from the vital
busbar vi.a the auxiliary contacts the contactor.
No.I d.c.b1.1s Vital d.c.bus
No.2 d.c. bus
No.3 cl.c.t
Mo9nel lc
•lr®r-------- ---,
t:xrernol supply
No. I bus-he
No. 3 bus-lie
- -- - - + a ! :
~ , . ,Aux.
--1.L~j--c-_ Maio
External power
Fig 4,S
Schematic of an external pOW\lr supply - multiple d.c.
busba.r system
In some aircraft, and as an example we may consider the Boeing 737, a separate external power
connector is installed for starting an auxiliary power
unit in the event that the aircraft's battery is inoperative. The circuit arrangement is shown in
The receptacle is located adjacent to the battery
together with two circuit breakers indicated as "A"
and "B" in the diagram. The positive pin of the
receptacle is coupled directly to the battery busbar
via circuit breaker "A", and forms a parallel circuit
with the battery . Before external power is applied,
circuit breaker "B" must be tripped in order to
prevent damage to the battery charger.
A.C. Systems
In aircraft which from the point of view of electrical
power are principally of the "a.c. type", then it is
essential for the external supply system of the installation to include a section through which an external
source of a.c. power may be supplied. The circuit
arrangements for the appropriate systems vary between aircraft types but in order to gain some understanding of the circuit requirements and operation
generally we may consider the circuit shown in
Fig. 4.7.
When external power Is coupled to the receptacle a
three-phase supply is fed to tho main contacts of the
external power breaker, to an external power transformer/rectifier unit (T.R.U.) and to a phase sequence
protection unit. The T.R.U. provides a 28 volt d.c.
feedback supply to a hold-in circuit of the ground
power unit. If the phase sequenco is correct the pro.
tection unit completes a circuit to the control relay
coil, thus energizing it. A single-phase supply Is also
fed to an amber light which comes on to indicate that
external power is coupled, and to a voltmeter and
Battery bus
,-- -,
·:-}~.I·--~ ~::~zr I
L __
I i" I
I ...J... I
r- - ,
~ 1'_!
'- - ~
A 'External
B ' Battery
Separate cxtcrnd d.c. supply for A.P.U. starting
d.e. busbor
frequency meter via a selector switch,
The circuit is controlled by an external power
switch connected to a busbar supplied with 28 volts
d.c. from the aircraft battery system. When the
switch is set to the "close" position current flows
across the main contacts of the energized control
relay, to the ''close" coil of the external power
breaker, thus energizing it to connect the external
supply to the three.phase a.c. main busbar. The
axtemal power supply is disconnected by selecting
the "trip" position on the external power switch.
This action connects a d.c. supply to the trip coil of
the external power breaker, thus releasing its main
and auxiliary contacts and Isolating the external
power from the a.c. main busbar.
Figure 4.8 illustrates an external a.c. power
receptacle and control panel arrangement generally
representative of that adopted in large public trans.
port aircraft. The receptacle is of the six-prong type;
three of the large prongs are for the corresponding
a.c. power phases, and a fourth large prong for the
ground connection between the aircraft structure
and external power unit . The two small shorter
prongs connect d.c. power for the operation of
interlocking relays which connect the external a.c.
power to the aircraft.
- ·,
c. main busbar
Amber i ndicolor
Extorngf pQWer
brco~cr relo y
..1--+----------1 sequence
prot ect ion
L1m,t1ng resistor
28· V l e!dback -
Fig 4.7
Schematic of an external power supply - a.c. system
E: ,tarnol power
The control panel contains three single.phase a.c.
circuit breakers, and three more breakers which
protect relay control and indicating light circuits
within the aircraft's external power supply circuit.
Indicator lights, interphone jack plug sockets, and
pilo.t 's call button switch are also contained on the
The wrote indicator light is only illuminated
whenever external a.c. power is connected but is
Externlll a.c. power receptacle and control pant:!
not supplying power to any a.c. load busbar on the
aircraft. The blue light is illuminated whenever a.c.
power is being supplied to the load busbars.
The pilot's call button switch and interphone
jack plug sockets provide for communication
between ground crew and flight crew.
Many of today's aircraft are desig11ed so that if neces·
sary, they may be independent of ground support
equipment. This is achieved by the incorporation of
an auxiliary power unit (A.P.U.) in the tail section
which, after being started by the aircraft's battery
system 1 provides power for engine starting, ground
air conditl_oning and other electrical services. In
some installations, the A.P .U. is also used for supply. power in flight in the event of an engine.driven
generator failure (see p. 16) and for supplement.
Ing the delivery of air to the cabin during take-off
and climb.
In general, an A.P.U. consists of a small gas turbine
engine, a bleed·aif control and supply system, and an
accessory gearbox. The gas turbine comprises a two·
stage centrifugal compressor connected to a single.
stage turbine. The bleed.air control and supply system automatically regulates the amount of air bleed
from the compressor for delivery to the cabin air
conditioning system. In addition to those accessories
essential for engine operation, e.g. fuel pump control
unit and oil pumps, the accessory gearbox drives a
generator which is of the same type as those driven
by the main engines, and having the same type of
control and protection unit.
A motor for starting the A.P.U. is also secured to
the gearbox and is operated by the aircraft battery
system or, when available, from a ground power unit.
In some types of A.P.U. the functions of enSiJ!e
starting and power generation are combined in a
starter/generator unH. In order to record the hours
run, an hour meter ls automatically driven by an
An external view of a typical unit and a typical
installation, are shown in Figs. 4.9 and 4.10 respectively.
C'Ofll ptes.'S(lr
Pneum3hC shll1 ol f YOive
Stor ~er mo1or
/Y 1\1
OJl1 •• l~er
E1cc l<iC<iA
Actess.o,y geo•l)O:l
Fig 4.9
Auxiliary power unit
Turb<ne i>lenum
Power Distribution
In order for the power available at the appropriate
generating sources, to be made available at the
inputs of the power-consuming equipment and
systems then clearly, some organized fonn of distribution throughout an aircraft is essential. The
precise manner in which this is arranged is governed
principally by the type of aircraft anq its electrical
system, number of consumers and Iodation of consumer components. For example, in a small light
aircraft, electrical power requirements may be
limited to a few consumer services and components
situated within a small area, and the power may be
distributed via only a few yards of cable, some
tem1inal blocks, ckcuit breakers or fuses. In a large
multijet transport aircraft on the other hand, literally
miles of cable are involved, together with multiple
load distribution busbars, protection networks,~
junction boxes and control panels.
ln most types of aircraft, the ou tput from the generating sources is coupled to one or more low Impedance
conductors referred to as busbars. These are usually
situated in junction boxes or distribution panels
located at central points within the aircraft, and they
provide a convenient means for connecting positive
supplies to the various consumer circuits; in other
words, they perform a "carry-all'' function. Busbars
vary in form dependent on the methods to be adopted in meeting the electrical power requirements of
a particular aircraft type. In a very simple system a
busbar can take the form of a strip of interlinked
terminals while in the more complex systems main
busbars are thick metal (usually copper) strips or rods
Lo which input and output supply connections can be
made. The strips or rods are insulated from the main
structure and are normally provided with some form
of protective covering. Flat, fiexitilc strips of braided
copper wire are also used in some aircraft and serve
as subsidiary busbars.
Busbar Systems.
The function of a distribution
system is primarily a simple one, but it is complicated
by having_to meet additional requirements which concern a power source, or a power consumer system
operating either sepacately or collectively. under
abnormal conditions. The requirements and abnormal
conditions, may be considered in relation to three
main areas, which may be summarized as follows:
l. Power,consuming equipment must no t be
deprived of power in the event of power source
failures unless the total power demand exceeds
the available supply.
2, Faults on the distribution system (e.g. fault
currents, grounding or earthing at a busbar)
should have the minimum effect on system
functioning, and should constitute minimum
possible fire risk.
3. Power-consuming equipment faults must not
endanger the supply of power lo other cquipmcrll.
TI1ese requirements are met in a combirtcd manner
by paralleling generators where appropria te, by providing adequate circuit protection devices, and by
arranging for faulted generators to be isolated from
the distribution system. The operating fundamentals
of these methods are described elsewhere in this book
but the method with which this Chapter is concerned
is lhe additional one of arranging busbars a.nd distribution circuits so that they may be fed from cLifferen
power sources.
· In adopling this arrangement it is usual lo
categorize all consumer services into their order of
Importance and, in general, they fall into three
groups: vital, essential and non.essential.
Fig 4 .10
A.P.U. Installation
6. Low oil•pressurc switches
1. CoQling duct
7. fuel filter
2. Generator
3. Immersion thermocouple switch 8. Oil tank
9. Combustion chamber
4. Oil cooler
10. Ignition exciter
S. Hour meter
Vital services are those wh.ich would be required
after an emergency wheels-up landing, e.g. emergency
lighting and crash switch operation of fire extingtilshers. These services are connected directly to the
Essential services are those required to ensure safe
flight in an in-flight emergency situation. They are
cormected to d.c. and a.c. busbars, as appropriate,
and in such a way that they can always be supplied
from a generator or from batteries.
Non-essential services are those which can be
isolated in an in-flight emergency for load shedding
purposes, and are connected to d.c. and a.c. busbars,
as appropriate, supplied from a generator.
Figure 5.1 illustrates ln much simplified form, the
No. 2
ron sumers
Ccnlre busbar
Fig S.1
Busbar system
principle of dividing categorized consumer services
between individual busbars. In this example, the
power distribution system Is one in which the power
supplies are 28-volts d.c. from engine-driven generators
operating in parallel, 115-volts 400 Hz a.c. from rotary
inverters, and 28-volts d.c. from batteries. Each
generator has its own busbar to which are connected
the non-essential consumer services. Both busbars are
in turn connected- to e single busbar which supplies
power to the essential services. Thus, with both
generators operating, all consumers requiring d.c.
power are supplied. The essential services busbar is
also connected to the battery busbar thereby ensuring
that the batteries are maintained in the charged condition. In the event that one generator should fail it is
automatically isolated from its respective busbar end
all busbar loads are then taken over by the operative
generator. Should both generators fail however, nonessential consumers can no longer be supplied, but
the batteries will automatically supply power to the
essential services and keep them operating for a predetennined period calculated on the basis of consumer load requirements and battery state of charge.
For the particular system represented by Fig. 5.1,
che d.c. supplies for driving the inverters are taken
from busbars appropriate to the importance of the
a.c. operated consumers. Thus, essential a.c. con·
sumers are operated by No. I inverter and so it is
driven by d.c. from the essential services busbar. No.
2 and No. 3 i.ilverters supply a.c. to non-essential
services and so they are powered by d.c. from the
No. I and No. 2 busbars.
Figure S .2 illustrates a split busbar method o(
power distribution, and is based on an aircraft utilizjng
non-paralleled constant-frequency a.c. as the primary
power source and d.c. via transformer-rectifier units
(T.R..U.'s) .
The generators supply three·phase power through
separate channels, to the two main busbars and these,
in turn, supply the non-essential consumer loads and ·
T.R. U.'s. The essential a.c. loads are supplied from the
essential busbar which under normaJ oporalli\g condi·
tions is connected via a changeover relay to the No. l
main busbar. The main busbars are normally isolated
from each other i.e., the generators are not paralleled,
but If the supply from either of the generators fails,
the busbars are automatically inter-connected by the
energizing of the "bus-tie" breaker and serve as one,
thereby maintaining supplies to all a.c. consumers
and both T.R.U.'s. lf, for any reason, the power
supplied from both generators should fail the nonessential services will be isolated and the changeover
relay between No . I main busbar, and the essential
busbar, will automatically de-energize and connect
No,2 a.c.busbor
No.I a.c busb~r
'----o i
.---oT ~
power relay
Externol oc
Bus~t,e breoker
Q~ comJl'ner$
Sotlery bus_
v,tol de
d C COf\5Umer
Fig 5,2
Split busba.r system (primary a.c. power source)
lhe essential busbar to an emergency static inverter.
The supply of d.c. is derived from independent
T.R.U. andfrom batteries, The No. l T.R:u. supplies
essential loads and the No. 2 unit supplies non•
essential loads connected to the main d.o. busbar;
both busbars are automatically interconnected by
an isolation relay. The batteries are directly connected
to the battery bus bar and this is interconnected with
the essential busbar. In the event of both generators
failing the main d.c. busbar will become isolated from
the essential d.c. busbar whlch will then be automatically supplied from the batteries to maintain
operation of essential d.c. and a.c. consumers.
External power supplies and supplies from an
auxiliary power unit can be connected Lo the whole
system in the manner indicated in Fig. 5.2.
Another example of a split busbar system , based
on that used in the B7 37, is shown in Fig. S.3. The
primary power source Is non-paralleled 11 5/200-volt
3-phase a.c. from two 40 kV A generators. A source
of a.c, power can be supplied from another 40 kV A
generator driven by an auxiliary power unif, and also
from an external power unit. Direct current is supplied
via three T.R.U.'s.
The four power sources are connected to the
busbars by six 3-phase breakers and two transfer
relays, which are energized and de-energized according to the switching selections made on the system
control panel shown in Fig. 5 ,4. An interlocking
circuit system between breakers and switches is also
provided to enable proper sequencing of breaker and
overall system Opl;lralion. A source of power switched
onto or entering the system always takes priority and
so will automatically disconnect any existing power
source. The switches are of the "momentary select"
type in that following a selection they·are returned
o f------e
receptacle 0
'-----OG~B1n-_ ___,B~B
~ --y
1 1
8TB 2
t " \ - -- - - . . r i
I t"\--
Ext. power bus
Gen . bus 1
115- V
,- - - - - -
Main bus 1
Gen . bus 2
Main bus 2
- transfer
bus 1
28-V a.c.
I ...__1----+----- -l..--
- - --e
GS bus
DC bus 2
DC bus 1
Fig 5."3
Main a.c. and tl.c. po,,•or distribution system (no11-parnllcll
Battery bus
r -====~J J
BUI 011
GEi! Off
;.:.'--''~r-OH IUl
t·®n ~®""'
Fig 5.4
Control panel
to a neutral position by spring loading, The bus
transfer switch is retained in the "auto" position by
a guard cover to provide a path for signals controlling the "normal" and "alternate'' positions of
the transfer relays. ln the "off' position the transfer
relays are prevented from being energized to the
"alternate" positions so that the two main generating
systems are completely isolated from each other.
The indicating lights on the control panel are
illuminated as follows:
Ground Power Available (blue) - when external
'Power is plugged
into the aircraft.
Transfer Bus Off (amber)
- when either the
normal coil or
alternate coil of a
transfer relay is
- if both the respect·
Bus Off (amber)
ive GCB and BTB
are open.
- if the respective
Gen Bus Off (blue)
GCB is open.
APU Gen Bus Off(blue)
- if APU engine is
running and over
95% rev/min, but
there is no power
from the generator.
The ammeters indicate the load current of both
m.tin generators.
When external power is connected lo the aircraft
and is switched on, the external power contactor
closes and energizes both bus-tie breakers (BTB's) to
connect power to the whole busbar system. The
connection between the generator and
transfer busbars is made via the transfer relays which
are energized to the "normal'' position by the BTB's.
After starting an engine, number l for example,
and switching on its generator, BTB I trips open to
allow GCB I to close so that all system 1 busbars are
supplied from the generator. The number 2 system
busbars are still supplied from external power. When
number 2 engine has been started and its generator
switched on, BTB 2 trips open, GCB 2 closes to
connect the generator to the number 2 system busbars, and the external power contactor also trips
lf it is only necessary for the services connected to
the ground service busbar to be operated from
external power, this may be effected by leaving the
ground power switch on the control panel in the
"ofr' position, and switching on a separate ground
service swHch. The switch energizes a ground service
relay the contacts of which change over a connection
from generator bus 1 to the external power busbar.
The APU generator is connected to the entire busbar system via its own three-phase breaker, thjs, in
turn, being energized by two generator switches (see
Fig. 5 .4). Placing the left or number J switch to "on"
closes the APU generator breaker and also BTB 1,
and with the righ t or number 2 switch placed to
"on'' the BTB 2 is closed. As i.n the case of connect·
ing an external power supply, the traJ1Sfer relays arc
energized to the "normal" position by the BTB's.
The normal in.flight configuration of the power
distribution system is for each generator to supply
its respective busbars through its own breaker,
i.e. GCB 1 and GCB 2. These breakers are then
energized by the generator switches, the interlock
circuits keep the DTB's 1 and 2 in the open position,
so that the generator systems are always kept entirely
separate. GCB 1 and GCB 2 have a set of auxiliary
contacts which in the closed position energize trans·
fer relays to their "normal'' positions and so provide
connections between generators and transfer busbars
1 and 2. As will be noled from the diagram , the
transfer busbars supply TRU's I and 2, while TRU 3
is supplied direct from the main busbar 2.
In the event of loss of power from one or other
generator, number 1 for example, GCB 1 will open
thus isolating the corresponding busbars. When
GCB 1 opens, however, another set of auxiliary contacts within the breaker permit a d.c. signal to flow
from the control unit of generator 2, via a bus transfer switch, to the ''alternate" coil of transfer relay I .
The contacts therefore change over so that power
can then be supplied to transfer bus l from generator
2 which is still supplying its busbars in the normal
way. A similar transfer of power takes place In the
event of loss of power from generator 2.
Generator busbar 1 and main busbar 1 which
carry non-essential loads, can not be supplied with
power from generator 2 under the above power
loss conditions. If, however, power to these busbars
is required , the APU may be started and its number 1
switch placed momentarily to "on", thereby closing
the APU breaker and BTB 1. At the same time,
transfer relay I contacts would change over from
"alternate'' to ''normal" so that the APU supplies
the whole number I system. If a lo.5S of power from
the number 2 system should then occur, it is not
possible to connect it to the APU since its number 2
switch is electrically locked out during in-flight
~- --
~~- -
GCB' s ~-- -
)ad busbars .
The three TRU's are connected in such a way
that the loss of any one unit will not result in the
loss of a d.c. busbar. The relay between TRU I and
TRU 3 is held closed by supplying d.c. signals from
the generator control units via the bus transfer
switch in its "auto" position .
A further variation of the split busbar concept,
as adopted In the a.c. power generating system of
the 874 7, is simoly illustrated in Fig. S.5. lt utilizes
a system of interlocking GCB's and BTB's, but in
this case various combinations of generator operation
are possible .
If the GCB's only are closed, then each generator
will only supply its respective load busbat ; in other
words, they are operated individually and un•
paralleled. The generators may, however, also be
operated in parallel when the BTB's are closed to
connect the load busbars to a synchronous busbar.
As will be noted from the diagram , thls busbar is
split into two parts by a splll system breaker (SSB)
which, in the open position allows the generators
to operate in two parallel pairs. Closing of the SSB
connects both parts of the synchronous busbar so
that all four generators can operate as a fully
paralleled system. By means of the interlocking
Synchroni~lng busbar
~- --
~-- -
Synchronizing busbar
Com b!na tions of parall\ll opera !ion
system between breakers and the manual and auto·
matic sequencing by which they are controlled, any
generator can supply power to any load busbar,
and any combination of generators can be operated
in parallel.
Wires and Cables
Wires and cables constitute the framework of power
distribution systems conducting power in its various
forms and controlled quantities, between sections
contained within consumer equipment (known as
''equipment" wires and cables), and also between
equipment located in the relevant areas of an aircraft (known as "airframe" wires and cables). The
differences between a wire and a cable relate
principally to their constructional features (and
indirectly to their applications also) and may be
understood from U1e following broad definitions.
A wire is a single solid rod or filament of drawn
metal enclosed in a suitable insulating material and
outer protective covering. Although the term properly
refers to the metal conductor, it is generally understood to include the insulation and covering. Specific
applications of single wires are to b'e found in consumer equipment; for example, between the supply
connections and the brush gear of a motor, and also
between the various components which together
make up the stages of an electronic amplifier.
A cable is usually made up of a conductor com·
posed of a group of single solid wires stranded together
to provide greater flexibility, and enclosed by insula·
ting material and outer protective covering. A cable
may be either of the single core type, i.e., with cores
stranded together as a single conductor, or of the
mullicore type having a number of single core cables
in a common outer protective covering.
Having highlighted the above defmitions , il Is
interesting to note that with the present lack of international standardization of tern1inology, they may
not be used in the same context. For example, in
the U.S. and some other countries, the tenn ''wire"
is used as an all-embracing one.
In connection with power distribution systems in
their various forms, such terms as ''wiring systems",
"wiring of components", "circuit wiring" are corn·
10nly -used, These are of a general nature and apply
equally to systems incorporating either wires, cables
or both.
Wires and cables are designed and manufactured for
duties under specific environmental conditions and
are selected on this basis. This ensures functioning of
distribution and consumer systems, and also helps to
minimize risk of fire and structural damage in the
event of failure of any kind. Table 5 .1 gives details of
some commonly used general service wires and cablei
of U,K. manufacture, while typical constructional
features arc illustrated in Fig. 5.6.
The names adopted for the various types are
derived from contractions of the names of the variou
insulating materials used. For example, ''NVVIN" is
derived from "NYion" and from polyVINyl-chloride
(P.V.C.); and ''TERSIL" is derived from polyesTER
and SILicone. Cables may also be further classified b
prefixes and suffixes relating to the number of cores
and any additional protective covering. For example,
"TRINYVIN" would denote a cable made up of thrc
single Nyvin cables, and if suffixed by "METSHEAT
the name would further denote that the cable is
enclosed in a metal braided sheath.
It will be noted from the Table that only two
metals are used for conductors, i.e. copper (which
may also be tinned, nickel-plated or silver-plated
depending on cable application) and aluminium .
Copper has a very low specific resistance and is
adopted for all but cables oflarge cross.sectional
areas. An aluminium conductor having the same
resistance as a copper conductor, has only two-third
of fue weight but twice the cross-sectional area of
the copper conductor. This has an advantage where
low-resistance short-term circuits are concerned; for
example, in power supply circuits of engine starter
motor systems.
The insulation materials used for wires and cable:
must conform to a number of rigid requirements su<
as, toug}rness and flexibility over a fairly wide tein·
perature range, resistance to fuels, lubricants and
hydraulic fluids, ease of stripping for ·terminating, n
flammability and minimum weight. These requirements, which arc set out in standard specifications,
met by U1c materials listed in Table 5. I and in the
selection of the correct cable for a specific duty anc
environmental condition.
To ensure proper identification of cables, standa
specifications also require that cable manufacturers
comply with a code and mark outer protective cove
ings accordingly. Such a coding scheme usually sig·
Table 5.t
S086A (Type 2)
& Covering
temperature range
Tinned Copper
*P.V.C. Compound
Glass braid
-7S°C to+6S°C
11nned Copper
GI.ass braid
-7S°C to+ 50°C
-75°C to +lS<l°C
Silicone Rubber
Polyester tapes
Glass braid
Polye.s ter fibre
Glass braid
P.T.F.E. +
Glass braid
P.T .F.E..
Asbestos felt
impregnated with
silicone nrnish
Tanned Copper
As for NYVIN plus
an o·yerall tinnedcopper braid overlaid with polyester
tape, nylon braid
and lacquer
-75°C to +6S"c
Silicone Rubber
Glass braid and
F.E.P. 0
-7S°C to +J90°C
• Pol vVinvlC'h.lnric1e: + PnfvTe.1r:i.Fh1n ... n J;thvlrn,- ~ ••m..,.._.,..;"-st.-4 r,J....,, 1. ..ui~ o ............. ,, ... _ _
-7S°C to +220°C
Up to 240"C
General services wiring
except whe~c ambient
temperatures arc.
high and/or extended
pro pcrties of flexibility
arc required.
In high operating
temperatures and .in
areas where resistmtce
to aircraft fluids
ncoessa.ry. Also where
severe fie under
low-temperature condilions is encoun tered e .g.,
landing gear shock strut
switch circuits.
In circuits required to
function during or after
a rue.
In areas where
nifies, in sequence, the type of cable, country of
origin ("G" for U.K. manufacturers) manufacturer's
code letter, year of manufacture also by a letter, and
its wire gauge size, thus, NYVIN G-AN 22. A colour
code scheme is also adopted particularly as a means of
tracing the individual cores of multicore cables to and
from their respective terminal points. In such cases it
is usual for the insulation of each core to be produced
ln a different colour and in accordance with the
appropriate speci.ficatlon. Another method of coding,
and one used for cables in three.phase circuits of some
types of aircraft, is the weaving of a coloured trace
into the outer covering of each core ; thus red (phase A); yellow - (pha_se B); blue - (phase C). 111e
code may also be applied to certain single-core cables
by using a coloured outer covering.
As noted earlier in this chapter, the quantity of wires
and cables required for a distribution system depends
on the size and complexity of the systems. However,
regardless of quantity, it is important that wires and
cables be routed through an afrcraft in a manner
wltich, is safe, avoids interference with the reception
and transmission of signals by such equipment as
radio and compass sytems, and which also permits a
systematic approach to their identification, installa·
tion and removal, and to circuit testing. Various
methods, dependent also on size and complexity, are
adopted but in general, they may be grouped under
three principal headings: (i) open loom, (ij) ducted
loom, and (iii) conduit.
Open Loom.
In this method, wires or cables to be
routed to and from consumer equipment in the
specific zones of the aircraft, are grouped parallel to
each other in a bundJe and bound together with
waxed cording or p.v.c. strapping. A loom is supported
at intervals throughout its run usually by means of
clips secured at relevant parts of the aircraft structure.
An application of the method to an aircraft junction
box is shown in Fig. 5.7.
The composition of a cable loom Is dictated by
such factors as (i) overall diameter, (Ii) temperature
conditions, I.e. temperature rise in cabjes when opera·
ting at their maximum current,carrying capacity in
varying' ambient temperature conditions, (iii) type of
current, i.e. whether alternating, direct, heavy-duty or
light-duty, (iv) interference resulting from inductive or
magnetic effects, (v) type of circuit with which cables
are associated; this applies particularly to circuits in
the essential category, the cables of which tnust be
safe-guarded against damage in the event of shortcircuits developing in adjoining cables.
Magnetic fields exist around cables carrying direct
current and where these cables must interconnect
equipment in the vicinity of a compass magnetic
detector element, it is necessary for the fields to be
cancelled out. This is achieved by routing the positive
and earth-return cables together and connecting the
Nici/e l p/1ted coppor str,nd
¥ii+< G
q ............ ....... ... . ;, .......... ...... ;... -., -, .·- ··sr•7r ,-...
Fil! . 5.6
Constructional features of some typical cables-
. ... .. ·-- :. ~.. :... . ·-
earth-return cable at an earthing point located at a
specific safe distance from the magnetic detector
element of a compass system.
Ducted Loom. This method is basically the same as
that of the open loom except that the bundles are
supported in ducts which are routed through the air·
craft and secured to the aircraft structure (see Fig.
5.8). Ducts may be of aluminium alloy, resin·
impregnated asbestos or moulded fibre-glass.
reinforced plastic. In some applications of this
method, a main duct containing several channels may
be used, each channel supporting a cable loom corresponding to a specific consume.r system, For identification purposes; each loom is bound ,vilh appropri,
ately coloured waxed cording,
plastic, flexible metal or rigid metal sheaths. In cases
where shielding against signal interference is necessary
the appropriate cables are conveyed by metal conduits
in contact with metal structural members to ensure
good bonding.
<:able Seals, ln pressurized cabin aircraft it is essential
for many cables to pass through pressure bulkheads
without a "break" in them and without causing leakage
Conduits are generally used for conveying cables in
areas where there is the posslbU!ty of exposure to oil,
hydraulic or other fluids. Depending on the particular
application, conduits may take the form of either
Fig 5.8
Ducted looms
Fig 5.7
Open looms
of cabin air. This is accomplished by sealing the necessary apertures with either pressure bungs or pressureproof plugs and sockets. An example of a pressure
bung assembly is shown in Fig. S.9. It consists of a
housing, perforated synthetic rubber bung, anti·
friction washer and knurled clamping nuts; the housing is flanged and threaded, having a tapered bore to
accept the bung. The holes in the bung vary in size to
accommodate cables of various diameters, each hole
being sealed by a thln covering of synthetic rubber at
the smaller iliameter end of the bung. The covering is
Clomp,r,g /Int, • lriet,on
Clomp support /
Fig 5.9
P~ssure bung assembly
pierced by a special tool when londing the bung with
The cables are a tight fit in the holes of the bung
which, when fully loaded and forced into the housing
by the clamping nut, is compressed tightly into the
housing and around the cables. 'The anti-friction
washer prevents damage to the face of the bung when
the clamping nut is turned, On assembly, holes not
occupied by cables are plunged with plastic plugs.
In instances where cable "breaks" are required at a
pressure bulkhead, the cables at each side of the bulkhead are terminated by specially-sealed plug or socket
assemblies of a type similar to those shown in Fig.
S.14 (items 3 and 4).
For certain types of electrical systems, cables are
required tci perform a more specialized function than
that of the cables already referred to. Some examples
of what are generally termed, special purpose cables,
are described in the following paragraphs.
Ignition Cables. These cables arc used for the transmission of hlgh tension voltages in both piston engine
and tu rbine engine ignition systems, and are of the
single-core stranded type suitably insulated, and
screened by metal braldcd sheathing to prevent interference, The number of cables required for a system
corresponds to that of the sparking plugs or igniter
plugs as appropriate, and they arc generally made up
into a complete ignition c.ible harness. Depending on
the type of engine installation, the cables may be
enclosed In .a metal conduit, which also forms part of
a harness, or they may be routed openly. Cables are
connected to the relevant system components by
special end fittings comprising either small springs or
contact caps secured to the cable conductor, insulation
and a threaded coupling assembly.
171ermocouple Cables. These cables arc used for
the connection of cylinder head temperature indicator:
and turbine engine exhaust gas temperature indicators
to their respective thermocouple sensing elements. The
conducting materials are normally the same as those
selected for the sensing element combinations, namely,
iron and constantan or copper and constantan for ·
cylinder head thermocouples, chrome! (an alloy of
cluomium and nickel) and alumel (an alloy of aluminium and nickel) for exhaust gas thermocouples.
ln the. case of cylinder head temperature indicating
systems only one thermocouple sensing element is
used and the cables between it and a firewall connec·
tor arc normally asbestos covered, For exhaust gas
temperature measurement a numbe-r of thermocouples
are required to be radially disposed in the gas stream,
and it is the usual practice therefore, to arrange the
cables in the form of a hamess tailored to suit a
specific engine installation. TI1e insulating material
of the harness cables is either slllcone rubber or
P.T.F.E. Impregnated fibre glass. The cables terminate
al an engine or fuewall junction box from which
cables extend to the indicator. The insulating material
of extension cables is normally of the polyvinyl type,
sinco they are subject to lower ambient temperatures
than the engine harness. In some applications exten·
sion cables are encased in silicone paste within metal·
b1aided flexible conduit,
Co-axial Cables. Co-axial cables contain two or more
separate conductors, The innermost conductor may
be of the solid. or stranded copper wire type, and may
be plain, tinned, silver-plated or even gold-plated
in some applications, depending on the degree of
conductivity required. The remaining conductors are
in the form of tubes, usually of fme wire braid. Toe
insulation Is usually of polyethylene or Teflon. Outer
coverings or jackets serve to weatherproof the cables
and protect them from fluids, mechanical and electrical damage. TI1e materials used for the coverings
are manufactured to suit operations under varying
environmental conditions.
Co-axial cables have several main advantages. First ,
they are shielded against electrostatic and magnetic
fields; an electrostatic field does not extend beyond
the outer conductor and the fields due to current
flow in inner and outer conductors cancel each other.
Secondly, since co-axial cables do not radiate, then
likewise they will not pick up any energy, or be
influenced by other strong fields. The installations in
which coaxial cables are most commonly employed
are radio, for the connection of antennae, and capacitance type fuel quantity indicating systems for the
interconnection of tank Wlits and amplifiers. The
construction of a typical coaxial cable and also the
sequence adopted for attaching the end fitting are
shown in Fig. 5.10. The ouler covering is cut back to
expose the braided outer conductor (step "A") which
Fig 5.10
Typical coaxi~I cable and end fitting
I. Outer braid conductof
2. Outer covering
3. Adapter
Coupling ring
Inner conductor
Plug sub-assembly
Solder holes
is then fanned out and folded back over the adapter
(steps "B" and "C"). At the same time, the Insulation
is cut back to expose the inner conductor. The next
step (D) is to screw the sub-assembly to the adapter
thereby clamping the outer conductor firmly between
the two components. Although not applicable to all
cables the outer conductor may also be soldered to the
sub-assembly through solder holes. The assembly is
completed by soldering a contact on to the inner con·
ductor and screwing the coupling ring on Lo the subassembly.
In the-literal sense, earthing or grounding as it is often
termed, refers to the re turn of current to the conducting mass of the earth, or ground, itself. If considered
as a single body, Lhe earth is so large that any transfer
of electrons between it and anothor body fails to produce any perceptible change in its state of electrifi·
cation. IL can therefore be regarded as electrically
neutral and as a zero reference point for judging the
stale of electrification of other bodies. For example,
if two charged bodies, A and B, both have positive
potentials relative to earth, but the potential of A is
more positive than that of B, then the potential of B
may be described as negative to that of A by the
appropriate amount.
ru we have already learned, the positive outputs
of aircraft power supplies and the positive in putter· of consumer components are all connected to
busbars which are insulated from the alrcraft structure, Since in most aircraft the structure is of metal
and of sufficient mass to remain electrically neutral,
then it too can function as an earth or "negative
busbar" and so provide the return path of cunent.
Thus, power supply and consumer circuits can be com•
pleted by coupling all negative connections to the
structure at various "earth stations", the number and
locations of which are predicted in a manner appropria te to the particular type of aircraft. As tltis results
in the bulk of cable required for the circuits being on
the positive side only, then such an electrical installa·
tion Is designated as a "single-wire, or single-pole, earth·
return system". For a.c. power supply circuits the airframe aJso serves as a connection for the neutral point.
1l1e selection of types of connection for earth
return cables ls based on such important factors as
mechanical strength, current to be carried, corrosive
effec ts, and ease with which connections can be made,
As a result, they can vary in form: sorne typical
arrangements being a single bolt passing through and
must be provided. The number of connections· involve
secured directly to a structural member, and either a
in any one system obviously depend$ on the type and
single bolt or a duster of bolts secured to an earthing
size of an aircraft and its electrical installation, but
plate designed for riveting or bolting to a structural
the methods of connection with which we are here
member. In order to ensure good electrical contact
concerned follow the same basic pattern .
and minimum resistance between an earthing bolt or
In general, there are two connecting methods
plate and the structure, protective film is removed from
adopted and they can be broadly categorized by the
the contacting surfaces before assembly. Protection
frequency with which units must be connected or disagainst corrosion is provided by coating the surfaces
connected. For example, cable connections at junc\vith an anti-corrosion and solvent resistant compound
tion boxes, terminal blocks, earth stations etc. are of
or, in some cases by interppsing an electro-tinned
a more permanent nature, but the cable tenninations ari
plate and applying compound to the edges of the
such that the cables can be readily disconnected when
joint. An example of a cluster arrangement with a
occasion demands. With equjpment of a complex
corrosion plate is illustrated in Fig. 5.7.
nature liable to failure as the result of the failure of
Earth-return cables are connected to earthing bolts
any one of a multitude of components, the connec.
by means of crimped ring type connectors, each bolt
tions are made by some form of plug and socket thus
accommodating cables from several circuits. For some
facilitating rapid replacement of the component.
circuits, however, it is necessary lo connect cables
Furthermore, the plug and socket method also
separately and applies particularly to those of the
sensitive low current-carrying type, e.g. resistance type facilitates the removal of equipment that has to be
inspected and tested al intervals specified in maintemperature indicators in which errors can arise from
tenance schedules.
varying earth return currents of other circuits.
In aircraft in which the primary structure is of non·
metallic construction, a separate continuous maln
earth and bonding system is provided. lt consists of
four or more soft copper strip-type conductors ex111ere are several methods by which cable tem1inat!ons may be made, but the one most commonly
tending the whole length of the fuselage and disposed
so that they are not more than six feet apart as
adopted in power distribution systems is the soldermeasured around the periphery of the fuselage at the
less or crimped termination. The soldering method of
position of greatest cross-sectional area. The fuselage
making connections is also adopted but is more
ea1thing strips are connected to further strips which
generally confined to the joining of internal circuit
follow the leading and trailing edges from root to tip
connections of the various items of consumer equipment and in some cases, to the connec tions between
of each wing and horizontal stabilizer, and also to a
strip localed on or near the leading edge of the·vertical single-core cables and plug and socket cont.acts.
stabilizer. Earthing strips are provided in the trailing
edges of the rudder, elevators and ailerons, and are
Oimped Terminals. A crimped terminal is one
connected to the fuselage and wing systems 'Via the
which has been secured to Its conductor by compresouter hinges of the control surfaces. The strips are
sing it in such a way that the metals of both terminal
arranged to run with as few bends as possible and are
and conductor merge together to form a homogeneou1
connected to each other by means of screwed or
mass. Some of the advantages of the crimping method
riveted joints.
l. Fabrication is faster and easier, and uniform operaUghtning strike plates, extending round the tips of
each wing, horizontal and vertical stabilizers, fuselage
tion is assured.
nose and tail, are also provided. TI1ey consist of copper 2. Good electrical conductivity and a lower voltage
strips and are mounted on the exterior of the strucdrop is assured.
3. Connections are stronger (approachlng that obtainc
with cold welding); actually as strong as the condu<
tor itself.
4. Shorting due to solder slop and messy flux problenare eliminated.
ln order to complete the linkages between the various
5. "Wicking" of solder on conductor wires and "dry"
units comprising a power distribution system, some
join ts are eliminated.
appropriate means of connection and disconnection
6. When properly formed a seal against the Ingress of
air is provided and a corrosion:proof joint thereby
obtained .
A typical terminal (see Fig. 5.11) is comprised of
two principal sections; crimping barrel and tongue.
For :i particular size of conductor the copper or aJu.
mlniurn barrel is designed to fit closely over the barrel
end of the conductor so that after pressure has been
applied a large number of point contacts are made .
The pressure is applied by means of a hand-operated
or hydraulically.operated tool (depending on the size
of conductor and terminal) fitted with a die, shaped to
give a particular cross-sectional form, e.g. hexagonal,
diamond or "W". The barrels are insulated by plastic
sleeves which extend a short distance over the con.
ductor insulation and provide a certain amount of
support for the conductor allowing it to be bent in
any direction without fraying of the conductor
insulation or breaking of wire strands. In certain types
from an efglas cable to a nyvin cable may be necessary, a variant of the crimped terminal is used . This
variant is known as an In-line connector and consists
essentially of two crimping barrels in series, one con.
ductor entering and being crimped at each end. A
plastic insulating sleeve is also fitted over the connector and is crimped in position.
A selection of terminals and in·llne connectors are
shown in Fig. 5.12 .
Aluminium Cable Connections. The use of alumin·
lum wire as an electrical conductor for certain systems
is due chiefly to the important weight advantage of
Fig S,11
Crimped terminals
I. Tongue.
2. lnsv la tion sleeve
3, Barrel
4. Stainless stscl support
(hugo.diameter cable terminals)
of terminal the inside surface of the barrel is serrated
so that under the crimping pressure the strands of the
conductor "flow" into the serrations to make a con·
nection of high tensile strength, The serrations have
the additional function of assisting in the breaking
down of the oxide layer that forms on conductor
wires during the crimping operation. To facilitate
inspection of the crimped joint, the barrel is fre.
quontly left open at the tongue end, or in some
cases, is provided with an inspection hole through
which sufficient insertion of the conductor into the
barrel may be visually verified.
The design of the tongue end depends on where
and how the terminal is to be attached, The most
common forms are the ring type and fork type.
Where a connection between the ends of two cables
has to be made, for example , in a cable run from the
engine nacelle to the fuselage of an aircraft, a change
'""'! · • · : :
Fig 5.12
Terminals and in-lino connectors
this metal over copper. However, in order to acquire
satisfactory electrical connections, certain installation
techniques are necessary to compensate for two other
principal characteristics of aluminium, namely the
rapidity with which it oxidizes, an d its softness.
The oxide film is formed as soon as aluminium is
exposed to the atmosphere and il not only acts as an
insulator, but also increases in thickness as heat is
generated by the flow of current, still further increasing the electrical resista nce and causing corrosion at
connectingjoints. The method most commonly em·
ployed for eliminating the oxide film is the one in
which a special zinc granular compound is applied to
the exposed ends of the cable and the appropriate terminal. Aluminium terminals are normally of the
crimped type and the barrel is filled with compound;
in some cases the barrel contains a pre-fi1led cartridge.
When crimping Lakes place the compound is forced
around and between the wire strands of the cable, and
penetrates the oxide film to assist in breaking il down.
In this manner, clean metal-Lo-metal contacts arc pro·
vided and the high electrical resistance of the oxide
film is bypassed: Sealing of the terminal/cable joint
is also achieved so that the oxide film cannot reform.
In cases where an aluminium cable terminal is to
be bolted directly lo the aircraft structure, a busbar, or
surface of a component, the surfaces are first cleaned
and a coating of compound applied. To compensate
for the relative softness of aluminium as compared
wilh copper, flat washers with larger diameters than
the tongue end of a terminal are used to help <;!is·
tribute the clamping pressure over a wider surface.
For reasons of softness also, tightening torques applied
to bolted connections are maintained within specific
be fixed or free items, l.c. fixed in a junction box,
panel or a consumer component, or free as part of a
cable to couple into a fixed item . There are many vari·
ations in the design of plugs and sockets governed
principally by the distribution circuit requirements,
number of conductors to be terminated, and environ•
mental conditions. In general, however, the conventional construction follows the pattern indicated in
exploded form in Fig. 5.13. The bodies or shells, are
mostly of light alloy or stainless steel finished overall
with a cadmium plating; they may be provided with
either a male or female thread . Polarizing keys and
Plugs and sockets (or receptacles) are connecting devices which respectively contain male and female contact assemblies~.They may
Plugs and Sockets.
Fig 5.14
Fig 5,13
Plug nnd sockot construction
Typical plugs ~nd sockets
(1, 2) l"ixed equipment ~nd pone! types
(3, 4) Fi.>.cd tJ1rough· IYpe (bulkhead)
(5) Free cypc with cable cl ~rnp
(6) Fixed type angle fitting ·
(7) Fixed type r~ck equipment
keyways are also provided to ensure that plugs and
sockets and their corresponding conductors, mate
correctly; they also prevent relative movement
between their contacts and thereby strain, when the
coupling rings are being tightened. The shells of "free"
plugs and sockets are extended as necessary by the
attachment of outlets or endbells. These provide a
means of su pporting the cable or cable loom at the
point of entry to the plug or socket thereby preventing straining of the conductor, and pin or socket
joints, they prevent displacement of the contacts in
the softer material insulators, and the ingress of
moisture and dirt. In many cases a special cable clamp
Is also provided (see Fig. 5.14, item 5).
Plug contacts are usually solid round pins, and
socket contacts have a resilient section which is
arranged lo grip the mating pin. The contacts are re.
talned in position by Insulators or inserts as they are
often called, which are a sliding fit in the shells and
are secured by retaining rings and/or nuts. lnsulators
may be made from hard plastic, neoprene of varying
degrees of hardness, silicone rubber or fluorosilicone
rubber depending on the application of a plug and
socket, and on the environmental conditions under
which they are to be used. Attachment of conductors
to pin and socket contacts Is done by crimping (see
p. 89) a method which has now largely superseded
that of soldering. The socket contacts are designed so
that their grip on plug pin contacts is not reduced by
repeated connection and disconnection.
ln most applications, plugs and sockets are secured
in the matod condition by means of threaded
coupling rings or nuts ; in some cases bayonet-lock
and push-pull type couplings may also be employed.
Some typicl!,I fixed and free type plugs and
sockets are illustrated in Fig. 5.14. The rack type
unit (item 7) Is used principally for the interconnection of radio and other electronic equipment
which Is normally mounted in special racks or trays.
One of the elements, either the plug or the socket, is
Fig 5.15
Pin/socket sequencing
.,. Polarising key positions
fixed to the back of the equipment and the mating
unit is fixed to the rack or tray; electrical connection is made when the equipment is slid into the rack
or tray.
In addition to Identifying pins and sockets by
numbers or letters. usual in many types of
connectors to signify the numerical or alphabetical
sequencing. As shown ln Fig. 5.15, this is done by a
spiralling "guideline" embossed on the faces of
inserts. Bveiy tenth pin or socket cavity is identified
with parentheses.
This ls a technique usually applied to plugs and sockets which are to be employed in situations where
there is the possibility of water or other liquids
passing through the cable entry. Lt eliminates elaborate
cable ferrules, gland nuts, etc, by providing a simple
plastic shroud with sufficient height to cover the
terminations, and filling the cavity with a special
compound which though semi-fluid in its initial con·
clition, rapidly hardens into a rubbery state to form a
fairly efficient seal. In addition to sealing it provides
reinforcement for the cable connections.
The potting compound consists of a basic material
and an alkaline or acid base material (known as an
"accelerator'') which are thoroughly mixed in the
correct proportion to give the desired consistency and
hardness of the compound. Once mixed, the compound
is injected into a special mould and allowed to set.
When the mould is removed, the resilient hemlspherically-shaped insulation extends well into the plug or
socket, bonding itself lo the back of the insulanl
around the contact and conductor joints and partly
out along the conductor insulation.
l:lectrical Bonding
During flight , a build-up of electrical energy occurs in
the structure of an aircraft, developing in two ways:
by precipitation static charges and by charges due to
electrostatic induction. Precipitation static charges are
built up on the outer surfaces of an aircraft due to
frictional contact with rain particles, snow and ice
crystals, dust, smoke and other air con lamination .
As the particles flow over the aircraft negative charges
are left behind on the surfaces and positive charges
are released to flow into the airstream. ln addition,
particles of foreign impurities which are themselves
charged, make physical contact and transfer these
charges to the surfaces of the aircraft, increasing or
decreasing the charged state already present by virtue
of the frictional build·u p.
Charges of the electrostatic type are those induced
into an aircraft when flying into electric fields created
by certain types of cloud formation . This condition
of charge is the result of the disruption of water par·
ticles which increases the strength of a field and builds
up such a high voltage that a discharge occurs in the
familiar form of lightning. The discharge can take
place between oppositely charged pockets in one
cloud or a negatively charged section and the top of
the cloud, or between a positively charged pocket
and earth or ground. A well developed cloud may have
several oppositely charged areas, which will produce
several electric fields in both the horizontal and ver,
tical planes, where voltages ofup to 10,000 volts per
centimetre can be achieved. The relative hazard created
by these high potentials can be readily appreciated
if it is realized that by electrostatic induction, up to
10 million volts with possibly several thousand amperes of current, may be permitted to pass through
the aircraft when flying in or near Lhe aforementioned
Regardless of how an aircraft acquires its static
charges the resultant potential difference between it
and the atmosphere produces a discharge which tends
to adjust the potential of the aircraft to that of the
atmosphere. The charge is therefore being dissipated
almost as it is being acquired, and by natural means .
One of the hazards, however, is the possibility of
discharges occurring within the aircraft as a result of
differences bet ween the potentials of the separate
parts which go to make up the aircraft, and all the
systems necessary for its operation . It is essential, then
fore, to incorporate a system which will form a
continuous low-resistance link between all parts and
in so doing will :
(0 limit the potential dlffere.nce between all
(ii) elimlnate spark discharges and fire risks.
(iii) carry the high voltages and
currents so that they will discharge to atmosphere a l the extremities of the aircraft.
(iv) reduce interference wl!h radio and navigational aid signals.
(v) prevent the possibility of electrical shock
hazards to persons contacting equipment
and parts of the alrcraft.
Such a system is called a bonding system and
although differing in its principal functions. it will be
(o) la Yer& ond control rods
Bondinq strip
(b) Pipes with non-metall ic couplings
(c) Flight cont11)1 svfoces
(d) flexible auplino at bukheods
(el Shock-mounted eQuipment
Fig 5.16
Bonding methods
clear from the fact that electrical continuity is obtained, lhe requirements of the system overlap those
of the earthing system described on p. 87.
The continuous link is formed by metal strip
conductors joining fixed metal parts, e.g. pipes joined
either side of a non-metallic coupling, and by short•
length flexible braid conductors for joining moving
parts such as control rods, Oight control surfaces, and
components mounted on flexible mountings, e.g.
instrument panels, mounting racks for electronic
equipment. Some typical examples of the method of
joining bonding strips or "jumpers" as they arc some·
limes called, are shown in Fig. 5 .16.
In general, bonding is classified as Primary and
Secondary, such classifications being determined by
the magnitude of current to be expected from electro·
statically induced charges, and precipitation static
charges respectively. ?rimary bonding conductors
are used between major components, engines, external
surfaces, e.g. mght control surfaces, and the main
structure or earth. Secondary bonding conductors are
used between components and earth for which primary conductors are not specifically required, e.g.
pipelines carrying flammable fluids, metal conduits,
junction boxes, door plates, etc.
Some static charge is always liable to remain on an
aircraft so that after landing a difference In potential
between the aircraft and the ground could be caused.
This obviously is undesirable, since it creates an elec·
tric shock hazard lo persons entering or leaving the
aircraft, and can cause spark discharge between the
aircraft and external ground equipment being coupled
to it. In order to provide the necessary leakage path,
two 1ncthods are generally adopted either separately
or in combination. In one, the aircraft is fitted with a
nosewheel or tail-wheel tyre as appropriate, the
rubber of which con tains a compound providing the
tyre with good electrical conductivity. The ·second
method provides a leakage path via short flexible steel
wires secured to the nose wheel or main wheel axle
members and making physical contact with the ground.
During refuelling of an aircraft, strLngen t pre·
cau tions are necessary to minimize the risk of fire or
explosion due to the presence of static charges. The
aircraft itself may be charged, the fuel Oowing
through the hose generates electrical poten lials, and
the fuel tartker may be charged. Thus rsotential dif·
ferences must be prevented from occurring and which
could otherwise result in the generation of sparks and
ignition of flammable vapours. The equalizing of
potentials is achieved by providing a bonding connec·
tion between the aircraft and tanker which themselves
are bonded to the ground, and by bonding the hose
nozzle to a point specially provided on the aircraft.
During the refuelling operation physical contact
between the hose nozzle and tank filler is always
In a number of current " new technology" aircraft,
high-performance non.metallic composite materials
are used in major structural areas. Although they
obviously have advantages from the structural point
of view, the reduction in what may be termed the
"metallic content" of the ai rframe does, unfortunately. reduce the effectiveness of shielding the
aircraft, and electrical and electronic systems from
the effects of lightning strikes.
Since the operation of these systems is based on
sophhlicaled digital computing techniques, then
the many computers in control of an aircraft are
most vulnerable. This is because the magnetic
induction created by large lightning curren ts f1owing
nearby, could completely destroy microprocessors
and other vital integrated circuit packs comprising
the units, Thus, further lightning protection must
be built into the aircraft, and currently this takes
the form of special suppression fllters and metallic
shielding over system cables.
As noted earlie r, the discharge of static takes place
continuously in order to equalize the potentials of the
charges in the atmosphere and the aircraft. However,
it is often the case that the rate of discharge is lower
than the actual charging rate, with the result that the
aircraft's charge potential reaches such a value it permits what is termed a corona discharge, a discharge
which if of sufficient magnitude, will glow in poor
visibility or al night. Corona discharge occurs more
readily at curves and sections of an aircraft having
minimum radii such as wing tips, trailing edges,
propeller tips, horizontal and vertical stabilizers,
radio an ten nae, pi tot tubes, etc.
Corona discharge can cause serious interference
with radio frequency signals and means must therefor
be provided to ensure that the illscharges occur at
points where interference will be minintized. This is
accomplished by devices called static discharge wicks
or more simply, static dischargers . They provide a
relatively easy exit for the charge so that the corona
breaks out at predetermined points rather than hap·
hazardly at points favourable to its occurrence. Static
dischargers are fitt ed to the trailing edges of ailerons,
elevators and rudder of an aircraft. A typical static
discharger consists of nichrome wires formed in the
manner of a ''brush" or wick thereby providing a
number of discharge point~. In some instances, static
dischargers may also take the form of small metal rods
for trailing edge fitting and short flat metal blades for
fitting at the tips of wings 1 horiwntal and vertical
stabilizers. Sharp tun~ten needles extend at right
angles to the discharger tips to keep corona voltage
low and to ensure that discharge will occur only at
these points.
Screening performs a similar function to bonding in
that it provides a low resistance path for voltages
producing unwanted radio frequency interference.
However, whereas a bonding system is a conducting
link for voltages produced by the build up of static
charges, the voltages to be conducted by a screening
system are those stray ones due to the cou pling of
external fields originating from certain items of
electrical equipment, and circuits when in operation.
Typical examples are: d.c. generators, engine ignition
systems, d.c. motors, time switches and similar
apparatus designed for making and breaking circuits
at a controlled rate,
The methods adopted for screening are generally
of three main types governed principally by the equipmen t or circuit radiating the in terfcrencc fie lds. [n
equipment such as generators, motors and time
switches several capacitors, which provide a low resis·
tance path, are interconnected across the inter.
ference source, i.e. brushes, commutators and contacts, to form a self-contained unit known as a suppressor. The other methods adopted are the enclosing
of equipment and circuits In metal cases and the enclosure of cables in a metal braided sheath, a method
used for screening the cables of ignition systems. The
suppressors and metal screens are connected lo the
main earth or ground system of an aircraft.
As mentioned in the introduction to this chapter,
an organized form of power distribution throughout an aircraft ls essential, and as 811 example of this
we may conclude by considering the sequence illustrated in Figs. 5 .17 to S.2 1. The diagrams, although
based on the B737 power distribution and control
system, are generally represents tive of the standard
approach adopted in other types of public transport
Figure 5.17 shows the routing of the feeder lines
From APU
CSD Generator 2
Fig 5.17
Generator fcci:l~r lines
from the main generators and the APU generator. At
the wing/fuselage junction, the lines pass through
sealed connectors into the underfloor area. All lines
are then routed through an electrical/electronics
compartment (see Fig. 5.18). Those from the main
generators pass through sealed connectors into the
unpressurized nosewheel well to connect up with
the generator breakers, The feeder lines from the
APU generator are connected to its breaker located
above floor level within a special compartment
(designated. P6 panel) to the rear of the captain's
position. This compartment contains most of the
a.c. and d.c. busbars, the bus.tie breakeis, voltage
control and protection units for all three generators,
and an external power control unit. The feeder lines
from the main generator breakers pass into this
compartment to connect with the a.c. busbars.
A circuit breaker panel is mounted
the front side
of the compartment. As a complete unit therefore,
the compartment or P6 panel, establishes what is
tenned the load control centre of the aircraft.
The electrical/electronics compartment serves
as a centralized area for the rack mounting of the
many ''black boxes" associated with autornatic
flight control, compass, radio, and certain other
airframe systems. Removal, installation and main·
tenance checks of these boxes are thereby facilitated
by this arrangement.
In order to establish an organized fonn of system$
control by each member of the flighl crew, and also
of circuit protection, an appropriate number of
control panels are strategically located on the flight
deck as illustrated in Figs. 5.19, 5.20, and 5.21. The
panels are oesignated by the letter "P" prefixing the
panel numbers, and in the example considered they
are as follows :
PS panel
Gonerato r
APU generator breaker
ilusbar protccrion panel
(ext. power control unit)
Exterr;a l
E>(tcrnal d.c.
Access door 10
Fig 5.18 Eleclrical/clcctronics compartment
.... - --- .r.--Pt;-4
Pig 5. 19
Fll!lht deck control panels
Fig S.20
OverhClld control panel
Glare shield panel containing annunciator lights
fo r each pilot
P8 Fire protection system control panel
P9 Weather radar display indicator and radio
communication system selector controls
Circuit breakers are located on panels behlnd
!ach pilot as shown in Fig. 5 .2 1, and since they are
tllied to load control, then they are part of the load
:ontrol centre, as for example, the P6 panel. The
>reakers are grouped appropriate to each system as
ndicated in Fig. 5.21.
>J Captain's flight Instrument panel
Centre engine instrument panel
First Officer's flight instrument panel
Overhead panel from wWch electrical power
generation systems and other major systems
of the aircraft are controlled. Reference to
Fig. 5.20 shows that the panel is subdivided
into sections so that control switches, meters
and indicating lights are grouped appropriate to
their respective systems. For example, section
P5-4 is the primary one for switching the main
generators, APU generator and external power
onto the busbars .
·: . ~
·§ 5 C
·; e
I ,L
I :,,
Receptacles [ 115.y a.c.
for test
25-V d.c ..
bu 6
Gen bus
Gen bus
No . I
PG -11
PG (behind First Officer)
Fig 5.2 1
Circuit breaker pn11els (load control ccn t ros)
P18 (behind Captain!
Circuit Controlling Devices
In aircraft electrical Installations the function of
initiating, and subsequently controlling the operating
sequences of constituent circuits is performed principally by switches and relays, and the construction
and operation of some typical devices form the subject
of lhis chapter. [t may be noted that although circuit
breakers may also come within the above functional
classification, they are essentially circuil protection
devices and, as such, are separately described in the
appropriate chapter.
Switches and relays are constructed in a variety of
f..,, ms, and although not exhau·stive, the details given in
Table 6.1 may be considered a fairly representative
summary of the types and the actuating methods commonly employed.
In its simplest form, a switch consists of two contacting surfaces which can be isolated from each other
Table 6.l
Primary mothod of octuating conlact assemblies
Switching Device
Certain types
lncorpor;itc a
"hold-in" coil;
Mechanical timing
device operated in
tum by an elec1ric
Effects of metal
expansion and also
of eleciiic current.
Trilnsl$tor type
"on~ff''. Used in
the intemal circuits
of units such as
control and
protection ..
Cont actors
Electromagnetic, in
turn controlled by
a circuit incorpont·
ting one or more
manual switches,
switches oi a com·
binalion of these.
or brought together as· required by a movable con·
necting link. This connecting link is referred to as a
pole and when it provides a single path fo r a flow of
current as shown in Fig. 6.l(a), the switch is designated as a single-pole, single-throw switch. The term
throw thus indicates the number of circuits each pole
can complete through the switch. ln many circuits,
various switching combinations are usually required,
and in order to facilitate the make and break operations, the contact assemblies of switches (and certain
relays) may be constructed as integrated units. For
example, the switch at (b) of Fig. 6..1 can control
two circuits in one single make or break operation, and
is therefore known as a double-pole, single-throw
switch, the poles being suitably insulated from each
other, Two further examples are illustrated in diagrams
(c) and (d) and are designated single-pole, doubleth1ow and double-pole, double-throw respectively.
( b) Double•pole I s1ngle-l~row
purpose" switching functions and are used extensive!)
in the various circuits. A typical switch is illustrated h
Fig, 6 ,2.
to99le seal
Toggle switch
In some applications it may be necessary for the
switches in several independent circuits to be actuate,
simultaneously. This is accomplished by ''ganging" n
switches together by means of a bar linking each
toggle as shown in Fig. 6.3(a). A variation of this
method is used in certain types of aircraft for simultaneous action of switch toggles in one direction onl}
( e I $ ,ng le•pole, double•l hrow
( d) Double-pole, double ·lhrow
Fig 6 .J
Switch contact arran1,emcnts
In addition to the number of poles and throws,
switches (toggle types in particular) are also desig•
nated by the number of positions they have. Thus, a
toggle switch which is spring-loaded to one position
and must be held at the second to complete a circuit,
is- called a single-position switch. lf the switch can be
set at either of two positions, e.g. opening the circuit
in one position and completing il in another, it is then
called a two-position switch. A switch which can be
sot at any one of three positions, e.g. a centre "off'
and two "on" positions, is a three-position switch,
also known as a selector switch.
Toggle or tumbler.type switches, as they are sometimes
called, perform what may be regarded as "general-
Fig6 .3
''Cancing'' and locking or switches
(usually to a "system ofr' position). This is_accomplished by a separate gang-bar mounted on the control
panel in such a way that it can be pulled down to bear
against the toggles of the switches to push them in the
required direction. When the bar is released if is returned under the action of a spring.
A further variation is one in which the operation of
a particular switch, or all in a series, may be constrained,
A typical application to a triple generator system is
shown ln Fig. 6.3(b), the switches being used for the
alternative disposition of busbar loads in the event of
failure of any of the tluee generators.
A-. locking bar is free to rotate iri'mounting brackets
anchored by the locking nuts of the No. I and No. 2
switches. The radiused cut-outs, at 90 degrees to each
other, are provided along the length of the bar at
positions coincident with the toggles of each switch, A
steel spring provides for tensioning of the bar at each
selected position, and is inserted around the circumference at the right-hand end. Markin~ I, 2, 3 and
"N'' corresponl to the positions of the cut-outs on the
bar relative to the switch toggles. If, for example, there
is a failure of No. 1 generator the bar is rotated to the
position 1 permitting operation of f~ure switch No. 1,
but constrairling the toggles of the other two switches.
The action for switch operation at positions 2 and 3
is similar. Thus, the busbar loads of a failed generator
can be distributed between remaining serviceable
generators at the same time avoiding inadvertent switch
operation. When the letter ''N'' is evident the bar and
the cut-outs are positioned so that none of the switches
can be operated.
Push-switches are used primarily for operations of
short duration, i.e. when a circuit is to be completed
or interrupted rnornentarlly, or when ari alternative
path is to be rnade available for brief periods . Other
variants are designed to close one or more circuits
(through separate contacts) while opening another
circuit, and in these types, provision may be made for
contact-action in the individual circuits to occur in
sequence instead of simultaneously. ln basic form a
push-switch consists of a button-operated spring·
loaded plunger carrying one or more contact plates
which serve to establish electrical connection between
fixed contact surfaces. Switches may be designed as
independent units for either ''push·to·make" or "pushto-break'' operation, or designed to be double-acting.
For certain wart\lng and indicating purposes, some
types contain miniature famps positioned behind a
small trans.lucent screen in the push-button. When
illuminated, legends such as "on", ''closed" or "fail"
are displayed on the screen and in the appropriate
The construction of a simple type of ''push-tomake" switch and the arrangement of an illuminated
type are shown in Fig. 6.4. ln some circuits, for
example in a turbopropeller engine starting circuit
(see also p. 156), switches are designed to be both
manual and electromagnetic in operation, A typical
example, normally referred to as a "push-in solenoid
Clomping Png
Pus~ button
Contact ptoto
Simple type
Sw,tch octuoling plunger
lllum1notcd lens
Lamp contocts
~ ''~''"""'
V sereen
lllum,no tcd type
Push switc hes
switch", is shown in Fig. 6.5. The components are
contained within a casing comprising an aluminium
housing having an integral mounting flange, a sleeve
and an end cover. The solenoid coil is located at the
flange-end of the housing, and has a plunger passing
through it, One end of the plunger extends beyond
the housing flange and has a knob secured to it, while
the other end terminates in a spring-loaded contact
assembly. A combined terminal and fixed contact
block Is attached to the end of the housing and is held
in place by a knurled end cover nut.
Con toe I assembly
- - Term,nol gnd f1,ed
contocl bloc~
' I
Push-in solenoid switch
When the plunger is depressed and held, the spring·
loaded contact assembly bears again8t the fixed contacts and connects a d.c. supply to.the starter motor.
The commencement of the starling cycle provides a
current flow through the hold.In coil of the switch,
thereby energizing it and obviating the necessity for
further manual control. The switch remains in the
' on" position until the starting cycle is comple ted.
At this stage, the current through the solenoid coil
will have dropped sufficiently to permit the spring to
return the plunger and contacts to the "ofr' position.
Rocker-button switches combine the action of both
toggle and push-button type switches and are utilized
for circuit control of some systems and equlpmen t.
A typical switch is shown in section in Fig. 6.6. For
certain warning and indicating purposes, some types
are provided with a coloured cap or screen displaying
legend information, !lluminated by a miniature lamp.
These are manually operated; and for certain operating
requirements they offer an advantage over toggle
switches in that they are less prone to accidental
operation, Furthermore, the rotary principle and
positive engagement of contacts made possible by the
constructional features make these switches more
adaptable to multi-circuit selection than toggle type
switches. A typical application is the selection of a
single voltmeter to read the voltages at several busbars. In the basic form a rotary switch consists of a
Prs molded
Removable plg5!,c
bullon - - - - - .
bu!ning and;i;;;., --':LJ:l- --,;"i-,5:o'~~
Lock,nq l'lul
H,gh $1rengl h
1,w1!ch1nQ c~ombe,
rtrnt,~rdl1,1re rn•i lanl
Molded ,n
f erm,nal 1n~~r t
plo;t,c ' co5'
Rocker-bu llo n switch
central spindle carrying one or more contact plates or
blades which engage with corresponding fixed con·
tacts mounted on the switch base. The movement is
usually spring-loaded and equipped with some form
of eccentric device to give a snap action and positive
engagement of the contact surfaces.
Micro-switches are a special category of switch and
are one of the most extensively applied electrical
devices in aircraft, performing a wide range of operations to ensure safe control of a variety of systems
and components. The term "micro-switch" designates
.a switching device in which the differential travel
between "make'' and "break" of the operating mech·
anism is of the order of a few thousandths of an inch.
Magnification and snap action of contact mechanism
movements are derived from a pre-tensioned mechanically biased spring. The principle is shown in Fig. 6.7.
Sprong ( long member)
Fixed contoet
lnormolly open)
• , _.Roller
IOC~ ring
Locking 111,,e hole -
au, h1nq k.eywoy -
.,,... Loe kwosher
Tab washer
The long member of the one-piece spring is cantilever
supported and the operating button or plunger bears
against the spring. Two shorter side members are
anchored in such a way that they are bowed in com·
pression . In the inoperative position the contact
mounted on the free end of the spring is held against
the upper fixed contact by the couple resulting from
both tension and compression force . Depre&.sion of the
operating button deflects the long member downwards
thereby causing a reversal of the couple which "snaps"
the spring and contact downward. Upon removal of
the operating force, ·cantilever action restores the
spring and contact system to its initial position with a
snap action.
The method of actuating micro-switches depends
largely on the system to which it is applied but
usually it is either by means of a lever, roller or cam,
these in turn being operated either manually or eiec•
trically. The operating cycle of a micro-switch is
defined in terms of movement of the operating
plunger. This has a specified amount of pre-travel, or
free movement before the switch snaps over. Follow·
ing the operating point, there is some over-travel,
while on the return stroke some differential travel
beyond the operating point is provided before the
release action of the switch talces piece. The contacts
of the switches shown in Fig. 6.7 operate within
sealed evacuated chambers filled with an inert gas,
e.g. nitrogen.
These are controlling devices containing a resistance
the magnitude of which can be Varied, thereby adjusting the current in the circuit in which it is connected.
A typical example of this method of control is the
one adopted.for varying the intensity of Instrument
panel and certain cockpit lighting.
Rheostats normally adjust circuit resistance with·
out opening the circuit, although in some cases, they
are constructed to serve as a combined on-off switch
and variable resistor.
Certain consumer services are required to operate on a
pre-determined controlled time sequence basis and as
this involves the switching on and off of various com,
ponents or sections of circuit, switches automatically
operated by liming mechanisms are necessary. The principle of time switch operation varies, but in general
it is based on the one in which a contact assembly is
actuated by a cam driven at constant speed by either
a speed-controlled electric motor or a spring-driven
escapement mechanism. in some specialized consumer
services, switches which operate on a thermal principle are used. In these the contact assembly is
operated by the distortion of a thermal element when
the latter has been carrying a designed current for a
pre-determined period.
An example of a motor.driven time switch unit is
shown in Fig. 6.8 . lt is designed to actuate relays
which, in tum, control the supply of alternating current to the heating elements of a power unit de-icing
system (see p. 170), Signals to the relays are given in
repeated time cycles which can be of short or long
duration corresponding respectively to "fast" and
"slow" selections made on the appropriate system
control switch.
; wtl~h
Time switch unit
The unit comprises an assembly of five cam and
lever-actuated micro-switches driven by an a.c. motor
Uuough a reduction gearbox.
The motor runs at constant speed and drives the
camshaft at one revolution per 240 seconds. Two of
the cams are of the three-lobed type and they switch
on two micro-switches three times during one revolu·
lion, each "on'' period corresponding to 20 seconds.
Two other cams are of the single-lobed type and they
switch on two associated micro-switches once during
one revolution, the "on" periods in this case corresponding to 60 seconds. Thus the foregoing cam and
micr<rswitch operations correspond respectively to
"fast" and ''slow" selections of power to the heating
clements, which are accordingly heated for short or
long periods. The fifth cam and its micro-switch con·
stltute what is termed a "homing" control circuit, the
purpose of which is to re-set the time switch after use
so that It will always re-commence at the beginning of
an operating cycle.
When the "homing" micro-switch closes, it com·
pletes an external relay circuit whose function is to
continue operation of the motor whenever the de·
icing system is switched off. On completion of the
full revolution of the camshaft, the homing micro-
switch is opened, thereby stopping the motor and
resetting the timer for the next cycle of operation.
Mercury switches are glass tubes Into which stationary
contacts, or electrodes, and a pool of loose mercury
are hermetically sealed. Tilting the tube causes the
mercury to flow in a direction to close or open a gap
between the electrodes to "make" or ''break" the
circuit in which the switch is connected,
The rapidity of "make" and "break'' depends on
the surface tension of the mercury rather than on
externally applied forces. Thus, mercury switches are
applied to systems in which the angular position of a
component must be controlled a narrow band
of operation, and in which the mechanical force
requi.rcd to tilt a switch is very low. A typical appli·
cation is in torque motor circuits of gyro horizons
in which the gyros must be precessed to, and main·
tained in, the vertical position.
Mercury switches are essentially si.ngle,pole, singlethrow devices but, as will be noted from Fig. 6.9, some
variations in switching a.rrangements can be utilized.
in many of the aircraft systems in which pressure
measurement is involved, it is necessary that a warning
be given of either low or high pressures which might
constitute hazardous operating conditions. In some
systems also, the frequency or operation may be such
that the use of a pressure-measuring instrument is not
justified since it is only necessary for some indication
that an operating pressure has been attained for the
period during whic11 the system is in operation. To
meet this requirement, pressure switches are installed
in the relev:1nt systems and are connected to warning
or indicator lights located on the cockpit panels.
A typical switch Is illustrated in Fig. 6.10. It consists of a metal diaphragm bolted between the flanges
of the two sections of lhe switch body. As may be
seen, a chamber is formed on one side of the dia·
pluagm and ls open to the pressure source . On the
other side of the diaphragm a push rod, working
tluough a sealed guide, bears against contacts fitted
in a terminal block connected to the warning or indicator light assembly. The contacts may be arranged
to "make'' on either decreasing or increasing pressure,
and their gap settings may be preadjusted in accord·
ance with the pressures at which warning or indication
is required.
Pressure switches may also be applied to systems
To control
ontrol _ _._.....,.,....""'
.....'O_ __
Mercury switches
Fig6 .10
Typical pressure switch unit
requiring that warning or indication be given of
changes in pressure with respect to a certain datum
pressure; in other words, as adlfforcnliaJ pressure
warning device, The construction and operation are
basically the same as the standard type, with the exception that the diaphragm is subjected to a pressure
on each side.
Thermal switches are applied to systems in which a
visual warning of excessive temperature conditions,
automatic temperature control and automatic operation of protection devices are required. Examples of
such applications are, respectively, overheating of a
generator, control of valves in a thermal de-icing
system and the automatic operation of fire extin·
A principle commonly adopted for thermal switch
operation is bas~d on the effects of differences of
expansion between two metals, usually invar and steel.
In some cases mercury contact switches may be
An example of a differential expansion switch
employed in some cases as a fire detecting device, is
shown in Fig. 6.11. The heat-sensitive element is an
alloy steel barrel containing a spring bow assembly
of low coefficient of expansion. Each limb of the
bow carries a silver-rhodium contact connected by
fire.resistant cable to a tenninal block located within
a steel case.
ln the event of a fire or sufficient rise in temperature at the switch location (a typical temperature is
300°C) the barrel will expand and remove the compressive force from the bow assembly, pennitting the
contacts to close the circuit to its relevant warning
lamp. When the temperature drops, the barrel contracts, thus compressing the bow assembly and re·
opening the contacts.
$pr ,ng •bow
Mounling lug- -- = = = = =
Fig 6.11
Fire detector switch
These switches are used in several types of aircraft as
part of circuits required to give warning of whether or
not passenger entrance doors,.freight doors, etc. are
fully closed and locked. Since they have no moving
parts they offer certain advantages over micro-switche1
which are also applied to such warning circuits.
A typical switch shown in Fig. 6.12 consists of two
Proximity switch
main components, one of which is a11 hermetically·
sealed permanent magnet actuator, and the other a
switch unit comprising two reeds, each having ·
rhodium-plated contacts connected to the warning
circuit. The two·components are mounted in such a
manner that when they contact each other, the field
from the permanent magnet closes the reeds and contacts together, to complete a circuit to the "door
closed" indicator.
Relays are in effect, electromagnetic devices
by means of which one electrical circuit can be in·
directly controlled by a change in the same or another
electrical circuit.
Various types of relay are in use, their construction
operation, power ratings, etc., being governed by their
applica tions, which are also varied and numerous, In
the basic form, however, a relay may be considered as
being made up of two principal elements, one for
sensing the electrical changes and for operating the
relay mechanism, and the other for controlling the
changes. The sensing and operating element is a
solenoid and armature; and the controlling element
is one or more pairs or contacts.
A$ in the case of switches, relays are also liesignated
by their "pole" and "throw" arrangements and these
can range from the simple single-pole, single-throw
type to complex multiple contact assemblies controlling a variety of circuits and operated by the one
1n many applications the solenoid is energized
directly from the aircraft power supply, while in
others it may be energized by signals from an automatic device such as an amplifier in a cabin tempera·
ture-control system, or a fue detector unit·. When the
solenoid coil is energized a magnetic field is set up
and at a pre-determined voltage level (called the
"pull·in" voltage) the armature is attracted to a pole
piece against spring restraint, and actuates the contact
assembly, this in turn either completing or interrupting the circuit being controlled. When the solenoid
coil circuit is interrupted at what is termed the "dropout'' voltage, the spring retums the armature and contact assembly to the inoperative condition,
In addition to the contact assembly designations
mentioned earlier, relays a re also classified by the
order of making and breaking of contacts, whether
normally open ("NO") or normally closed ("NC")
in the de-energized position, rating of the contacts in
amperes and the voltage of the energizing supply. The
design of a relay is metaled by the function it is re·
quired to perform In a particular system or component,
and as a result many types are available, making It
difficult to group them neatly into specific classes.
On a very broad basis, however, grouping is usually
related to the basic form of construction, e.g.
attracted core, attracted-armature; polarized armature,
and "slugged", and the current-carrying ratings of the
controlling element contacts, i.e. whether heavy-duty
or light-duty. The descriptions given in the following
paragraphs arc therefore set out on this basis and the
relays selected arc typical and generally representative
of applications to aircraft systems.
The designation "heavy.duty" refers specifically to
the amount of current to be carried by the contacts.
These relays are therefore applied to circuits involving
the use of heavy-duty motors which may take starting
currents over a range from 100 A to I 500 A, either
short-term, as for starter motors for example, or continuous operation.
A relay of the type used for the control of a
typical turbopropeller engine starter motor circuit is
illustrated in Fig. 6.13 . The contact assembly consists
of a thick contact plate and two suitably insulated
fixed contact studs connected to the main terminals.
Corban oreiMQ contoci
Contact plate
Spindle ond
collar O\&ornbly
Attracted core heavy-duty relay
¢ ,. . -,.,
Fig 6.14
Att1acted armature light-duty relay {sealed)
The contact plate is mounted on a supporting spindle
and this also carries a soft inner core located inside
the solenoid coil. The complete moving component
ls spring-loaded to hold the contact plate from the
fixed contacts and to retain the core at the upper end
of the coil. When the coil is energized the polarities
of the magnetic fields established in the coil and core
are such that the core moves downwards against
spring pressure, until movement is stopped by the .
contact plate bridging across the fixed contacts, thus
completing the main circuit. Carbon contacts are
provided lo absorb the initial heavy current and
thereby reduce arcing to a minimum before positive
connection with the main contacts is made.
A relay designed for use in a 28-volt d.c. circuit and
having a contact rating of 3 A Is shown ln Fig. 6.14.
Flg6, 15
Attracted armature relny (unsealed type)
The contacts are of a silver alloy and are actuated in
the manner shown in the inset wagram, by a pivoted
armature. In accordance with the practice adopted
for many currently used relays, the principal elemen ts
arc enclosed in an hermetically-sealed case tilled with
dry nitrogen and Ute connection in the circuit is made
via a plug-in type base. Fig. 6.15 illustrates another
example of attracted armature relay. This is of the
unsealed type and is connected into the relevant circuit by means of terminal screws in the base of the
In certain specialized applications, the value of con·
trol Circuit currents and voltages may be only a few
milliamps and millivolts, and therefore relays of excep·
tional sensitivity are required. This requirement cannot
always be met by relays which employ spring·
controlled armatures, for although loading may be
decreased to permit operation at a lower ''pull·in"
voltage, effective control of the contacts is decreased
and there is a risk of contact flutter. A practical
solution to this problem resulted in a relay in which
the attraction and repulsion effects of magnetic
forces are subslituted for the conventional spring·
control of the armature and contact assembly. Fig.
6.16 shows, in diagrammatic form, the essential
features and operating principle of such a relay.
The armature is a permanenl magnet and 1s
pivoted between two sets of pole faces formed by a
frame of high permeability material (usually mumetal), lt is lightly biased to one side to bring the
contact assembly into the static condition as in Fig.
6. l 6(a). The centre limb of the frame carries a low·
inductance low-current winding which exerts a small
magnetizing force on the frame when it is ene rgized
from a suitable source of direct current. With lhc
armature in the static condition, the frame pole-faces
acquire, by Induction from the armature, the polarities
shown, and the resulting forces of magnetic attraction
retain the armature firmly in position.
When a d.c. voltage is applied to the coil the frame
becomes, in effect, the core of an electromagnet. The
flux established in the core opposes and exceeds the
flux due to the permanent magnet armature, and the
frame pole-faces acquire the polarilies shown in Fig.
6.16(b). As the armature poles and frame pole-faces
are now of like polarity, the armature is driven to the
position shown in Fig. 6.16(c) by the forces of repulsion. ln position it will be noted that poles and
pole-faces are now of unUke polarity, and strong forces
of attraction hold the armature and contact assembly
in the operating condition. The fluxes derived from
the coil and the armature act in the same direction to
give a flux distribution as shown in .Fig. 6.16(c). When
the coll circuit supply is interrupted, the permanent
magnet flux remains, but the force due to it is weaker
than the armature bias force and so the armature and
contacts are returned to the static condition (Fig.
For some applications requirements arise for the use
of relays which are slow to operate the con tact assembly either at the stage when the armature is being
attracted, or when it is being released.
Some relays are therefore designed to meet these
requirements, and they use a simple principle whereby
the build-up or collapse of the main electromagnet
Ar mo l ur e
· -·
+ d.c.
+ -
+ -
Fig 6.16
Principle of a polarized armature reloy
Auxiliary contacts
con ta cts
,- -
~ -- --- -
Inputs ---
- -- -...,
------i"" -li
flux is slowed down by a second and opposing magnetizing force. This procedure is known as ''slugging"
and a relay to which it is applied is called a "slug"
relay. The relay usually incorporates a ring of copper
or other non-magnetic conducting material (the
"slug") in the magnetic circuit of the relay, in such
a way that changes in the operating flux which is
linked with the slug originate the required opposing
magnetic force. In some slug relays the required result
is obtained by fitting an additional winding over the
relay core and making provision for short-circuiting
the winding, as required, by means of i.ndependenl
contacts provided in Lhc main contact assemblies.
These devices sometimes referred tc> as contaclors, are
commonly used in power generation systems for the
connection of feeder liJles to busbars, and also for
interconnecting or "tying" of busb ars (see also
page 78). The internal arrangement of one such
breaker is shown in Fig. 6.17.
[l consists of main heavy-duty contacts for connecting the a.c. feeder lines, and a number of smaller
auxiliary contacts which carry d.c. for the control
of other breakers, relays, indicating lights as appropriate to the overall system. All contacts are closed
and/or tripped by a d.c .·operated electromagnetic
coil; a pennanent magnet serves to assist the coil in
closing, and also to latch the breaker in the closed
position. The coil Is also assisted in tripping by
means of a spring. Two zoner diodes are connected
across the coil lo suppress arcing of the coil circuit
contacts during closing and tripping.
When say, a main generator switch is placed in its
"on" position, a d.c. "closing" signal will now
through the relaxed contacts ''A" and then through
the coil to ground via relaxed contacts "B". With
the coil energized, the main and other auxiliary
contacts will therefore be closed and the spring will
be. compressed . The changeover of the coil contacts
''A" completes a hoJd.jn circuit to ground, and with
the assistance of the permanent magnet the breaker
remains latched.
A tripping signal resulting from either the generat,
switch being placed to "ofr', or from a fault con.
dition sensed by a protection unit, will flow to
ground in the opposite direction to that when
closing, and via the second set of the "close" con.
tacts. The spring assists the reversed electromagnetic
field of the coil in breaking the permanent magnetic
Breakers of this type are installed with their
opening-closing in the horizontal position,
Circuit Protection Devices
and Systems
In the event of a short circuit, an overload or other
fault condition occurring in the circuit formed by
cables and components of an electrical system, it is
possible for extensive damage and failure to result.
For ex.ample, if the excessive current flow caused by
a short circuit at some section of a cable is left un·
checked, the heat generated in the cable will continue
to increase until something gives way. A portion of
the cable may melt, thereby opening the circuit so
that the only damage done would be to the cable
involved. The probability exists, ho.wever, that much
greater damage would result ; the heat could char and
bum the cable insulation and that of other cables
forming a loom, and so causing more short circuits
and setting the stage for an electrical fire , It is essential therefore to provide devices in the network of
power distribution to systems, and having the common
purpose of protecting their circuits, cables and components. The devices normally employed are fuses,
circuit breakers and current limiters . In addition,
other devices are provided to serve as protection
against such fault conditions as reverse current,
overvoltage, undervoltage, overfrequency, underfrequency, phase unbalance, etc. These devices may
generally bo considered as part of main generating
systems, and those associated with d.c . power genera.
lion, in particular, arc norma..lJy integrated with the
generator control units.
~ fuse is a thermal device designed primarily to pro.ect the cables of a circuit against the, flow of short·
:ifcuit and overload currents. In its basic form, a fuse
:onsists of a low melting point fusible element or link,
mclosed in a glass or ceramic casing which not only
lrote~ts the element, but also localizes any flash which
nay occur when "fusing". The element Is joined to
end caps on the casing, the caps in turn, providing the
connection of the element with the circuit it is
designed to protect. Under short-circuit or overload
current conditions, heating occurs, but before this
can affect the circuit cables or other elements, the
fusible element, which has a much lower current·
carrying capacity, melts and interrupts the circuit, 1l1e
materials most commonly used for the elements are
tin, lead, alloy of tin and bismuth, silver or copper in
either the pure or alloyed state.
The construction and current ratings of fuses vary,
to permit a suitable choice for specific electrical installations and proper protection of individual circuits.
Fuses are, in general, selected on the basis of the
lowest rating consistent with reliable sytem operation,
thermal characteristics of cables, and wl th out resulting
"nuisance tripping". For emergency circuits, i.e., ·
circuits the failure of which may result in the inability
of an aircraft lo tain con trolled fligh l and effect a
safe lancling, fuses are of the hlghest rating possible
consistent with cable protection. For these circuits It
is also necessary that the cable and fuse combination
supplying the power be carefully engineered taking
into account short-term transients in order to ensure
maximum utilization of the vital equipment without
circuit in tcrruption.
Deing therniaJ devices, fuses are also influenced by
ambient temperature variations. These can affect to
some extent the minimum ''blowing" current, as well
as ''blowjng" tim e at higher currents, and so must also
be taken in accoun t. Typical examples of fuses cur·
rently in use in light and heavy-duty circuits, are
shown in Fig. 7.l(a)·(b) respectively, The light-duty
fuse is screwed in to its holder (in some types a bayonet
cap fitting is used) which is secured to the fuse panel
by a fixing nut. The circuit cable is connected to
terminals located in the holder, the temli.nals making
contact with corresponding connections on the
oxide), kiesclguhr, and calcium carbonate (chalk).
When an overload current condition arises and each
element is close to fusing point, the element to go
first immediately transfers its load to the remaining
elements and they, now being well overloaded, fail in
quick succession.
In some transport aircraft, the fuseholders are of
t[le self.indicating type incorporating a lamp and a
resistor, connected in such a way that the lamp lights
when the fusible element ruptures.
L~r: 1'
Fig 7.J
Typical fuses
(a) Light-duty circuit fuse
~b) High-rupturing capacity fuse
element cartridge. A small hole is drilled through the
centre of the cap to permit lhe insertion of a fuse test
Fuses are located accessible for replacement, and as
close to a power dlstribution point as possible so as to
acruevc the minimum of unprotecte'd cable.
The heavy-duty or high rupturing capacity fuse
(Fig. 7, l(b)) is designed for installation al main
power di~tribution points (by means of mounting
lugs and bolts), It consists of a tubular ceramic cartridge within which a number of identical fuse
elements in parallel are conn~cted to end contacts.
Fire-clay cement and metallic end caps effectively
seal the ends of the cartridge, which is com pletely
filled with a packing medium to damp down the
explosive effect of the arc set up on rupture of the
fusible elements. The material used for packing of
the fuse illustrated is granular quartz; other materials
suitable for trus purpose are magnesite (magnesium
Current limiters, as the name suggests, are designed t•
limit the current to some pre-determined amperage
value. They are also thermal devices, but unlike or·
clinary fuses they have a high melting point, so that
their time/current characteristics permit them to carr
a considerable overload current before rupturing. For
this reason their application is confined to the pro·
teclion of heavy-duty power distribution circuits.
A typical current limiter (manufactured under the
name of" Airfuse") is illustrated in Fig. 7.2. It incor·
porates a fusible element which is, in effect, a single
strip of tinned copper, drilled and shaped at each end
to form lug type connections, with the central portio
"waisted" to the required width to form the fusing
area. The central portion is enclosed by a rectangular
ceramic housing, one side of which is furnished with
an Inspection window which, depcnd!ng on the type,
may be of glass or mica .
·,· ]·.,··,·~
.. \ -t'\ ~···
Fig 7.2
Typical cummt limiter ("Airfuse")
These provide another form of protection particular!:
in d.c. circuits in which the initial current surge is
very high, e.g. starter motor and inverter circuits,
circuits containing highly-capacitive loads. When suet
circuits are switched on they impose current surges
of such a magnitude as to lower the voltage of the
complete system for a time period, the length of
which is a function of the time response of the genera·
ting and voltage regulating system. In order therefore
to keep the current surges within limits, the starting
sections of the appropriate circuits incorporate a
resistance element which is automatically connected
in series and then shorted out when the current has
fallen to a safe value.
Figure 7,3 illustrates the applicatjon of a limiting
resistor to a turbine engine starter motor circuit
incorporating a time switch; the initial current flow
may be as high as 1500 A. The resistor is shunted
across the contacts of a shorting relay which is con·
trolled by the time switch. When the starter push
switch is operated, current from the busbar flows
through the coil of the main starting relay, thus
energizing it. Qosing of the relay contacts completes
a circuit to the time switch motor, and also to the
starter motor via the limiting resistor which thus
reduces the peak current and initial starting torque
of the motor. After a pre-determined time interval,
which allows for a build-up of engine motoring
speed, the torque load on the starter motor decreases
and the time switch operates a set of contacts which
complete a circuit to the shorting relay. As will be
clear from Fig. 7.3; with the relay energized the
current from the busbar passes direct to the starter
motor, and the limiting resistor is shorted out. When
ignition takes place and the engine reaches what is
termed "self-sustaining speed", the power supply to
the starter motor circuit is then switched off.
Circuit breakers, unlike fuses or current limiters,
isolate faulted circuits and equipment by means of a
mechanical trip device actuated by the healing of a
bi-metallic element through which the current passes
to a switch unit. We may therefore consider them as
being a combined fuse and switch device. They are
used for the protection of cables and components
and, since they can be reset after clearance of a fault,
they avoid some of the replacement problems associ,
ated with fuses and current limiters. Furthe rmore,
close tolerance trip time characteristics are possible,
because the linkage between the bi-metal element
and trip mechanism may be adjusted by the manufacturer to suit the current ratin~ of the element. The
mechanism is of the "trip-free'' type, i.e. it will not
allow the contacts of the switch unit to be held closed
while a fault current exists in the circuit.
The factors governing the selection of circuit
breaker ratings and locations, are similar to those
already described for fuses.
The design and construction of circuit breakers
varies, but in general they consist of tluee main
assemblies; a bi·metal thermal element, a contact type
switch unit and a mechanical latching mechanism. A
push-pull button is also provided for manual resetting
after thermal tripping has occurred, and for manual
tripping when it is required to switch off the supply
to the circuit of a system. The construction and opera·
tion is illustrated schematically in Fig. 7.4. (At (a)
i>u!.h• pull
bull on
•, Mo,ler sw,ICh
l!lt rnent
·" - --
- - H--41.
I C:..+ T
lo I
Fig 7.4
Schematic diagram of circuit breaker operation
(a) Closed
(b) Tripped condition
Fig 7.3
S1or1er motor
Application of a limiting resistor
the circuit breaker Is shown in its normal operating
position; current passes through the switch unit con·
tacts and the thermal element, which thus carries the
IOOOrr-...-..-,,......-- - - -- - -- - Curve
Arnbient oir temp
+ s1°c
+ 20°c
- 40°c
applled. The ambient temperature under which the
circuit breaker operates also has an influence on circuit breaker operation and this, together with operating
current values and tripping times, is derived from
characteristic curves supplied by the manufacturer. A
set of curves for a typical 6 A ci1cuit breaker is shown
in Fig. 7.5. The current values are expressed as a
percentage of the continuous rating cif the circuit
breaker, and the curves are plotted to cover specified
tolerance bands of current and time for three atnb·
lent temperatures. If, for example, the breaker was
operating at an ambient temperature of +57°C, then
in say 30 seconds it would trip when the load current
reached a value between 140 and 160 per cent of the
normal rating, i.e. between 8,4 and 9,6 A. At an
ambient temperature of +20°C it would trip in 30
seconds at between 160 and 190 per cent of the
normal (ating (between 9,6 and 11 -4 A) while at
-40°C the load current would have to reach a value
between 195 and 21S per cent of the normal rating
(between 11 •7 and 12-9 A) in order to trip in the same
time interval.
After a circuit breaker has tripped, the distorted
I .__.__.__,....._,....._,.._.__.__.__..__..__..__.,_.,_.,__,
Normol roted curren1 (per cent)
Fig 7.S
Characteristic curv~s of a typical circuit breaker tripping times
full current supplied to the load being protected. At
normal current values heat is produced In the thermal
element, but is radiated away fairly quickly, and after
an initial rise the temperature remains constant. If
the current should exceed the normal operating value
due to a short circuit, the temperature of the element
begins lo build up, and since metals comprising the
thermal element have different coefficients of expansion, the element becomes ilistorted as indicated in
Fig. 7.4(b). The distortion eventually becomes suf·
ficlent to release the latch mechanism and allows the
control spring to open the switch unit contacts, thus
isolating the load from the supply, At the same time,
the push·pull button extends and in many types of
cirpuit breaker a white band on the button is exposed
to provide a visual indication of the tripped condition.
n,.,, temperature rise and degree of distortion pro·
duced in the thermal element are proportional to the
value of the current and the time for which it is
Te1m1nol cover
Tr,p bullon cove
( b)
f'ig 7.6
Ciicuit breakers
(Q) Typical
(b) Circuit breaker with a "manual lrlp" button
11 5
element begins to cool down and reverts itself and
the latch mechanism back to normal, and once the
fault which caused tripping has been cleared, the cir•
cult can again be completed by pushing in lhe circuit
breaker button. This "resetting" action closes the
main contacts and re-engages the push.button with
the latch mechanism. If it is required to isolate the
power supply to a circuit due to a suspected fault,
or during testing, a circuit breaker may be used as a
switch simply by pulling out the button. In some
designs a separate button is provided for tltls purpose.
The external appearance of two typical single-pole,
single-throw "trip-free" circuit breakers is illustrated
in Fig. 7.6. The circuit breaker shown at (b) incorporates a separate manual trip push button , A cover
may sometimes be fitted to prevent inadvertent operation of the button.
In three-phase a.c. circuits, triple-pole circuit
breakers are used, and their mechanisms are so arranged
that in the event of a fault current In any one or all ·
three of the phases, all three p·otes will trip simul·
taneously. Similar tripping will take place should an
unbalanced phase condition develop as a result of a
phase becoming "open-circuited". The three trip
mechanisms actuate a common push-pull button.
ln all types of electrical systems the current flow is, of
course, from the power source to the distribution
busbar system and fmally to the power consuming
equipment; the interconnection throughout being
made by such automatic devices as voltage regulators
and control units, and by manually controlled
switches. Under fault conditions, however, it is possible for the current flow to reverse direction, and as
this would be of detriment to a circuit and associated
equipment, it ls therefore necessary to provide some
automatic means l)f protection. In order to illustrate
the fundamental principles we may consider two
commonly used methods, namely reverse current
relays and reverse current circuit breakers.
Reverse Current Cut-Out Relay
A reverse current cut-out relay is used principally in
a d.c. generating system either as a separate unit or
as part of a voltage regulator, e.g. the one described
on p. 18. The circuit arrangement, as applied to the
generating system typical of several types of small
aircraft, is shown in Fig. 7.7. The relay consists of
two coils wound on a core and a spring-controlled
armature and con tact assembly. The shunt winding
is made up of many turns of fine wire connected
across the generator so that voltage is impressed on it at
all limes. The series winding, of a few turns of heavy
wire, is in series with the main supply line and Is
designed to carry the entire line current. The winding
,Is also connected to the contact assembly, which under
&u! bOr
To conwrner
To vollo90 and
Current regulator
~ - - --Seri es (~urrcnt)
Bollery , w,tch
- ··
Ar rn otur!
t~ormQI current flow
Re,cr,c current flow
Fig 7.7
Reverse current cut-out operation
static conditions is held in the open position by means
of a spring.
When the generator starts operating and the voltage
builds up to a value which exceeds that of the battery,
the shunt winding of the relay produces sufficient
magnetism in the core to attract the artnature and so
close the contacts. Thus the relay acts as an automatic switch to connect the generator to the busbar,
and also to Lhe battery so that it is supplied with
charging current. The field produced by the series
Winding ai.ds the shunt-winding field in keeping the
contacts firmly closed.
When the generator is being shut down or, say, a
failure in its output occurs, then the output falls
below the battery voltage and there is a momentary
discharge of current from the battery; in other words,
a condition of reverse current through the cut-out
relay series winding is set up. As this'also causes a
reversal of its magnetic field, the shunt winding.field
will be opposed, thereby reducing core magnetization
until the armature spring opens the, contacts. The
generator is therefore switched to the "off-line"
condition lo protect it from damaging effects which
would otherwise result from ''motoring" current dis·
charging from the battery.
Switched Reverse Current Relay
This relay is adopted in the d.c. generator systems of
some types of small aircraft, its purpose being to
pem1it switching of a generator on to the 'main bus,
bar, and at the same time retain the disconnect
function in the event of reverse current. The circuit
arrangement is shown in Fig. 7.8.
In addition to a current coil the relay has a voltage
coil, and a pair of contacts actuated via a contactor
coil, When the voltage output is at a regulated value\
the current through the voltage coil is sufficient to
actuate Its contacts which then connect the generator
switch and contactor coil to ground. The contactor
coil is thus energized from the A+ output of the
generator and so the auxiliary and main contacts
close to connect the generator output to the battery
and main busbar. The magnetic effect of the current
passing through the current coil assists that of the
voltage coil In keepirtg the pilot contacts closed.
During engine shut-down, the generator output
voltage decreases thereby initiating a reverse current
condition, and because the magnetic effect of the
current through the current coil now opposes that
of the voltage coil, the pilot contacts open to deCurrent
coi~I_ _ _ ,._:_-uo':" T:fu
i Contactor I
co:t: : _ _ _ _ _ _ _ _
Fig 7.8
Swllched reverse current relay
energize the conlactor coil ; thus, the main and
auxiliary contacts are opened to disconnect the
generator from the battery and main busbar.
Reverse Current Circuit Breakers
These circuit breakers are designed to protect power
supply systems and associated circuits against fault
currents of a magnitude greater than those at which
cut•outs normally operate. Furthermore, they are
designed to remain in a "locked-out" condition to
ensure complete isolation of a circuit until a fault has
been cleared.
An example of a circuit breaker designed for use
In a d.c. generating system is shown in Fig. 7.9. It
consists of a magnetic unit, the field strength and
__. Te tffltftOI
Mo1n fe rFI\M,al' -
l0u;~ J
h ,m,nol
{qene,oto, )
Fig 7.9
current circuit breaker
direction of which are controlled by a single-tum coil
connected between the generator positive output and
the busbar via a main contact assembly. An auxiliary
contact assembly is also provided for connection i.n
series with the shunt•field vinding of the generator.
The opening of both con ta, ' assemblies is controlled
by a latching mechanism actuated by the magnet
unit under heavy reverse current conditions. In com·
mon with other circuit breakers, resetting after a
tripping operation has to be done manually, and is
accomplished by a lever which is also actuated by the
latching mechanism. Visual indication of a tripped
condition is provided by a coloured indicator flag
which appears behind a window in the circuit breaker
cover. Manual tripping of the unit is effected by a
push-button adjacent to the resetting lever.
Figure 7.l O is based on the circuit arrangement of a
d.c. generating system used in a particular type of air·
craft, and is an example of the application of a reverse
current circuit breaker in conjunction with a cut-out
relay, Unlike the circuit shown in Fig.?.?, the relay
controls the operation of a line contactor connected
In series with the coil of the reverse current circuit
breaker. Under normal current flow conditions closing
of the relay energizes the line contactor, the heavy•
duty contacts of which connect the generator output
to the busbar via the coil and main contacts of the
normally closed reverse current circuit breaker. The
magnetic field set up by the current flow assists that
of the magnet unit, thus maintaining the breaker contacts in the closed position. The generator shunt field
circuit is supplied via the auxiliary contacts.
When the generator is being shut down, or a failure
of its output occurs, the reverse current resulting
from the drop in output to a value below that of the
battery flows through the circuit as indicated, and the
cut-out relay Is operated lo de-energize the line contactor which takes the generator "off line" . Under
these conditions the reverse current circuit breaker
will remain closed, since the current magnitude is
much lower than at which a specific type of
breaker is normally rated (some typical ranges are
200-250 A and 850-950 A).
Let us consider now what would happen in the
event of either the cut-out relay or the line contactor
failing to open under the above low magnitude
reverse current conditions, e.g. contacts have welded
due to wear and excessive arcing. The reverse current
would feed back to the generator, and in addition to
its motoring effect on the generator, it would also
reverse the generator field polarity. The reverse current
passing through the circuit breaker coil would continue
to increase in trying to overcome mechanical loads
due to the engine and generator coupling, and so the
increasing reverse field reduces the strength of the
magnet unit. When the reverse current reaches the
pre•set trip value of the circuit breaker, the field of
the magnet unit is neutralized and repelled, causing
the latch mechanism to release the main and auxiliary
contacts to completely isolate the generator from the
busbar. The breaker must be reset after the circuit
fault has been cleared.
Overvoltage is a condition which could arise in a
generating system in the event of a fault in the field
excitation circuit, e.g. internal grounding of the field
windings or an open-circuit in the voltage regulator
sensing lines. Devices are therefore necessary to pro-
Main conloc Is
,-- -·-,
c==r ==:::]
Reverse Curren!
circuil breaker
I! I
I, Line
_ _J ,QnloclQf
' 1-----~
Normal current flow
- - - Rtverse currenl flow
Fig 7.10
Reverse current
circuit breaker operation
·tect consumer equipment against voltages higher than
those at which they are normally designed to operate.
The methods adopted vary between aircraft systems
and also on whether they supply d.c. or a.c. An exam·
pie of an ovcrvoltage relay method applied to one
type of d.c. system is shown in Fig. 7 .11.
The relay consists of a number of contacts connected In all essential circuits of the generator system,
and mechanically coupled to a latching mechanism.
This mechanism is electromagnetically controlled by
a sensing coil and armature assembly, the coil being
connected in the generator shunt-field circuit and in
series with a resistor, the resistance of which de·
creases as the current through it is Increased. Under
normal regulated voltage conditions, the sensing coil
circuit resistance is high enough to prevent generator
shunt-field current from releasing the relay latch
mechanism, and so the contacts remain clo.sed and
the generator remains connected to the busbar. lf,
however, an open circuit occurs in the regulator voltage coil sensing line, shunt-field current increases
and, because of the inverse characteristics of the relai
sensing coil resistor, the electromagnetic field set up
by the coil causes the latch mechanism to release all
the relay contacts to th<: open position, thereby
isolating the system from the busbar. After the fault
has been cleared, the con tacts are reset by depressing
the push button.
Figure 7.12 illustrates a method employed in a
frequency-wild a.c. generating system, the full contrc
of which is provided by magnetic amplifiers (sec also
Chapter 2). The output of the overvoltage protection
magnetic amplifier is fed lo a bridge rectifier and to ·
the coil of a relay, via a feedback winding. The main
contacts of the relay are connected in the normal d.c
supply switching circuit to the line contactor.
Under normal voltage output conditions the impedance of the magnetic ampli!lcr is such that its a.c.
~" .... -1
1-....-.1,i,11,1,,,~ Sensing
'-- · - ·
relay ,
Fig 7.11
Qvervoltagc protaction d.c. generating system
output, and the rectified a.c. through the relay coil,
maintain the relay in the de-energized condition. When
an overvoltage condition is produced the current
through the relay coil increases to a pre-determined
energizing value, and the opening of the relay contacts
interrupts the d.c. supply to the line contactor, which
then disconnects the generator from the busbar. At
the same time, the main control unit interrupts the
supply of self-excitation current to the generator,
11.C, ~uDPly
Con trol
,.,no "Q --'~++--- -.r:s---
fle e1,t ,ed
Reio y
To 1one
co ..1uctor
Fig 7.12
OC ~uDDlt
Overvoltage protection n.c ..gcncr~ting system
causing its a.c. output to collapse to zero. The relay
resets itself and after the fault has been cleared the
generator output may be restored and connected to
the busbar by carrying out the normal starting cycle.
An overvoltage protection system adopted In one
example of a constant frequency (non-paralleled)
a.c. generating system is shown in basic form i!l
Fig. 7.13 .
The detector utilizes solid-state circuit elements
which sense all three phases of the generator output,
and. is set to operate at a level greater than
130 ± 3 volts. An overvoltage conclition is an
excitaUOn•type fault probably resu.lting from loss
of sensing to, or control of, the voltage regulator
such that excessive field excitation of a generator
is provided .
The signal resulting from an overvoltage ls supplied
through an inverse time delay to two solid-state
switches. When switch S1 is made it completes a
circuit through the coil of I.he generator control
relay, one contact of which opens to interrupt the
generator excitation field circuit. The other contact
closes and completes a circuit to the generator
breaker trip relay , this in turn, de-energizing the
1 - - - - t t - - - - -- - - - -- - -- - ----<!""':""t>--- - I
GB trip
O.V light relay
Generator control relay
--- --
To gene.rator
d.c. - - ~
Field supply
from voltage
Fig 7.13
Overvoltage protection (constant frequency $ystem)
generator breaker to illsconnect the generator froni
the busbar. The making of solid -state switch S2
energizes the light relay causing it to illuminate the
annunciator light which is a white one in the actual
system on which Fig. 7.13 is based. The purpose of
the inverse time delay is to prevent nuisance tripping
under transient conditions.
Undervoltage occurs in the course of operation when
a generator is being shut down, and the flow of reverse
current from the system to the generator Is a normal
indication of this condition . In a single d.c. generator
system undervoltage protection is not essential since
the reverse current is sensed and checked by the reverse current cut-out. It is, however, essential In a
multi,generator system with an equalizing method of
load-sharing, and since a load-sharing circuit always
acts to raise the voltage of a lagging generator, then
an undervoltage protection circuit Is Integrated with
that o( load-sharing. A typical circuit normally comprises a polarized relay which disconnects the load·
sharing circuit and then allows the reverse current
cut-out to disconnect the generator from the busbar.
In a constant frequency a.c . system, and considering the case of the one referred to on p. 78,
the circuit arrangement for undervoltage protection
is· similar in many respects to that shown in Fig, 7.13,
since it must also trip the generator control relay,
the generator breaker, and must also annunciate
the condition. The voltage level at which the
circuit operate$ is less than 100 ± 3 volts. A time
delay is also included and is set at 7 ;t 2 seconds; its
purpose being to prevent tripping due to transienl
voltages, and also to allow the CSD to slow down
to an underfrequency condition on engine shutdown
and so inhibit tripping of the generator control
When generators are operating in parallel , under·
voltage protection circuits are allied to reactive load.
sharing circuits, an example of which was described
on p. SO.
Over-excitation and under-excitation are conditions
which are closely associated with those of overvoltage and undervoltage , and when generators are
operating in parallel, the conditions are also associ -
ated with reactive current. Protection is therefore
afforded by a mixing circuit. If the reactive current
is the same in the generators paralleled, there will
be no output from the circuit, When an unbalance
occurs, e.g. a genetator is over-excited, voltages
will be produced in both over-excitation and underexcitation sections of the circuit, and these voltages
will be fed to the overvo)tage and undervoltage
circuits. As a result, the overvoltage circuit will be
biased down so that it will trip the generator breaker
at a lower level. The undervoltage circuit will also
be biased down so that it will trip the breaker at a
lower voltage. Since the overall effect of overexcitation ls to raise the busbar voltage then the
overvoltage circuit provides the protective function.
With an under-excited generator, the voltages
fed to the overvoltage and undervollage circuits
cause the biasing to have the opposite effect to
over-excitation. Since under-excitation lowers the
busbar voltage, then the undervoltage circuit
provides the protective function.
Protection against these faults applies only to a.c.
generating systems and Is effected by the real loadsharing circuit of a generating system (see p. 48).
The purpose of a differential current protection
system is to detect a short-drcuited feeder line or
generator busbar which would result in a very high
current demand on a generator, and possibly result
in an electrical fire. Under these conditions, the
difference between the current leaving the generator
and the current arriving at the busbar is called a
differential fault or a feeder fault. In an a.c. system,
current comparisons are made phase for phase, by
two three-phase current transfom1ers, one on the
ground or neutral side of the generator (ground
DPCT) and the other (the load DPCT) on the downstream side of the busbar. Figure 7.14 illustrates
the arrangement and prlnclple of a system as applied
lo a single-phase line.
control relay
--- r
.)••Fault lr
~-!I--I-i)-+-(I-1)--! -_-
--4.....__ _ _....__ I-- I-, - -....
DP detector
in generator
control unit
Fig7.14 •
l) iffcrential current protection
If the current from the generator is I, and the
fault current between the generator and busbar
equals Ir, then the net current at the busbar will
be equal to I - Ir, The fault current will flow
through the aircraft structure and back to the
generator through the ground DPCT. The remainder
of the current J - Ir, will f1ow through the load
DPCT, the loads, the aircraft structure and then
back to tne generator via the ground DPCT. Thus,
the ground DPCT will detect the generator's total
current (I - Ir)+ Or) wruch is equal to I, and the
load DPCT will detect I - Ir,
If the difference in cunent (Le , the fault current)
between the two current transformers on the phase
line is sensed to be greater than the specified limit
(20 or 30 amperes are typical values) a protector
circuit within a generator control unit will trip the
generator control relay.
fir· ·'---------__.J
i~ I
Titis system Is applied to some a.c. generating systems
to provide protection against faults between phases or
between one of the phases and ground. The connectio,
for one phase are shown in Fig. 7.15; those for other
phases (or other feeders in a single-phase system) beini
exactly the same. Two similar current transformers are
connected to the line, one at each end, and their
secondary windings are connected together via two
relay coils. Since the windings are in opposition, and
as Jong as the currents at each end of the line equal
the induced e, are in balance and no current flow1
through the relay coils. When a fault occurs, the fault
current creates an unbalanced condition causing
curre~t. to flow through the coils of the relays thereby
energizmg them so as to open the line at each end.
Fig 7.15
Merz- Price protection syst Grn
Measuring Instruments and
Warning Indication Systems
In order to monitor the operating conditions of the
various supply and utilization systems, it is necessary
for measuring instruments and warning devices, in the
form of indicators and lights, to be included jn the
systems. The number of indicating devices required
and the types employed depend on the type of ai rcraft and the overall nature of its electrical installation .
However, the layout shown in Fig. 8.1 is generally
representative of systems monitoring require ments
and can usefully serve as a basis for study of the appropriate indicating devices.
These instruments are provided in d.c. and a.c. power
generating systems and in most Instances arc of the
permanent magnet moving-coil type shown in basic
form in Fig. 8.2.
An instrument consists essentially of a permanent
magnet with soft-iron pole pieces. between which a
soft-iron core is rnounted. A coil made up of a num·
ber of turns of fine copper wire is wound on an
aluminiurn for mer which in turn is mounted on a
spindle so that it can rotate in the air gap between the
pole pieces and the core. The magnetic field in the
air gap is an intense uniform radial field established
by the cylindrical shape of the pole pieces and core.
Current is led into and out of the coil thro ugh two
haJrsprings which also provide the controlling force.
The hairsprings are so mounted that as the coll rotates,
:me spring Is unwound and the other is wound. A
pointer is attached to the spindle on which the moving
:oil is mounted.
When current flows through the coil a magnetic
leld is set up which interacts with the main field in
.he air gap such that it is strengthened and weakened
is shown in the diagram. A force (Fd) Is exerted on
,ach side of the coil, and the couple so produced
:auses the coil to be rotated until it is balanced by
the opposing controlling force (Fe) of the hairsprings.
Thus, ro tation of the coil and pointer to the equilibrium position is proportional to the current nowing
through the coil at tha t instant. Thi$ proportionality
results in the evenly divided scale which is a characteristic of the moving coil type of indicator. When the
coil former rotates in the main field, eddy currents
are induced in the metal and these react with the main
field producing a force opposing the rota lion, thus
bringing the coil to rest with a minimum of oscillation.
lndicators of this kind are said to be "dead beat".
In order to protect the movements of these instru·
men ts against the effec ts of external magnetic fields
and also to prevent " magnetic leakage", the movements are enclosed In a soft-iron case which acts as a
magnetic screen. The soft-iron has a similar effect to
the core of the indicator, i.e. It draws in lines of force
and concentrates the 11eld within itseif.
Moving coil instruments are also generally employed
for the measurement of voltage and curre nt in an a.c.
system. Additional com ponents are necessary, of
course, for each measuring application; e.g. for the
measurement of voltage, the instrument must also
contairl a bridge rectifier while for the measurement
of current, a shunt 3nd a transformer arc required in
addition to the bridge rectifier.
Reference to Fig. 8.1 shows that all the inslrU·
men ts located on the control panel are of the circularscale type; a presentation which is now adopted in
many current types of aircraft. It has a number of
advantages over the more conventlonaJ arc-type scale;
namely, that the scale length is increased and for a
given measuring range, the graduation of the scale can
be more open, thus helping to improve the observa.
tional accuracy .
ln order to ca ter for this type of presentation, it is,
of course, necessary fo r some changes to be ~ade in
the arrangement of the magnet and movin g coll
Electrical syster11 con trot panel
systems, and one such arrangement is illustrated in
Fig. 8.3.
The magnet ls in the form of a block secured to a
pole piece which is bored out to accommodate a core
which itself is slotted and bored to permit the posi· of the moving coil. The coil former, unlike
that of a conventional instrument, is mounted to one
side of its supporting spindle, and under
conditions it surrounds the core and lies in the ah
gap at the position shown. The field flows from the
magnet to the core which, in reality, forms a North
pole, an~ then across the air gap to the pole piece
forming the South pole. The return path of the field
to the South pole of the magnet is completed tluous
the yoke, which also shields the flux from distortion
by external magnetic fields. When current flows
through the coil, a force is produced due to the· inte1
action between the permanent magnetic field and th,
induced field, but unlike the conventional instrumen
the coil is rotated about the core by a force acting 01
one side only; the opposite side being screened from
the flux by the core itself.
Po,l'\te, ol"ln
b~ ronc ,nq o•
Ze,o ull1u~trr
Pol t
Ccul former
l ield du,
io repulsron
Mogne r f,eld
Basic form of moving coil indicator
Magner ond
magnel space,
Fig 8.3
M~gnet system of a t ypical long-5cale moving coil inst rument
Shunts are used in conjunction with all d.c. system
ammeters, and where specified, in a.c. systems, and
thei r main purpose is to permit an ammeter to measure
a large number of possible values of current, i.e. they
act as range extension devices. Fundamentally, a shunt
is a resistor having a very low value of resistance and
connected external to the ammeter and in parallel
with its moving coll, The materials used for shu'nts are
copper, nichrome, manganin, rninalpha and tclcumen.
Typical shunts used in d.c. and a.c. generating sys,
terns are illustrated in Fig. 8.4 and although their
principal physical features differ, a feature common
to all shunts should be noted and that is they are
Current (mQ1n ) tern,,nols ....__,_
.-~ Potentiol
1erm 1r,0 1~
Potent,a I \ornrnete, )
Fig 8.4
each provided with four terminals. Two of these are
of laJge current.carrying capaci ty (''current" terminals) for connecting the shunt in series with the
main circuit, and two are of smaller size to carry
smaller current (''potential" terminals) when con·
necled to the associated ammeter. The unit sh.own at
(a) employs strips of lacquered minalpha spaced from
each other to promote a good circulation of air and
thus ensure efficient cooling.
When the ammeter Is in series with the main circuit only a fraction of the current passc:. Lhrough the
moving coil, the remainder passing through the shunt
which is selected to carry the appropriate load with,
out overheating. The scale of the ammeter is, however,
calibrated to indicate Uie range of current now in the
main circuit, since the now through the coil and the
shunl are in some pre-calculated ratio.
Transformers are used in conjunction with a.c.
measuring instruments, and they perform a similar
function to shun ts, i.e. they permit a "scaling-down"
of large currents and voltages to a level suitable for
handling by standardized types of ins1rumet1ts. They
fall in to two main classes: (i) current or series
transformers and (ii) potential or parallel transformers. The construction and operation of both
classes has already been dealt with in Chapter 3 and
at this stage therefore we shall only concern ourselves
with typical applications.
Current transformers are normally used with a.c.
ammeters and Fig. 8 .S illustrates a typical circuiL
arrangement. The main current-carrying conductor
passes through the aperture of the secondary windings,
the output of which is supplied to the ammeter via
a bridge rectifier, which may be a separate unit or
form part of the instrument itself.
An application of a potential transform~r is illustrated in Fig. 8 .6 and ii will be noted thal in this case
the transformer forms part of a shunt, the primary
winding being connected 10 the current terminals l
and 4. The voltage developed across the shunt is
stepped-up in the transformer to a ma.xJmum r.m.s.
value (2,5 volts in this particular example) when
the rated current is flowing through the shunt. The
transformer output is connected to the "potential"
terminals 2 and 3 and is rectified within the relevant
ammeter and then applied to the moving coil. The
scale of the ammeter used with this transformer
arrangement is non-linear because the deflection of
the moving coil is not proportional to the current
flowing through the shunt as a result of the sum of
non-Unear characteristics of the transformer and
Figure 8.7 Illustrates a circuit arrangement
adopted for the measurement of d.c. loads in a rectified a.c. power supply system . The ammeter is
utilized in conjunction with a three-phase current
transformer, bridge rectifier and a shunt, which form
r - ,
?----s:" ""-.
,_:. . .L
. .~ j
... -
r------- · -,
Main current .
corryiJ 'conduclor
L__ ._
shunl - - ~
Current transformer
Application of a curtent uansformer
d,C. busbor
Fig 8.7
MeaJurement of d.c. loads In a rectified a.c. system
Mo,n current·
corry,nq conducior
Application of a potential transformer
an integrated unit of the type shown In Fig. 8.8, and
also a main shunt similar to that employed in basic
d.c. generating systems. The ammeter is calibrated in
amperes d.c. and It may be connected into either one
of two circuits by means of a selector switch marked
"D.C." and "A.C.". In the ''D.C." position the
ammeter is selected in parallel with the main shunt
so that It measures the total rectified load taken from
the rnairi d.c. busbar.
When the "A.C." position is selected, the ammeter
is connected to the shunt of the currenl transformer
unit and as will be noted from the circuit diagram, this
unit taps the generator output lines at a point before
the main d.c. output rectifier. The transformer output
is rectified for measuring purposes, so therefore in the
" A.C." position of the switch, the ammeter will
measure the d.c. equivalent of the total unrectificd
Fig 8.8
'fhree•phase current transformer unit
These instruments form part of the ,metering system
required for main a.c. power generating systems, and
in some aircraft, they may also be employed in
secondary a.c. generating systems utilizing inverters.
The dial presentation and circuit diagram of a typical
meter are shown in Fig. 8.9. The indicating
elemcrll, which is used in a mutual inductance circuit,
is of the standard e!ectrodynamometer pallem con·
sisting essentially of a moving coil and a fixed field
coil. The inductor circuit includes a nickel-iron core
loading inductance, a dual fixed capacitor unit, four
current-limiting resistors connected in series-parallel,
and two other parallel-connected resistors which provide for temperature compensation . The electrical
values of a!J the inductor circuit.components are
The instrument also incorporates a circUit which
is used for the initial calibration of the scale. The
circuit is comprised of a resistor. used to govern the
total length of the arc over which the pointer travels
between the minimum and maximum frequencies,
and a variable inductor system which governs the
position of the centre of the arc of pointer travel
relative to the mid-point of the instrument scale.
In operation the potential determined by the
supply voltage and frequency is impressed on the field
coil, which in tum sets up a main magnetic field in
the area occupied by the moving coil. A second poten-
Varia ble
induc tor
3 ·2 kn(ol 37•8°C)
negQhve tempt!roture
2·5 kn
,n lest
co • eft1c,en1
Temperature co_m_p-en-so-..l-or--'
•tf- - - - 1
2-7 kfi
2·7 k!l
2 ·7 k.Cl
Curren1 llmilin9 r0&1SIOr$
Circuit arrangements of a frequency meter
Field coil
tial, whose value is also dependent on the supply
voltage and frequency, is impressed on the moving
coil, via the controlling springs. Thus, a second mag•
netic field is produced which interacts with the main
magnetic field and also produces a torque causing the
moving coil to rotate in the same manner as a conventional moving coil Indicator. Rotation of the coil
continues until the voltage produced in this windlng
by the main field is equal and opposite lo the impressed potential at the given frequency. The total
current in the moving coil and the resulting torque are
therefore reduced lo zero and the coil and pointer
remain stationary at the point on the scale which
corresponds to the frequency impressed on the two
In some a.c. power generating systems it is usual to
provide an indication of the total power generated
and/or the total reactive power. Separate instruments
may be employed; one calibra ted to read directly in
watts and the other calibrated to read in var's ( volt·
amperes reactive) or, as in the case of the instru,
ment illustrated in Fig. 8.10, both functions may be
combined in what is termed a watt/var meter.
The construction and operation of the meter, not
unlike the frequency meter described earlier, is based
on the conventional electrodynamometer pattern and its
scale, which is common to both units of measurement,
is calibrated for use with a current transformer and an
external resistor. A selector switch mounted adjacent
to the meter provides for it to be operated as either
a wattmeter or as a varmeter.
When selected to read in watts the field coil is
supplied from the current transformer which as will
be noted from Fig. 8.10 senses the load conditions
at phase "B" of the supply. The magnetic field produced around the field coil is proportional to the load.
The moving coil is supplied at 115 volts from phase B
to ground and this field is constant under all conditions. The currents in both coils are In phase with
each ot_her and the torque resulting from both mag·
netic fields deflects the moving coil and pointer until
balance between it and controlling spring torque is
In the "var" position of the selector switch the
field coil is again supplied from the current transformer
sensing load cond!lions at phase "B". The moving coll,
however, is now connected aero~ phases "A" and
"C" and in order to obtain the correct coil current, a
calibrated resistor is connected in the circuit and
C"; ,tnt
1 ~,,
:L 1J
I l-
11ov,n9 ,od
F 1tlO COI i
~--- -
Fig 8.10
Circuit arrangements of a wan/VAR meter
mounted external to the instrument. The current in
the moving coil is then at 90 degrees to the field coll
current, and if the generator Is loaded at unity power
factor, then the magnetic fields of both coils bear the
same angular relationship and no torque is produced.
For power factors less than unity there is inter·
action of the coil fields and a torque proportional to
the load current and phase angle error is produced.
Thus, the moving coil and pointer are rotated to a
balanced position at which the reactive power is indi·
Warning and indicator lights are used to alert the
flight crew to conditions affecting the operation of
aircraft systems. The lights may be divided into different categories according to the function they per·
form, and in general, we find that they fall into three
main categories: (i) warning lights, (ii) caution lights
and (iii) indicating or advisory lights.
Warning Lights. These are designed to alert the
flight crew of unsafe conditions and are accordingly
coloured red.
Caution Lights. These are amber in colour to indi·
cate abnormal but not necessarily dangerous condi-
tions requiring caution, e.g. hydraulic system pressure
running low.
/11dicati11g or Advisory Lights. These Hgh ls, which are
either green or blue, are provided to indicate that a
system is operable or has assumed a safe condition,
e.g. a landing gear down and locked.
Warning and indicator light assemblies are, basically,
of simple construction, consisting of a bulb contained
within a casing which incorporates electrical contacts
and terminals for connection into the appropriate cir·
cuit. The coloured lens is contained within a cap
which fits over the casing and bulb . Provision for
testing the bulb to ensure that its filament is intact
is also incorporated in many types of light assemblies.
The lens cap is so mounted on the casing, that it can
be pressed in to connect the bulb directly to the main
power supply. Such an arrangement is referred to as
a .ipress-to.test" facility .
Lights may also include a facility for dimming and
usually this may be done in either of two ways. A
dimming resistor may be included in the light circuit,
or the lens cap may incorporate an iris type diaphragm
which can be opened or closed by rotating the cap.
Lights used for warning purposes do not usually include the dimming facility because of the danger
involved in having a dimmed warning light escaping
The power supplies for wantlng and indicator
lights are derived from the d.c. distribution system
and the choice of busbar for their connection must
be properly selected. For example, if the failure of a
system or a component is caused by the loss of supply
to an auxiliary busbar, then it is obvious that if the
warning light systetn is fed from the same busbar
warning indications will also be lost. To avoid this
risk it is necessary for warning lights to be supplied
from busbars different from those feeding the assoc·
lated service, and preferably on or as close as possible
electrically to the busbar. Cau lion and indicating
lights may also, in some cases, be supplied in a similar
manner, but usually they are supplied from the same
busbar as the associated semce.
1n many types of aircraft system, components require
electrical control; for example, in a fuel system,
electric actuators position valves which permit the
sui,ply of fuel from the main tanks to the engines and
also for cross-feeding the fuel-supply . All such devices
are, in the majority of cases, controlled by switches
on the appropriate systems panel, and to confirm the
completion of movement of the device an in(Ucating
system is necessary.
The indicating system can either be in the form of
a scale and pointer type of instrument, or an indicator
light , but both methods can have certain disadvantages.
The use of an instrument is rather space-consuming
particularly where a number of actuating devices are
involved, and unless it is essential for a pilot or sys.
terns engineer to know exactly the position of a device
at any one time, instruments are uneconomical. lndi·
ca tor lights are of course simpler, cheaper and consume
less power, but the liability of ti1eir filaments lo
failure without warning contributes a hazard particularly in the case where "light out'' is intended to
indicate a "safe" condition of a system. Furthermore,
in systems requiring a series of constant indications of
prevailing conditions, constantly illuminated lamps
can lead to confusion and misinterpretation on the
part of the pilot or systems engineer.
Therefore to enhance the reliability of indication,
indicators containing small electromagnets operating
a shutter or similar moving element are installed
on the systems panels of many present-day aircraft.
In its simplest form,(see Fig. 8.1 l(a)) a magnetic
indicator is of the two-position type comprising
a ball pivoted on its axis and spring returned to the
"ofr' position. A ferrous armature embedded in the
ball is attracted by the electromagnet when energized, and rotates the ball through 150 degrees to
present a different picture in the window. The picture
can either be of the line diagram type, or of the
instructive type.
figure 8.1l(b) shows a development of the basic
indicator, it incorporates a second electromagnet
which provides for three alternative indicating positions. The ferrous armature is pivoted centrally above
the two magnets and can be attracted by either of
them. Under the influence of magnetic attraction the
arrnature tilts and its actuating arm will slide the
rack horizontally to rotate the pinions fixed lo the
ends of prisms. The prisms will then be rotated
th.rough 120 degrees lo present a new pattern in the
window. When the rack moves from the centre "rest''
position, one arm of the hairpin type centring spring,
located In a slot in the rack, will be loaded . Thus, if
the .electromagnet is de-energi~ed, the spring wUl
return to mid-position rotating tho pinions and prisms
back lo the "ofr' condition in the window.
The pictorial presentations offered by these indi·
cators is further improved by the painting of "flow
lines" on the appropriate panels so that they inter·
connect the indicators with the system control
switches, essential indicators and warning lights. A
typical application of" flow lines" is shown in
Fig. 8.1.
C.011lro1 u1rinq
Coble- 9ro mme-l
Fig 8.J I
Masnstic indicators
ln the development of large types of aircraft and
their associated systems, it became apparent that the
use of warning and indicator lights in increasing numbers, and widely dispersed throughout flight compart·
men ts, would present a problem and that a new
approach would be necessary. As a
referred to as "central warning systems" were
ln its basic form, a system comprises a centralized
group of warning and indicator lights connected to
signal circuits actuated by the appropriate systems
of the aircraft , each light displaying a legend denoting
the system , and a malfunction or advisory message,
All the lights are contained on an annunciator panel
installed within a pilot's visual range.
An example of a system containing master warning and caution annunciator lights is shown in
Fig. 8.12. The lights are centrally grouped according to systems, on a glare shield panel directly in
front of the pilots and over their main instrument
panel~. The lights are also interconnected with
systems indicating lights on an overhead control
When a fault occurs in one of the systems, the
overhead panel light for that system will illuminate,
but as this may not always be readily observed by
the pilots, their attention will be drawn to the fault
situation by the simultaneous illumination of the
annunciator li~ht for the system, and of the master
caution light. The lights are illuminated via a "fault
pulser" and SCR circuit arrangement. Identification
of the faulted system is cross-checked by observation
of its con LroJ section of.the overhead panel, and
once this has been made, it is unnecessary for the
master caution and annunciator lights lo remain
illuminated, They can therefore be extinguished
by pressing the cap of either master caution light.
If there is a need to recall the faulted system on an
annunciator panel this can be accomplished by
pressing the cap of the corresponding annunciator
light. rf the fault is nol corrected a ·• recall pulser"
circuit will relrigger tl1e SCR and so illuminate the
system annunciator lighi.
Push to
Push to
First Officer
Fig 8,12
Master warning and caution lights
System lights
[n aircraft carrying a flight engineer, a panel is
also installed at his station and is functionally inte.
grated with the pilot's panel. A flight engineer's panel
is illustrated in Fig. 8.13 and may be taken as an
example of central warning displays. In this case,
the panel is made up of a number of blue lights
which are advisory of nonnal operating conditions,
a number of amber lights, a red "master warning"
light and an amber "master caution" light.
When a fault occurs in a system, a fault-sensing
device transmits a signal which illuminates the.appro·
priate amber light. The signal is also transmHted to an
cilectronlc device known as a logic c-0n troller 1 the
function of which Is to determine whether the fault
is of a hazardous nature or is one requiring caution.
If the fault is haiardous, then the controller output
signal illuminates the red "master warning" light;
if caution is required, then the signal will illuminate
only the amber "master caution" light.
Each master light incorporates a switch unit so
that when the caps are pressed in, the active signal
circuits are disconnected to extinguish the lights and,
at the same time, they are reset lo accept signals from
fa"lts which might subsequently occur in any other
of the systems in the aircraft. The system lights are
not of the resetting type and remain illuminated until
the system fault is corrected. Dimming of lights and
testing of bulb ftlaments is carried out by means of
switches mounted adjacent to the annunciator panel.
1 • nut
I• 1(0
c:::J 1
,;,aac &1!".... IICi\1011,.CNT
Fig 8.!3
Csntralized warnln!l system annunciator panel
An electronic display system is one in which the
data necessary for the in-flight operation of aircraft
and systems, and also for their maintenance, is
processed by high-storage capacity computers and
then presented on the "screens" of colour cathode
ray tube display units In alphanumeric and symbolic
fonn. Advisory messages relating to faults In any
one of the systems can also be displayed In this
manner. Systems of this type are now in use in
various types of aircraft, both military and civil,
and in the latter category we may note as examples,
the Airbus A310, Boeing 757 and 767.
The data for computer processing and for display,
originates as signals (analogue and/or digital)
generated by sensors :wociated with each Individual
major system of an aircraft, and as is conventional,
data presentation falls into two broad areas: (j) flight
path and navigational, and (il) engine and airframe
systems operation. Electronic display systems are
therefore designed appropriate to each of these
areas and are known respectively as an Electronic
Flight Instrument System (tFlS) and either an
Electronic Centralized Aircraft Monitor (ECAM)
system or an Engine Indicating and Crew Alerting
System (EICAS).
L h ~MP'•r u ,•111
Fig 8,14
Schematic functional diagram - ECAM system
0 ti4bVI
As far as electrical systems are concerned, the
operational handled by either an
ECAM system or EICAS, and is confined to
electrical power generation. The operation of both
systems is understandably of a complex nature and
space precludes detailed descriptions of them . We
may however, gain some idea of their function ,
particularly in the role of fault annunciation by
briefly referring to the ECAM system. For readers
who may be interested in more details, reference
may be made to "Microelectronics in Aircraft
Systems" which is a companion volume to this.
A schematic functional diagram of the ECAM
system (as used in the Airbus A3 l 0) Is shown in
Fig. 8.14 . The CRT display units are mounted side
by side so that the left-hand unit is dedicated to
information in message form on systems' status,
warnings and corrective action required, whlle the
right-hand unit ls dedicated to associated information in diagrammatic form . There are four modes
of dis play, three of which are automatically selected
and referred to as: flight phase-related, advisory and
failure-related. The fourth mode is manual and
permits the selection of diagrams related to any of
the aircraft's systems, for routine checking and
also the selection of status messages. The selections
are made on the ECAM control panel.
In the context of this chapter, the failure-related
mode is appropriate and an example of a djsplay
presentation ls shown in Fig. 8.15. In this case, there
is a problem associated with the number one
generator. The left-hand display unit shows the
affected system in message fom1, and ill rod or
amber depending on the degree of urgency , and
also the corrective action required in blue. At the
same time, a diagram is displayed on the right-hand
cllsplay unit. When the number one generator has
been switched off, tho light in the relevant pushbutton switch on the ntght deck overhead panel is
illuminated, and simultaneously, the blue Instruction
on the left-hand display unit changes to white. The
diagram on the right-hand display unit is also "redrawn'' to depict by means of an amber line that
the number one generator is no longer available,
and that number two generator is supplying the
busbar system. This is displayed in green which is
the normal operating colour of the displays. After
corrective action has been taken , the message on the
left-hand djsplay unit can be removed by operating
a "clear" button switch on the ECAM control panel.
GEH1 FAULT. . .... .. ... . .. ... ... . ... ...OFF
£>,4£R 81JS
115 V
100 't ovr~
Ml !OO't
R.H. Display Unit
L.H. Display Unit
Display when warning detected
Fig 8.15
Display in failure•relat~d mode
£55 BVS
OVRO . - - ~ ~ - --,
IOG 11
Display when corrective action taken
Fig 8.16
CRT djjplay unit locations (Airbus A310)
~igure 8 .16 shows the location of display units
>n the Instrument panels of the Airbus A310. The
mits of the ECAM system are those occupying the
:entral position, while EFIS units are on the left and
ighl panels. There is also a third system which
utilizes the electronic display concept, and that is
known as a Flight Management System (FMS) the
units of which are on the pedestal between the two
Power UtilizationMotors
Our study of electrical systems thus far, has been
concerned primarily with the fundamental principles
of the methods by which power is produced and
distributed, and also of the circuit protection methods
generally adopted. This study, how~ever, cannot be
concluded without learning something of the various
ways in which the power is utilized within aircraft.
Utilization can extend over very wide areas depending
as it does on the size and type of aircraft, and whether
systems are employed which require fol.I or only partial
use of electrical power; therefore, ln keeping with the
theme of the book, we sh.all only concern ourselves
with some typical aspects and applications.
For the purpose of explanation, the subject is
treated in tllis Chapter and in Chapter 10 respectively
under two broad headings : (i) motors used in conjunction with mechanical systems, e.g. a motor,
driven fuel valve ; and (11) system.I' which are principally all-electric, e.g. an engine starting and ignition
A wide variety of components and systems depend
upon mechanical energy furnished by motors and the
numbers installed in any one type of aircraft depend
on the extent to which electrical power is in fact
utilized, A summary of some typical applications of
motors is given. in Table 9 .l.
ln most of the above applications the motors and
mechanical sections of the equipment form integrated units. The power supply required for operation
is 28 volts d.c. and/or 2~volts or 11 S·volts constant
frequency a.c. and is applied almost without exception,
by direct switching and without any special starting
equipment. Many motors are required to operate only
for a short time during a flight, and ratings between
15 and 90 seconds are common. After operation at
the rated load, a cooling period of as long as IO to
20 minutes may be necessary ln some cases, e.g. a
propeller feathering pump motor.
Table 9.1
Fuel "trimming"; Cargo door
operation; Heat Cl\changer conuol
!lap op1m1tion; Landing flap
Control Va.Ives
Hot and cold air mixing for airconditioning and thermal de-icing;
Fuel delivery; Propeller featheringi
De-icing fluid delivery;
Fuel shut-off.
Flight lnsquments
Hydraulic fluid ,
Gyroscope oper3lion; Servo
and Contiol Systems
Continuously-rated motors are often fan cooled
and, in the case of fuel booster pumps which are of
the immersed type, heat is transferred from the seale
motor casing to the fuel. Operating speeds are high
and in cases where the energy from motors must be
converted into mechanical movements, reduction
gear-boxes are used as the transmission system.
O.C. Motors
The function and operating_principle of d.c. motots
is the reverSe of generators, I.e . if an external supply
is connected to the terminals it will produce motion
of the armature thereby converting electrical energy
into mechanical energy. This may be seen from Fig.
9.1 whlch represents a motor in its simplest fom,,
i.e. a single loop of wire '' AB" ananged to rotate
between the pole pieces of a magnet. The ends of the
wire are connected to commutator segments which
are contacted by brushes supplied with d.c. With
D.C. motor principle
current flowing in the loop in the direction shown,
magnetic fields are produced around the wire which
interact with the main field and produce forces causing
the loop to move in a clockwise direction. When the
loop reaches a position at which the commutator
reverses the polarity of the supply to the loop, the
cfuection of current Oow is also reversed, but due to
the relative positions of the field around the wire and
of the main field at that instant, the forces produced
cause the loop to continue moving in a clockwise
direction. This action continues so long as the power
is supplied to the loop.
A.. far as construction fundamentals are concerned,
there is little difference between d.c. generators and
motors; they both consist of the same essential parts,
i.e. armature, field windings, commutator and brush·
gear, the same methods of classifying according to
various field excitation arrangements, and in the
majority of motors the armature and field windings
are supplied from a common power source, in other
wprds they are self.excited.
11me are three basic types of motors and as in the
case of generators they are classified according to
field excitation arrangements; senes,wound, shuntwound and compound-wound. These arrangements
and certain other variations are adopted for a number
of the functions listed in Table 9.1 and are illus.
trated in Fig. 9 .2.
The application of a motor to a particular function
is governed by two main characteristics; the speed
characteristic and the torque characteristic. The
former refers to the variation of speed with armature
current which is determined by the back e.m.f. , this,
in its tum, being governed by the mechanical load on
the motor. The torque characteristic is the relationship between the torque required to drive a given load
and the armatur~ current.
In series·Y.JOLmd motors, the field windings and the
armature windings are connected in series with each
other and the power supply. The currents flowing
through both windings and the magnetic fields pro.
duced are therefore the same, The windings are of
low resistance, and so a series motor is able to draw a
large current when starting thereby eliminating
building up the field strength quickly and giving the
motor its principal advantages: high starting torque
and good acceleration, with a rapid build-up of back
Fig 9.2
Types of d .c. motor
e.m.f. induced in the armature to limit the current
now through the motor.
The speed characteristic of a series wound motor
Is such that variations in mechanical load are accompanied by substantial speed variations; a light load
causing it to run at high speed and a heavy load
causing it to run at low speed.
The torque is proportional to the square of the
ar mature current, and as an increase in load results in
a reduction of the back e.m.f., then there Is an increase
in armature current and a rapid increase In drivmg
torque. Thus the torque characteristic is such that a
motor can be started on full load .
In shunt-wound motors the field windings are
arranged in the same manner as those of generat ors
of this type, i.e. in parallel with the armature. The
resistance of the winding Is high and since it is connected directly across the power supply, the current
through it is constant. The armature windings of some
motors are of relatively high resistance and although
their overall efficiency is low compared to the
majority of shunt motors, they can be started by con·
necting them directly to the supply source. For the
starting of motors having low-resistance armature
windings it is necessary for a variable resistance to be
connected in series with the armature. At the start
full resistance would be in circuit to limit the armature
current to some predetermined valul?. As the speed
builds up the armature current is reduced by the in·
crease in back e.m.f. and then the resistance is progressively reduced until, at normal speed of the motor,
all resistance is out of the armature circuit .
In operating from a "no-load" to a ''full-load"
condition the variation in speed of a mo tor with a
low-resistance armature is small and the motor can
be considered as having a constant-speed characteristic. In the case of a motor with a high-resistance
armature there is a more noticeable difference in
speed when operating over the above load conditions,
The torque is proportional to the arma ture current
until approaching full-load condition when the increase in armature reaction due to full-load current
has a weakening effect. Starting torque Is small since
the field strength is slow to build up; thus, the torque
characteristic is such that shunt-wound motors must
be started on light or no load.
For many applications it is necessary to utilize the
principal characteristics of both series and shunt
motors but without the effects of some of their
normally undesi ra ble fea tures of operation. For
example, a motor may be required to develop the
high starting torque of a series type but without the
ten dency to race when load is removed. Other appli·
cations may require a motor capable of reducing
speed with increased load to an extent sufficient
to prevent excessive power demand on the supply,
while still retaining the smooth speed control and
reliable "off.load" ru nning characteristic of the
shunt motor. These and other requirements can be
met by what is termed compounding, or in other
words, by combining both series and shwlt field
windings in the one machine . In most compound·
wound motors the series and shunt windings are
wound to give the same polarity on the pole faces so
that the fields produced by each winding assist each
·other. This method of connection is known as cumu·
lativc compounding and there are three forms which
may be used; normal, stabilized shunt and shunt
ln normal compounding a motor is biased towards
the shunt·wound type, the shunt winding producing
about 60 to 70 per cent of the total flux, the series
winding producing the remainder. The desired charac·
teristlcs both series and shunt-wound motors are
In the stabilized shunt form of compounding a
motor is also biased towards the shunt-wound type
but has a relatively minor series winding. The purpose
of this windin g is to overcome the tendency of a
shunt motor to become unstable when running at or
near its highest speed and then subjected to an in·
crease in load.
The sllunt.fimited motor is biased towards the
series.wound type and has a minor shunt field winding
incorporated in the field system, The purpose of the
winding is to limit the maximum speed when running
under "off-load" conditions while leaving the torque
and general speed characteristics unaltered. Shunt
limiting is applied only to the larger sizes of compound
motors, typical examples being engine starter motors
(see Fig. 9,3). The speed/load characteristics of
series, shunt and compound motors are shown in
Fig. 9.4.
ln a number of applications involving motors it is
required that the direction of motor rotation 'be
reversed in order to perform a particular function,
e.g. the opening and closing of a valve by an actuator.
This is done by reversing the direction of current flow
Tprm,nol posts
Boll beor1ng
0,1 seol
F,eld coils
Brush gear
Fig 9.3
Typical starter motor
circuit is shown in Fig. 9.5. When the switch is
placed in the "Forward" position then current will
flow in section "A" of the field winding and wiU
establish a field in the iron core of appropriate polar·
ity. Current also flows through the armature winding,
the interaction of its field with that established by
field winding section "A" the armature to
Pig 9.5
Split field motor circuit
' ' ' .... ....
- - - Shunt
• Compound
- - Series
00:---'-- ~ ~- -- ------ - -~_.._~1~00%
Output power or load
Fig 9.4
D.C. motor char~cteri5tics
and magnetic field polarity, in either the field win din~
or the armature.
A method based on this principle, and one most
commonly adopted in series-wound motors, is that in
which the field winding is split into two electrically
separate sections thereby establishing magnetic fields
flowing in opposite directions. One of the two
windings is used for each direction of rotation and is
controlled by a single-pole double-th.row switch. The
rotate in the for ward direction. When "Reverse" is
selected on the control switch, section "A" is isolated
and current flows th.rough section "B" of the field
winding in the opposite direction. The current flow
through the armature is in the same direction as before1 but as the polarities of the iron.~ore pole pieces
are now reversed then the resultant interaction of
fields causes the armature to run in the reverse direction. Some split-field series motors are designed with
two separate field windings on alternate poles. The
armature in such a motor, a 4-pole reversible motor,
rotates in one direction when current flows through
the windings of one set of opposite pole. pieces, and
in the reverse direction when current flows through
the other set of windings.
The reversing of motors by interchanging the
armature connections is also employed in certain
applications, notably when the operating characteris·
tics of compound machines are required. The circuit
diagram illustrated in Fig. 9 .6 is based on the :
arrangement adopted in a compound motor designed
for the lowering and raising of an aircraft's landing
flaps (see Fig. 9 .7). Current flows to the armature
winding via the contacts of a relay, since the current
demands of the motor are fairly high.
Motor Actuators
Motor actuators are self-contained units combining
electrical and mechanical devices capable of exerting
reversible Linear thrust over short distances, or rever·
slble low-speed turning effort. Actuators are thereby
classified as either linear or rotary and may be
powered by either d.c. or a.c. motors. In the majority
of cases d.c. motors are of'the split-field series-wound
Curre11f flow•e 1ockw1S~
.__.. Cur n::ml I h)w· onllclotkW•~e
Fig_ 9.6
Reversing of a compound motor
Becr111Q hou~•9
Bro ke dr~m o;,~e mbly
Clulch ~ ft
ln lt: rm11dtolt
housmq ossern1llr
Fig 9. 7
Reversible compound motor
Linear actuators may vary in certain of their design
and constructional features dependent upon the
application, load requirements and the manufacturer
responsible. In general, however, they consist of the
motor which is coupled through reduction gearing to
a lead screw which on being rotated extends or retracts a ram or plunger. Depending on the size of
actuato r, extension and retraction Is achieved either
by the action of a conventional screw thread or by
what may be termed a ''ball bearing thread". In the
former case, the lead screw is threaded along its
length with a square•form thread which mates with a
corresponding thread in the hollow ram. With the
motor in operation the rotary motion of the lead
screw is thereby converted into Linear motion of the
ram, whlch is linked to the appropriate movable
The ball bearing method provides a more efficient
thread and is usually adopted in large actuators de·
signed for operation against heavy loads. In this case,
the conventional male and female threads are re·
placed by two semi-circular helical grooves, and the
space between the grooves is filled with steel balls.
As the lead screw rotates, the balls exert thrust on the
ram, extending or retracting it as appropriate, and at
the same time, a recirculating device ensures that the
balls are fed con tinuously into the grooves.
A typical linear actuator is shown in Fig. 9 .8.
Rotary actuators are usually utilized in components
the mechanical elements of which are required to be
rotated at low speed or through lirn.lted angular travel.
As in the case of linear actuators the drive from the
motor is transmitted through reduction gearing, the
output shaft of which is coupled directly to the relevant movable component, e.g. valve flap. Some
typical examples of the application of rotary actuaton
are air,conditioning system spill valves and fuel cocks.
Linear actuator
The reduction gearing generally takes the form of
multi-stage spur gear trains for small types of linear
and rotary actuator~. while in the larger types it is
more usual for epicyclic gearing to be employed. The
gear ratios vary between types of actuator and specific
Both linear and rotary type actuators are equipped
with limit switches to stop their respective motors
when the operating ram or output shaft, as appropriate, has reached the permissible limit of travel. The
switches are of the micro type (seep. 104) and are
usually operated by a cam driven by a shaft from the
actuator In some cases, limit switch contacts are also utilized to complete circuits to indicator
lights or magnetic indicators. The interconnection of
the switches-is shown in Fig. 9 .9, which is based on
the circuit of a typical actuator-controlled valve
In the valve closed position, the cam operates the
micro switch "A" so that it interrupts the "close''
winding circuit of the motor and completes a circuit
to the "closed" indicator. The contacts of the micro
switch "B" are at that moment connected to the
"open" winding of the motor so that when the control switch is selected, power is supplied to the winding.
In running to the valve open position the cam causes
micro switch "A" contacts to change over, thereby
interrupting the indicator circuit and connecting the
''close" winding so that the motor is always ready for
operation in either dlrectlon. As soon as the "open"
L1m1f , witch
Mo()'ll!tic indicator;
t----<1-- . 0 Of I
0---------------·x .,
Limit switch
Fig 9.9
Limit swit ch operation
position is reached the cam operates micro switch "B",
the contacts of which then complete a circuit to the
"open'' indicator.
The majority of actuators are fitted with electro·
magnetic brakes to prevent over-travel when the motor
is switched off. The design of brake system varies with
the type and size of the actuator, but in all cases the
brakes are spring-loaded to the "on" condition when
the motor is de-energized, and the operating solenoids
are connected in series with the armature so that the
brakes are withdrawn immediately power is applied.
Friction clutches, which are usually of the single-plate
type or multi·plate type dependent on size of actuator,
are incorporated in the transmissiorl systems of actuators to protect them against the effects of mechanical
D.C. motors are not widely used ln aircraft Instruments, and in present-day systems they are usually confined to one o, two types of tum-and-bank indicator
to form the gyroscopic element . The motor armature
together with a concentrically mounted outer rim
forms the gyroscope rotor, the purpose of the rim
being to increase the rotor mass and radius of gyration.
The armature rotates inside a cylindrical two-pole
permanent magnet stator secured to the gimbal ring.
Current is fed to the brushes and commutator via
flexible springs to permit gimbal ring movement. An
essential requirement for operation of the instrument
is that a constant rotor speed be main tamed . This is
achieved by a centrifugal cut-out type governor consisting of a fixed contact and a movable contact,
normally held closed by an adjusting spring, and in
series with the armature winding. A resistor is connected In parallel with the contacts.
When the maximum speed is attained, centrifugal
force acting on the movable contact overcomes the
spring restraint, causing the contacts to open. Current
to the armature therefore passes th.rough the resistor
and so reduces rotor speed until it again reaches the
nominal value.
A.C. Motors
In aircraft employing constant-frequency alternating
current either as the primary or secondary source of
electrical power, it is of course logical to utilize a.c.
motors, and although they do not always serve as a
complete substitute for d.c. machines, the advantage:
and special operating characteristics of certain types
are applied to a number of systems which rely upon
mechanical energy from an electromotive power
The a.c. motor most commonly used is the induc·
lion type, and dependent upon the application may
be designed for operation from a three-phase, twophase or single-phase power supply.
An induction motor derives Its name from the fact
that current produced in the rotating member, or
rotor, is due to induced e.m.f created by a rotating
magnetic field set up by a.c. flowing in the windings
of the stationary member or stator. Thus, inter·
connection between the two members is solely magnetic and as a result there is no necessity for a comm
tator, slip rings and brushes.
The rotor consists of a cylindrical laminated-iron
core having a number of longitudinal bars of copper
or aluminium evenly spaced around the circumferenc
These bars are joined at either end by copper or
aluminium rings to form a composite structure com·
monly called a "squirrel-cage" . The stator consists of
a number of ring-shaped laminations having slots
formed on the inner surface and into which seriesconnected coil windings are placed. The number of
windings and their disposition within the stator is
directly related to the number of poles and phases of
the power supply, e.g. more windings are required in
a 4-polc motor than in a 2-pole motor both of which
are to be operated from a 3-phase supply.
The operating principle may be undersJood from
Fig. 9.10, which represents a 2-pole 3,phase motor
arrangement. Assuming that the relationship betweer
phases (phase rotation) is as indicated, then at the
instant 0°, phases "A" and "C" are the only two
carrying current and they set up magnetic fields whic
combine to form a resultant field acting downward
through the rotor core. The field thus passes the bars
of the squirrel-cage, and since they form a closed cir,
cull of low resistance the e.m.f. induced Ln the bars
sets up a relatively large current flow in the directlbn
indicated. As a result of the current flow magnetic
fields are produced around the bars. each field inter·
acting with the main field to produce torques on the
rotor. This action is, in fact, the same as that which
takes place in a d.c. motor and also a moving coil
stator is wound, and the frequency of the power
SU pply,
Synchronous Speed _,
f (Hz) x 60
No, of pairs of poles
The difference bet ween the synchronous and rotor
speeds, measured in rev/min., is called the slip speed
and the ratio of this speed to synchronous speed,
expressed as a percentage, is called quite simply the
A:, the name indicates these motors have only one
At instoM o•
At 60°
stator winding, and as this alone cannot produce a
rotating field to tum the rotor then some other
Phose "ti.'
Phose ·a·
Phos! ··c "
method of self-starting is necessary. The me thod most
'' ,/
commonly adopted Is the one in which the main
., .,
winding of the stator is split to produce a second
starting winding. Thus we obtain what is usually
called a split-phase motor, and by displacing the
windings mutually at 90 electrical degrees, and arranging
that the current In the starting winding either
Fig 9,10
leads or lags on the winding, a rotating field can
Induction motor principle
be produced in the manner of a two-phase motor.
After a motor has attained a certain percentage of its
rated speed, the starting winding may be switched out
Assuming now that the power supply frequency
of the circuit; it then continues to run as a single·phase
has advanced through 60 degrees, then phase "A"
curreo t falls to zero, phases "B" and "C" are the two
now carrying current and so the resultant field proThe lagging or leading of cunents in the windings
duced also advances through 60 degrees. In other
is obtained by arranging that the ratio of inductive
reactance to resistance of one winding differs con·
words, the field starts rotating in synchronism with
the frequency and establishes torques on the rotor
siderably from that of the other winding. The variasquirrel-cage bars, thereby turning the rotor in the same tions in ratio may be obtained by one of four methods,
direction as the rotating field in the stator. This
namely resistance starting, inductance starting,
action continues throughout the complete. power
resistance/Inductance starting or capacitance starting;
supply cycle, the field making one complete revoluthe application of each method depends on the power
tion. ln the case of a 4-pole motor the field rotates
output ratings of the particular motor. For example,
only 180 degrees during a full cycle and a 6-pole
horsepower ratings of capacitance starting motors are
motor only 90 degrees.
usually fractional and less than 2 h.p.
As the speed of the rotor rises, there is a correThe first three methods are used only during
sponding decrease of induced e.m.f. and torque until
starting of a motor, because if both windin~ rethe latter balances the torques resulting from bearing
mained in circuit under running conditions, the per•
friction, wind·resistance, etc., and the speed remains
formance would be adversely affected. Moreover, a
constant. Thus, the rotor never accelerates to the
motor is able to run as a single-phase machine once a
synchronous speed of the stator field; if it were to do
certain speed has been reached. The starting winding
so the bars would not be cut by the rotating field,
circuit is normally disconnected by a centrifugal
there would be no induced e.m.f. or current flow, and
switch. The fourth method can be used for both
no torque to maintain rota tion.
starting and running, and with suitably rated capacito1
The synchronous speed of an induction motor is
the running performance of capacitor motorS, as they
are called, approaches that of two-phase motorS.
determined by the number of poles for which the
,, , . --, -I ,,,,.---......
Figure 9.11 illustrates the application of a
squirrel-cage capacilor motor lo an axial-flow blower
designed for radio rack cooling or general air circulation. It utilizes two capacitors connected In parallel
and operates from a 11 5-volts single-phase 400 Hz
supply. The capacitive reactance of the capacitors is
greater than the inductive reacta.nce of the starting
winding, and so the current through this winding
CQPOc11or bQI
Floshprool tub!
Sw1 l~h lmPE:ll! r
So,dor mo,mhnq
lnn; r CQ~1ng
o,,ve enQ
Ori ...~ plol a
Fig 9. 11
Motor-Qriven blower
thereby leads the supply voltage. The current in the
running winding lag$ on the supply voltage and the
phase difference causing field rotation is therefore the
sum of the lag and lead angles,
These find their greatest applications in systems
requiring a servo control of synchronous devices, e.g.
as servomotors In power follow-up synchro systems.
The windings are also al 90 degrees to each other but,
unlike the motors thus far described, they are con·
nected to different voltage sources. One source is
the main supply for the system and being of constant
magnit·ude it serves as a reference voltage; the other
source serves as a control voltage and is derived from
a signal amplifier in such a way that it is variable in
magnitude and its phase can either lead or lag the
reference voltage, thereby controlling the speed and
direction of rotation of the field and rotor.
Hysteresis motors also consist of a stator and rotor
assembly, but unlike other a.c. motors the operation
is directly dependent on the magnetism induced in
the rotors and on the hysteresis or lagging characteris,
tics of the material (usually cobalt steel) from which
they are made.
A rotating field is produced by the stator and if
the rotor is stationary, or turning at a speed less than
the synchronous speed, every point on the rotor is
subjected to successive magnetizing cycles. As the
stator field reduces to zero during each cycle, a cer·
taln amount of flux remains in the rotor material, and
since it lags on the stator field it produces a torque at
the rotor shaft which remains constant as the rotor
accelerates up to the synchronous speed of the stator
field. This latter feature is one of the principal advan·
tages of hysteresis motors and for this reason they are
chosen for such applications as autopilot servomotors,
which produce mechanical movements of an aircraft's
flight control surfaces.
When the rotor reaches synchronous speed, it is
no longer subjected lo $UCcessive magnetizing cycles
and in this condition it behaves as a permanent mag·
Power Utilization
Lighting plays an important role in the operation of an
aircraft and many of its systems, and in the main falls
into two groups: external lighting and internal lighting.
Some of the principal applications of lights within
these groups are as follows:
Extemal Lighting
The marking of an aircraft's position by
means of navigation lights.
(ii) Position marking by means of flashing
{iii) Forward illumination for landing and
{iv) lliurninatlon of wings and engine air
intakes to check for icing.'
(v) lliumination to permit evacuation of
passengers after an emergency landing.
Internal lighting
illumination of cockpit instruments and
control panels.
(vil) illumination of passenger cabins and
passenger information signs.
(viii) Indication and warning of system operating
The plan view of external lighting given In Fig. 10.1
ls based on the Boeing 747 and, although not all the
lights shown would be standard on all other types of
aircraft, it serves to illustrate the disposition of externaJ lights generally.
The requirements and characteristics of navigation
lights are agreed on an international basis and are set
out in the statutory Rules of the Air and Orders for
Air Navi,gatlon and Air Traffic Control regulations.
Briefly, these requirements are that every aircraft in
flight or moving on the ground during the hours of
darkness shall display :
(a) A green light at or near the starboard wing tip,
visible in the horizontal plane from a point
directly ahead through an arc of 110 degrees to
{b) A red light at or near the port wing tip, with a
similar arc of visibility to port,
(c) A white light visible from the rear of the aircraft
in the horizontal plane through an arc of 140
degrees, The conventional location of this light
is in the aircraft's tail, but in certain cases,
notably such aircraft as the Douglas DC· l Oand
Lockheed l O11 "Tristar'', white lights are
mounted in the trailing edge sections of each
wing tip.
The above angular settings are indicated in
Fig. 10,1 ,
The construction of the light fittings themselves
varies in order to meet the installation requirements
for different types of.aircraft. In general, however,
they consist of a filament type lamp, a.ppropriate
fitting and transparent coloured screen or cap. The
screen is specially shaped and, together with the
method of arranging the filament of the lamp, a sharp
cot-off of light at tlie required angle of visibility is
obtained. The electrical power required for the lights
is normally 28 volts d.c. but In several current types
of 11all a.c." aircraft, the lights are supplied with 28
volts a,c. via a step-down transformer. The operation
of navigation lights, and their circuit arrangements,
are factors which are dictated primarily by the regula·
tions established for the flight operation of the types
of aircraft concerned. Originally lights were required
to give steady lighting conditions, but in order to im•
prove the position marking function, subsequent
Wing 5Con ligh t
~reen no111go !ton light
Red ont•tol li s•on ligh ts
Bott om
\.._ Ove rwing
egre55 light s
LH ond RH
l urnof I ond
Fig 10, l
Disposition of external lightlns
developments provided for the lights to flash in a
controlled sequence. However, following the adoption
of flashing anti-collision beacons the requirement for
flashing navigation lights was discontinued and the
requirement for steady lighting conditions reintroduced to become the order of the day once more.
It is possible, however, that flashing navigation lights
may still be observed on occasions: these are installed
in some aircraft below a certain weight category,
registered before current requirements became effec.
tive and thereby permitted alternative lighting
Anti-collision lighting also fulfiJs a position marking
function and, in conjunction with navigation lights
giving steady lighting conditions, permits the position
01 an aircraft to be more readily determined. A
lighting system may be of the type whlch emits a
rotating beam of tight, or of the strobe type from
which short-duration Oashes of hlgh-intensity light
are emitted. ln some current types of aircraft both
methods are used in combination, the,strobe lighting
forming what is termed "supplementary lighting".
Rotating Beam ltghts.
These tights or beacons as
they are often called, consist of a ·mamenl lamp unit
and a motor, which in some cases drives a reflector
and in others the lamp unit itself; the drive transmission system is usually of the gear and pinion type
and of a specific reduction ra tio. All components are
contained within a mounting enclosed by a red glass
cover. The power required for beacon operation is
normally 28-volts d.c.• but a number of types are
designed for operation from an a.c. supply, the motor
requiring 115 volts and the lamp unit 28 volts supplied via a step-down transformer . The motor speed
and gear drive ratios of beacons are such that the
reflector or lamp unit, as the case may be, is operated
Lo establish a beam oftight which rotates at a constant frequency, Typical speeds are 40- 45 rev/min
giving a frequency of 80-90 cycles per minute. There
are several variations in the design of beacons, but
the two types described here usefully serve as examples of how the rotating reflector and rotating lamp
techniques are applied.
The beacon shown in Fig. 10.2 employs a V·
shaped reflector which is ro·tated at about 45 rev/min
by a d.c. motor, over and about the axis of a sealed
beam lamp. One half of the reflector is flat and emits
a narrow high-intensity beam of light near the horizontal, while the other half is curved to Increase the
up and down spread of its emitted beam to 30 degrees
above and below the horizontal, and thereby reducing
the light intensity.
Figure 10.3 illustrates a beacon employing two
filament lamps mounted in tandem and pivoted-on
their own axes. One half of each lamp forms a
reflector, and the drive from the motor Is so arranged
that the lamps oscillate through 180 degrees, and as
may be seen from the Inset diagram, the light beams
are 180 degrees apart at any instant. The power
supply required for operation is a.c.
Strobe Lighting. This type of lighting system is
based on the principle of a capacitor-discharge flash
tube. Depending on the size of the aircraft, strobe
lighting may be Installed in the wing tips to supplement the conventional red beacons, they may be used
to function solely as beacons, or may be used in combination as a complete strobe type anti.collision high·
intensity lighting system,
The light unit takes the form of a quartz or glass
tube filled with Xenon gas, and this Is connected to a
power supply unit made up essentially of a capacitor,
and which converts input power of 28 volts d.c. or
Setu• ,n9 ~;, ew&
115 volts a.c. as the case may be, into a high d.c. out·
put, usually 4SO volts. The capacitor is charged to this
voltage and periodically discharged between two
electrodes in the Xenon-filled tube, the energy producing an effective high,intensity flash of light having
a characteristic blue.white colour. A typical flashing
ftequency Is 70 per minute.
The unit shown in Pig. 10.4 is desJgned for wing
tip mounting and consists of a housing containing the
power supply circuitry, the tube, reflector and glass
lens. When used as supplementary lighting or as a
compfete strobe anti-collision lighting system, three
units are installed in trailing positions in each wing tip,
and all lights are controlled in a flashing sequence by
controllers and flasher timing units.
As their names indicate these lights provide essential
illumination for the landing of an aircraft and for
taxi-Ing it to and from runways and terminal areas at
night and at other times when visibility conclitions are
poor. Landing lights are so arranged that they illuminate the runway immediately ahead of the aircraft
from such positions as wing leading edges, front fuse·
lage sections and nose landing gear structure. The
lights are of the sealed beam type and in some air·
craft are mounted to direct beams of light at pre·
Mo! Qr
3·Pin connector
Fig 10.2
Rotating reflector be~con
Pig 10.3
Rotating lamp beacon
determined and fixed angles. In other types of aircraft, the lights may be extended to preselected
angles, and retracted, by an electric motor and gear
mechanism, or by a linear actuator. Micro-type limit
switches are incorporated in the motor circuit and are
actuated at the extreme limits of travel to interrupt
motor operation,
Fig 10.4
iyplcal suobe light u.nll
A typical power rating for lights is 600 watts, and
depending on the design the power supply required
for operation may be either d.c. or a.c. at 28 volts,
the latter being derived from a 115-voJts supply via a
step.down transformer. In lights of the retractable
type which require a.c. for their operation, the moto1
is driven directly from the 115-volts supply. The
supplies to the light and motor are controlled by
switches on the appropriate control panel in the
cockpit. An example of a retractable type landing
light is shown in Fig. 10 .5 .
The circuit of an extending/retracting light systen
is shown in Fig. 10.6. It ls drawn to indicate the
retracted position, and so the "retract" and "extend·
limit switches controlling the motor, are open and
closed respectively. The supply circuit to the light
itself ls automatically interrupted when it is retra.ctec
When the control switch ls placed in the "extend"
posiiion, the 115-volt supply passes through the
corresponding field winding of the motor until
interrupted by the opening of the extend limit
switch. The retract limit switch closes soon after the
motor starts extendJng the light. The switch in the
supply circuit to the light also closes but the light is
not illuminated until it is fully extended and the
control switch placed in the ''on" position. The
power supply to the light is reduced from 115 to
1S volts by a step-oown transformer.
In some aircraft, a fixed.type landing light is
located in the leading edge of each wing near the
fuselage, and an extending/retracting type Is located
in the fairing of each outboard landing flap track.
In lights located in flap track fairings, additional
switches are included in the "retract'' and "extend"
circuits, The switches are actuated by a mechanical
coupling between the wing and flap track fairings.
Thus, when the landing flaps are lowered, and the
landing lights extended, the circuits of the motor will
be signalled to adjust the positions of the lights so
that their beams remain panillel to a known fore
and aft datum regardless of flap positions.
Taxi lights are also of the sealed beam type and are
located In the fuselage nose section, ln most cases on
the nose landing gear assembly. The power rating of
the lights is normally lower than that of landing lights
(250 watts is typical) and the supply required is
either d.c. or a.c. at 28 volts.
In certain cases the function of a taxi light ls
combined wi~h that of a landing light. For example,
in the unit illustrated in Fig. 10.S, the light has two
filaments, one rated at 600 watts and the other at 400
watts; both filaments provide the illumination for
landing, whlle fof taxi·ing only the 400 watt filament
is used.
Pig 10.5
Typical landing light
8 ,C,
ff) ' Ii•·
Fig 10.6
Extending/retracting Ught circuit
In addition to taxi lights some of the larger types
of transport aircraft are equipped with lights which
direct beams of light to the sides of the rWlway (see
Fig. 10.1). These are known as runway tum-off lights,
their primary function being to illuminate the points
along the runway at which an aircraft must turn to
leave the runway after landing.
Ice inspection or wing-scan llghts are fitted to most
types of transport aircraft, to detect the formation of
Ice on the leading edges of wings and also at the air
intakes of turbine engines. Lights are also of the
sealed beam d.c. or a,c. type and with power ratings
varying from 60 watts to 250 watts depending on the
lighting intensity required for a particular aircraft
type. They are recessed into the sides of the fuselage
and are preset to direct beams of light at the required
angles. In some aircraft having rear-mounted engines
lights are also recessed into the tr;lillng edge sections
of the wings.
Internal Lighting
The Internal lighting of aircraft can be broadly divided
into three categories ; cockpit or operational lighting,
passenger cabin lighting, and servicing lighting whkh
includes galleys, toilet compartments, freight com·
partments and equipment bays.
The· most Important requirements for cockpil lighting
are those necessary to ensure adequate illumination of
all instruments, switches, controls, etc., and of the
pmels to which these items are fitted. Some of the
Fig 10,7
Boeing 74 7 cockpit under night Ugh ting conditions
methods adopted to meet these requirements are as
(i) integral lighting, Le. one in which the light
source is within each instrument;
(ii) pillar and bridge lighting, in whi·; h a number
of lights are positioned on panels to iUuminate small adjacent areas, and to provide
flood-lighting of individual instruments;
(ill) flood-lighting, whereby lamps are positioned
around the cockpit to nood-light specific
panels or a general area.
(iv) tra,ns-illuminated panels which permit
etched inscriptions related to various controls, notices and instructions to be read
under night or poor visibility conditions.
A view of the Boeing 747 cockpit under night
lighting conditions is shown In Fig. 10.7.
The principal form of integral lighting for instruments
is that known as wedge or front lighting; a form deriving its name from the shape of the two portions which
, ,,
- -'----/
~ig 10.8
Wedgc•lype lighting
together make up the instrument cover glass. lt relies
for its operation upon the physical law that the angle
at which light leaves a reflecting surface equals the
angle at which il strikes that surface, The two wedges
are mounted opposite to each other and with a
narrow airspace separating them as shown in Fig.
10,8 . Light is introduced into wedge "A" from two
6-volt lamps set into recesses in its wide end. Acer·
tain amount of light passes cfuectly through this
wedge and on to the face of the dial while the remainder is reflected back into the wedge by its
polished surfaces. The angle at which the light rays
strike the wedge surfaces governs the amount of light
reflected; the lower the angle, the more Ught is reflected.
The double wedge mechanically changes the angle
at which the light rays strike one of the reflecting
surfaces of each wedge, thus distributing the light
evenly across the dial and also limiting the amount of
light given off by the instrument, Since the source of
light is a radial one, the initial angle of some light
rays with respect to the polished surfaces of wedge
"A" is Jess than that of the others. The low-angle light
rays progress further down the wedge before they
leave and spread light across the entire dial. Light
escaping into wedge "B" is confronted with con.
stantly decreasing angles, and this has the effect of
trapping the light within the wedge and directing it to
its wide end. Absorption of light reflected into the
wide end of wedge "B" is ensured by painting its
outer part black.
Pillar lighting, so called after the method of construction and attachment of the lamp, proviaes Illumination for individual instruments and controls on the
various cockpit panels. A typical assembly, shown
in Fig. 10.9, consists of a miniature centre-contact
filament lamp inside a housing, which is a push fit
in to the body of the assembly . The body is threaded
externally for attachment to the panel and has a hole
running through its length to accommodate a cable
which connects the positive supply to the centre con·
tact. The circuit through the lamp is completed by a
ground tag connected to the negative cable.
Ught is distributed through a filter and an aperture
in the lamp housing. The shape of the aperture distributes a sector of light which extends downwards
over an arc of approximately 90 degrees to a depth
slightly less than 2 in. from the mounting point.
The bridge-type oflighting (Fig. I0.9(b)) is a
multi-lamp development ,of the individual pillar lamp
Fig 10.9
Pillar and bridge Ughting
already described. Two or four lamps are fitted to a
bridge structure designed to fit over a variety of the
standardized instniment cases, The bridge fitting Is
composed of two light aUoy pressingS secured to,
gether by rivets and spacers, and carrying. the requisite
number of centre contact assemblies above which the
lamp housings are mounted. Wiring arrangements pro·
vide for two separate supplies to the lamps thus
ensuring that total loss of illumination cannot occur
as a result of failure of one circuit.
These panels or ' 'lightplates", provide for the
illumination of system nomenclature, switch
positions etc. They are of plastic th rough which
light from many very small incandescent bulbs is
passed. The light can only be seen where appropriate
characters have been etched through a painted
surface of a panel. The bulbs are soldered in place
and are not replaceable when installed . More than
one bulb provides illumination in each relevant area
so that failure of a bulb will not impair illumination.
Flood-lighting Is used for the general illumination of
instru ments, control panels, pedestals, side consoles
and areas of cockpit floors . TI1e liS}lts usually take
the form of incandescent lamp units and fluorescent
tube units and depending on the type of aircraft, both
forms may be usod in combination.
This form of lightin g is employed in a number of aircraft as passenger information signs and also, i.n some
cases, for the illumination of instrument dials and
selective positions of valves or switches. An elec troluminescent light consists of a tlun laminate structure
in which a layer of phosphor is sandwiched between
two electrodes, one of which is transparent. The light
requires a.c. for its opera tion, and when this is applied
to the electrodes the phosphor particles Juminesce,
i.e. visible light is emitled through the transparent
electrode. The luminescent intensi ty· depends on the
voltage and frequency of the a.c. supply. The area of
the phosphor layer which becomes "electrolumines·
cent" when the current is applied is that actually
sandwiched between the electrodes; consequently if
the back electrode is shaped in the ionn of a letter
or a figure the pattern of light emitted through the
transparent electrode is an image of the back electrode
The extent to which lighting is used in a passenger
cabin depends on the size of a cabin and largely on
the interior decor adopted for the type ofaircrafl;
thus, it can vary from a small number of roof-mounted
incandescent lamp fittings to a large number of fluorescent fittin~ located in ceilings and hat racks so as to
give concealed, pleasing and functional lighting effects.
The power supplies required are d.c. or a.c. as appropriate, and in all commercial passenger transport air·
craft the lights are controlled from panels at cabin
attendant stations. In addition to main cabin lighting,
lights are also provided for passenger service panels
:see p. 182).and are required for the illumination of
!!Ssential passenger information signs, e.g. "Fasten
Seat Belts" and "Return to Cabin". The lights for
these signs may be of the incandescent type or, in a
number of aircraft, of the electroluminescent type
described earlier. They are controlled by switches on
1 cockpit overhead panel.
but because a transistor requires only very low
voltage levels over its conducting range, the rheostat
can be smaller from the point of view of electrical
characteristics and physical dimensions.
D.c. power is supplied to the rheostat and also to
the collector "C" of the transistor. When the rl1eostat
wiper is at contact position ''A" , the voltage at the
:ontrol of Lighting Intensity
:ertain internal lighting circuits must have a means
)f varying the light intensity and so they are pro•
rided with an intensity control system. The methods
)f control, and their application, depends largely on
he extent of the lighting required, this in turn,
leing dependent on the type of aircraft. The fundanental operating principles of each method are
:hown in Fig. 10.10.
The most basic of dimming circuits is the one
Jtilizlng a panel-mounted rheostat which is con1ected in series with the lights whose intensity is
.o be controlled (diagram (a)). Power from the d.c.
)usbar is fed to the rheostat wiper which, at contact
Josition ' 'A'' isolates the lights from the supply.
Nhen moved to contact position "B" , the-circuit is
:witched on but as current must flow through the
1,1hole of the rheostat resistance, the lights will be
:lirnly illuminated. As the wiper Is moved towards
:ontact position ''C'' the resistance 111 the circuit
lecomes Jess and less and so the lighting intensity
ncreases, At position "C" maximum current flows
:hrough the clrcull to provide maximum lighting
ntcnsity .
Diagram (b) illustrates a circuit development of
;he basic rheostat method and is one which Is widely
1dopted in many aircraft since it permits the use of
ess "bulky'' rheostats, and control of an increased
1umber of lights in any one circuit. The circuit
1tilizes an NPN transistor which functions as a
·emotely controlled resistor unit. A rheostat is still
·equ!red to vary the voltage input to the transistor,
28-V a.c.
- --------0-.. Bright
I 1 - - --c>-' Off
Fig 10,10
Control oflighting intensity
base of the transistor is zero, and no current flows
through the collector to the emitter "E" or out to
the lights. Movement of the wiper from contact
position ''A", causes a positive voltage to be applied
to the base of the transistor, and a small amount of
current flows from the collector, and through the
emitter to the lights as a result of a reduction in
resistance of the collector-emitter junction, Further
·movement of the wiper increases the positive voltage
at the transistor base' and the resulting decrease in
collector-emitter junction resistance increases the
current now to the lights and therefore, their
Diagram (c) of Fig. 10.10, shows a method in
which lighting intensity may be cont rolled by means
of a variable transfonner. This is commonly adopted
in aircraft whose maln power generating systems are
An essential requirement concerning lighting is that
adequate illumination of the cockpit and the various
sections of the cabin, exits, escape hatches, chutes
etc., must be provjded under emergency conditions,
e.g. a crash-landing at night. The illumination is nor·
mally at a lower level than that provided by the standard lighting systems, since the light units are directly
powered from an emergency battery pack or direct
from the aircraft battery in some cases. The batteries
are normally of the nickel-<:admium type although
in some aircraft silver-zinc batteries are employed,
Under nonnal operating conditions of the aircraft,
emergency battery packs are maintained in a fully.
charged condition by a trickle charge from the aj.r·
craft's main busbar system .
Primary control of the lights is by means of a
switch on a cockpit overhead panel.
Engine Starting Systems
Throughout the development of aircraft engines a
number of methods of starting them have been used
and the prime movers involved have varied from a
mechanic manually swinging a propeller, to electric
motors and electric control of sophisticated turbo.
starter units, Although there are still one or two types
of light aircraft in service requiring the manual
swinging technique, the most widely adopted starting
method for reciprocating engines utilizes electric
motors, while for the starting of gas turbine engines
either electric motors or turbo-starter units may be
utilized as the prime movers .
ln basic form, these systems consist of a motor, an
engaging gear, a relay and a starter switch; in some
systems a clutch mechanism is also incorporated in
the engaging gear mechanism. The motors employed
may be of the plain series-field type or may be com•
pounded with a strong series bias (see pp. 138 and l ::
Fig. 10.11 shows the interconnection of the princ
electrical components typical of those required for
the starting of reciprocating engines installed In man
types of light aircraft. When the starter switch is clo
direct current from the battery and busbar energize!
the starter relay, the closed contacts of which connc
the motor to the battery. The relay contacts are of
the heavy-duty type to carry the high drawn
the motor.during the period of cranking over the
The method of engaging a motor with an engine
varies according to the particular engine design. For
most types of light aircraft engines, a pinion is enga1
with a starter gear ring secured to the engine crank·
shaft in a manner similar to that employed for starti
automobile engines. When the engine starts, it over·
runs the starter motor and the pinion gets "kic.ked
out" of engjgemen t. In other versions used for
starting more powerful engines, a jaw engages with,
similar member on the engine and the drive is trans.
mitted via a clutch and reduction gear traiJ1 in the
starter motor and in an accessories gearbox in the
The gear ratio between a starter motor and a re·
c!procating engine is such that it provides a low crar
ing speed of the engine; a typical reduction ratiq is
about 100 : 1. Cranking speed is not critical because
the fuel priming provisions made in the starting drill
also because there is a good stream of sparks avallab
at the plug points for the power stroke. Thus, once
the engine has "fired" and gets away under its own
power further assistance from the starter motor is
rendered unnecessary . Although the moment of
inertia of an engine's moving parts is comparatively
light during cranking, a starter motor has to overcor
some heavy frictional loads, i.e. loads of pistons and
bearings, and also loads due to compression.
Compared with a reciprocating engine, the starting c
a turbine engine represents a relatively severe duty
for the starter motor. TI1is stems mainly from the
starting principle involved and also from the constn
tion of the rotating assembly, e.g. whether the com·
! """·1..______________;-r-,er--------.
Fig 10. 11
Simple engine 5torting system
pressor and turbine are on a single shaft (single-spool
engine) or whether high-pressure compressor/turbine
assemblies and low·pressure compressor/turbine
assemblies on separate shafts are employed (two·
spool engine), Another factor also is whether the
compressor and turbine assembly is designed to drive
a propeller. In general, turbine engines have a high
moment of inertia, and since IL is a requirement that
starting shall be effected as quickly as possible, then
high gear ratios and therefore high cranking speeds
are necessary.
The process of starting a turbine engine involves
the provision of an adequate and continuous volume
of air to the combustion system, effective atomiza•
tion of fuel at the burners of the combustion system,
and the initiation of combustion in the combustion
chambers. To provide the necessary volume of air
the starter motor must be capable of developing
sufficient power to accelerate the compressor smoothly
and gently from a static condition to a fairly high
speed. At some stage in the cranking operation , fuel
is injected into the combustion system and the fuel/
air mixture is ignited, i.e. the engine "fires" or "lights·
up" as it is more usually stated in turbine engine
terms. Unlike reciprocating engine starting, however,
the starter motor does not disengage at this poin1 but,
assisted by the engine, continues to accelerate it up
to a speed at which the engine alone is capable of
maintaining rotation. This is known as the self·
sustaining speed of the engine. Eventually a condition
is reached where the starter motor is no longer
required and its torque, and the current consumed,
start decreasing fairly rapidly. Its speed wiU tend to
increase, but this is limited by the retarding torque
provided br the shunt field when there is no longer a
load on the motor (see also p. 138). Depending on the
type of starter system, the power supply to the
starter motor is interrupted automatically either by
the decrease in current causing the starter relay to de.
energize, or by the opening of contacts in a time
switch unit.
Figure 10.12 illustrates the circuit diagram of a
system based on that employed in a. current type of
twin turbopropeller aircraft for the starting of its
engines. The starter motor ls a 28-volts d.c. four-pole
compound-wound machine having a torque output of
16·5 lbf.ft(22,37 Newton metre) at a speed of 3800
rev/min and a time rating of 90 seconds. It drives the
engine through a clutch, pawl mechanism and reduc·
tion gear. The clutch is held in the driving position
until the engine has accelerated above the starter
motor speed and until the centrifugal force acting
on the pawl mechanism is sufficient to release the
pawls, The starter motor is disengaged by the action
of an overs peed relay.
When the master switch is set to the "start" position, and the starter push switch is depressed, direct
current flows through the coil of the main starter
relay thereby energizing It. At the same time current
also flows to contact "1" of the overspeed relay. The
closing of the heavy-duty contacts "A" and "B" of
the starter relay completes a circuit from the main
busbar to the starter motor via the coil of the over.
speed relay, which on being energized, allows current
to flow across its contacts to the coil of the push
switch thereby holding this switch closed. During
initial stages of starting the current d!awn by the
starter motor is high, and as this is carried by the
coil of the overspeed relay continued cranking of
the engine is assured. A~ the engine accelerates, the
starter motor draws less current until, at a value pre·
determined by the speed at which the engine becomes
self-sustatning, the overspeed relay is de-energized, this
in turn de-energizing the starter switch and main
starter relay. The overspeed relay therefore prevents
the starter motor from overspeeding by ensuring that
the power supply is disconnected before the starter
drive ls disengaged from the engine.
The purpose of the "blow out" position of the
master switch is to permit the engine to be cranked
over in order to blow out unburnt fuel resulting from
an unsuccessful start or "light up". When the position
is selected, the circuit is operated in a similar manner
to normal starting except that the starter switch must
be pulled to the "off' position after the motor has
been running for 30 seconds. The reason for this is
:- · -i
r------ - - ------11ri&2·
~-- ----- ---:-- 4<,1 . f---JBl~·- ~----r·~:I I'
: "A>-+--- ~- fo~o-- ------c,-v-;,
L. .J
. ;:,o
: ·art
I• ,J
• sw,tcll
- '
---. \
'"- ---- ,
~ -- - -- - - - To ,gn1!10n $y$!Cm
Fig 10.12
Basic circuit of a turboprop engine staxting system
that since the ignition system is isolated, the starter
motor is still heavily loaded and so the current
through the overspeed relay remains too high for the
relay to de.energize of its own accord.
purposes, the unit functions as a fully compounded
motor, the shunt winding being supplied with current
via a field changeover relay . When the engine reaches
self.sustaining speed and the starter motor circuit is
isolated from the power supply, the changeover relay
is also automatically de-energized and its contacts
connect the shunt-field winding to a voltage regulator.
The relay contacts also permit d.c. to flow th.rough
the shunt winding to provide initial excitation of the
field. Thus, the machine functions as a conventional
d.c. generator, its output being connected to the busbar on reaching the regulated level.
With the development of more powerful turbine en·
gines ever-increasing power output from starter systems was required for effective starting action. As
far as electrical methods of starting were concerned
this presented Increasingly difficult problems associated notably with high current demand, increased
size and weight of motors and cables, These problems
therefore led to the discontinuance of electric motors
for the starting of powerful engines, and their functions were taken over by turbo-starter systems
requiring a simpler control circuit consuming only a
few amperes.
There are three principal types of turbo-starter
systems; air, cartridge and monofuel, the application
of each being governed largely by the operational role
of the aircraft, i.e. civil o'r military. The basic principle
is the same for each system, that is, a gas is made to
impinge on the blades of a turbine rotor within the
starter unit, thereby producing the power required to
turn the engine shaft via an appropriate form of
The gas may be (i) compressed air supplied to a
turbine air motor from either an extemal supply unit ,
an A.P.U. in the aircraft or the compressor of a
running engine; (ii) the cordite discharge from a11
electrically fired cartridge or (iii) the result of igniting
a monofuel, in other words a fuel which bums freely
without an oxidant such as air ; a typical fuel is iso.
The electrical control circuits normally require d.c.
for their operation, their function being to energize
solenoid-operated air control valves, to fire cartridge
units and to energize a fuel pump motor and ignition
systems as appropriate to the type of turbo-starter
All types of aircraft engines are dependent on electrical
ignition systems. ln reciprocating-type engines, the
charges of fuel vapour and air which arc induced and
compressed in the cylinders, are ignited through the
medium of sparks produced by electric discharges
across the gaps between the electrodes of a spark plug
fitted in each cylinder, and a continuous series of
high-voltage electrical impulses, separated by intervals
which are related to engine speed, must be made
available to each of the plugs throughout the period
the engine is running. A basically similar electrical
ignition system is also used to initiate combustion of
the fuel/air mixture in the combustion chambers of
gas turbine engines. It is, however, of much simpler
form for the reasons that impulse intervals are not
related to engine speed, and as combustion is continuous after "light up". the ignHion system is only
required during the starting period.
Reciprocating-type engine ignition systems fall into
one or other of two main categories; coil ignition and
magneto ignition. The former derives its power from
an external source, e.g. the main power supply, while
the magneto is a self-contained unit driven by the
engine and supplying power from its own generator.
In aircraft engine applications, magneto Ignition is
the system most commonly adopted.
Several types of turbine-powered air~raft are
equipped with starter systems which utilize a prime
mover having the dual function of engine starting and
of supplying power to the aircraft's electrical system.
Starter-generator units are basically compound-wound
machines with compensating windings and lnterpoles,
and are permanently coupled with the appropriate
engine via a drive shaft and gear train . For starting
Magneto Ignition systems, which operate on the
principles of electromagnetic induction, are classified
as either high tension or low tension, and they consist
of the principal components shown schematically in
Fig. 10.13. Most of these components .ue contained
within the magneto, which is basically a combination
of permanent-magnet a.c. generator and autotransformer.
Ignition Systems
The high tension system Is the one most widely
used, and the requisite alternating fluxes and voltages
are induced either by rotating the transformer
windings between the poles of a permanent magnet,
by rotating the magnet between fixed transformer
windings or by rotating soft-iron inductor bars be·
tween fixed permanent magnet and transformer
windings. These arrangements, respectively, permit
further classification of magnetos as (i) rotating·
armature, (ii) rotating magnet and (iii) polar inductor.
The rotating portion of a magneto is driven by the
engine through a coupling and an accessory gear drive
shaft. ~ the windings are cut by the alternating
magnetic flux from the appropriate source, a low
voltage is induced in the primary winding to produce
a current and flux of a strength directly proportional
to the rate at which the main flux is cut. At this point
the primary circuit is broken by the contact breaker,
the contacts, or points, of which are opened by a
cam driven by the rotating assembly. The primary flux
therefore collapses about the secondary winding, which
produces a high voltage output. The output is, how·
ever, not sufficient to produce the required discharge
at the spark plugs and it is necessary to speed up the
rate of flux collapse . This is effected by connecting a
capacitor across the contact breaker so that the capaci
tor is shorted out when the breaker points are closed
and is charged by primary winding current when the
points are open. When the potential difference
acrosS the capacitor reaches the point whereby it discharges itself, the correspondingly high current flows
through the primary winding in the reverse direction
and thereby rapidly suppresses the primary flux to
produce the required higher secondary output vol·
tage. In addition to this function, the capacitor also
prevents arcing between the contact breaker points
as they begin to Open, thereby preventing rapid
deterioration of the points.
The secondary winding output is supplied to the
distributor, the purpose of which is to ensure that the
high voltage impulses are conducted to the sparking
plugs in accordance with the order in which combus·
tion must take place in each cylinder, i.e. the "firing"
order of the engine . A distributor consists of two
main parts, a rotor made up of an insulating and a
conducting material, and a block of insulating materla
containing conducting segments corresponding in
number to the number of cylinders on the engine.
The conducting segments are located circumferentiall:,
around the distributor block i.n the desired firing
order. so that as the rotor turns a circuit is
· w1nd1ng
swi t ch
Fig 10.13
Magneto ignition system
completed to a sparking plug each time there i~
alignment between the rotor and a segment.
Distributors usually form part of magnetos, and the
rotors are rotated at the required speed by a gear
driven from the main magneto shaft. In some cases,
however, distributors may be separate units driven
by an engine gear train and drive shaft. To prevent
ionization, and to minimize "flashover", the distribu·
tor casing is vented to atmosphere , and in many types
of magnet a flameproof wire mesh screen is provided
to prevent combustion of any tlanunable vapours
round the engine.
Ignition of the combustible mixture is required In each
cylinder once in every two revolutions of the engine
crankshaft, and as a result there must be a dellnitie
relationship between such factors as the number of
sparks produced by a magneto and the speeds of the
magneto, distributor and engine. Magneto speed may
be calculated from the relation:
number of cyl..i.nders
2 x magneto sparks per rev.
A rotating armature magneto, which is normally
only used on engines having up to si.)(. cylinders, produces
two sparks per rev. Thus, assuming that one is fitted
to a four-cylinder engine then it must be driven at the
same speed as the engine. A rotating magnet or polar
inductor magneto produces four sparks per rev and
is normally used on engines having more than six
cyllnders. Thus, for a twelve-cylinder engine the
magneto must be driven at one and a half limes the
engine speed. Distributor rotors are driven at half
engine speed irrespective of magneto speed.
As mentioned earlier, during starting, a piston engine
is cranked over at very low speeds, and as a result its
magnetos are driven much too slowly for the e.m.f.
induced in the primary winding to produce a spark of
adequa te energy-content at the instant the contact
breaker points open. It is therefore necessary to pro·
vide auxiliary means for boosting the magneto output
during the engine starting period, when it is advan·
tageous to have the spark retarded to some extent.
Two methods widely adopted arc Impulse starters and
booster coils, which arc described in the following para.
graphs. The retarding of the spark is effected by a
secondary brush in the distributor arm which "trails"
the main brush.
Impulse couplings, or impulse starters as they are
sometimes called, are u~ed in some small piston
engine ignition systems and are fitted between the
magneto shaft and drive shaft. The unit produces a
heavy spark by giving a magneto armature or magnet
a bcief acceleration at the moment of spark produc.
tion. In one type of unit the coupling between the
magneto and engine is a spring-loaded clutch device
which flicks the armature or magnet through the
positions at which a spark normally occurs, thus
momentarily increasing its rotational speed and the
voltage generated, After the engine Is started and the
magneto reaches a speed al which it furnishes suf.
flcient output, flyweights in the coupling fly ou tward
due to centrifugal force and overcome lhe springs, so
that the coupling functions as a solid drive shaft and
the magneto continues to operate in the normal
Booster coils, which may be either of the high tension
impulse or low tension impulse type , derive their
power from the aircraft's system via either the battery
or the ground power supply source. The supply is
controlled either by a separate booster coil or the
engine starter switch. High tension booster coils
supply a stream of impulses to the trailing brush of the
distributor, while in a low tension system a stream of
impulses is fod to the magneto primary windings either
to augment or to replace the voltage induced by the
magnetic flux. In some low tension systems, the
supply to the primary winding is fed via a second
contact-breaker, which is retarded in relation to the
main contact·breaker but connected in parallel with ·
it. With this arrangement intermitten t high tension
curren t is induced in the secondary winding of the
Ignition systems arc controlled by "on-ofr' switches
connected in the magneto circuit, but unlike the basic
and conventional switching arrangements, an ignition
system switch completes a circuit by closing its contacts in the "ofr' position. The circui t in this case is
between the magneto primary winding and ground,
and since the contact-breaker becomes short·
circuited, then in the event the magneto is rotated, .
there can be no sudden collapse of the primary winding flux and therefore no high voltage spark across
the spark plug gap.
Oi1 dual-ignition systems each magneto may be
controlled by a separate toggle switch or, as is more
usual, by a rotary type four-position switch controlling
both magnetos. The four positions are "off', "left",
"right" and "both". The left and right positions allow
one system to be turned off at a time for carrying
out "magneto-drop" checks during engine ground
These systems were developed for use on engines
having a large number of cylinders and designed for
hig_h altitude ope.ration. They overcome certain
problems which can occur with high tension systems,
e.g. breakdown of Insulation within a magneto due
to decreased atmospheric pressure and electrical
leakage, particularly when ignition harnesses are
enclosed in metal conduits. Further'rnore, the amount
of cable carrying high voltages is considerably
reduced. The magneto is similar to a polar inductor
type of magneto but does not embody a secondary
coil. Low voltage impulses from the magneto primary
winding are supplied directly to the distributor, which
also differs from the types normally employed, in
that voltage impulses are received and distributed via
a set of brushes and segmented tracks. The distributor
output is supplied to transformers corresponding In
number to the number of spark plugs and located near
the plugs. Thus, high voltage is present in only short
lengths of shielded cable. Low tension magnetos are
switched on and off in the same manner as high ten·
sion magnetos.
The function of a spark plug is to conduct the high
voltage impulses from the magneto and to provide an
air gap across which the impulses can produce a spark
discharge to ignjte the fuel/air charge within the
The types of spark plugs used vary in respect to
heat range, thread size, or other characteristics of the
instaJlation requirements of different engines, but in
general they consist of three main components: (i)
outer shell, (ii) Insulator and (iii) centre electrode.
The outer shell, threaded to fit into the cylinder, ls
usually made of high tensile steel and is often plated
to prevent corrosion from engine gases .and possible
thread seizure . The threads are of close tolerance and
together with a copper washer they prevent the very
high gas pressure escaping from the cylinder. Pressure
that might escape through the plug is retained by inner
seals between the outer shell, the insulator, and cent
electrode assembly.
The materials used for insulators vary between
plug designs and applications to specific engines: tho
most commonly adopted are mica, ceramic and
aluminium oxide ceramic, the latter being specificall
developed to withstand more exacting mechanical,
thermal and electrical requirements. Insulation Is als,
extended into a screen tube which is fixed to the ou1
shell and provides attachment for the ignition harne!
cable to ensure suppression of radio interference .
The centre electrode carries the high tension volt:
from the distributor and is so secured that the requi·
site spark gap is formed between It and a negative or
ground electrode secured to the "firing" end of the
outer shell. Elec.trodes must operate under very sever
environmental conditions, and the materials normall
chosen arc nickel, platinum and iridium.
Almost all piston engines employ two entirely in·
dependent ignition systems; thus each cylinder has
two spark plugs, each supplied from a different
magneto. The purpose of dual ignition is to (i)
reduce the possibility of engine failure because of an
engine fault and (ii) reduce the time taken to bum
the full charge enabling peak gas pressure to be
reached and thereby increasing engine power output
Both magne tos are normally switched by a rotary
switch in the manner already described .
The ignition system of a turbine engine is much simr
ler than that of a piston engine due to the fact that
fewer components are required and that electrical
ignition of the air/fuel mixture Is only necessary whe
starting an engine. Another difference is that the
electrical energy developed by the system is very mu
higher in order to ensure ignition of atomized fuel
under varying atmospheric and air mass flow conditions and to meet the problems of relighting an engin
in the air.
The principal components of a system are a high·
energy ignition unit and an igniter plug interconnect,
as shown in Fig. 10.14. Two such systems are norma
fitted to an engine, the igniter plugs being located in
diametrically-opposed combustion chambers to ensu:
a positive and balanced light-up during starting. Dire
current from the_ aircraft's main busbar is supplied to
an induction coil or a transistorized high tension
generator within the ign1tion unit in conjunction will
cases, relighting is automatic by having one of the
two ignition units of a low rating (usually 3 joules)
and keeping it in continuous operation. Where this
method is not desirable a glow plug is sometimes
fitted in the combustion chamber where ii is heated
by the combustion process and remains incandescent
for a sufficient period of time to ensure automatic
In some types of aircraft the high energy ignition
units are dependent on an initial power supply of
115 volts a.c., and the simplified circuit diagram of
one such unit employed on the Boeing 747, is illustra·
led in Fig. 10.16.
The a.c. power supply [s applied to the primary
winding of a step·u p power transformer T 1, via a
radio noise filter network consisting of inductor LI
and a capacitor Cl. The high voltage induced in the
secondary winding of Tl is then rectified by the
diodes I and 2, the current passing through them
being limited by resistors RI and R2. The rectified
ou tput charges the capacitor C2 until the stored
voltage reaches the ionization po ten Lial of the discharge
gap. The discharge fl ows through the primary winding
of the high-tension auto-transformer T2, and is further
boosted by a charge developed across capacitor C4.
The voltage induced in the secondary winding is then
of sufficient potential to provide the requisite discharge
flashovcr across the igniter plug gap. Resistors R3 and
R4 provide the means of dissipating the energy of the
circuit in the event that the ou tput of the igniter unit
is open-circuited. In addition, they serve to "bleed-off"
any residual charge on capacitor C4 between successive
flashovcrs, and so provide a constant level of triggering
voltage from the secondary winding of transformer T2.
Fire Detection and Extinguishing Systems
Fire is, of course, one of the most dangerous threats
to an aircraft and so precautions must be taken to
reduce the hazard, not only by the proper choice of
materials and location of equipment in potential
fire zones, but also by the provision of adequate fire .
detection and extinguishing systems. These systems
may be broadly classified as (i) fixed , some examples
of which are used mainly for engine fire protection,
and detection of smoke in baggage compartments,
or (ii) portable, for use in the event of cabin fires.
Both systems are employed in all aircraft except certain small low·powercd piston engined types which,
having been certificated as constituting a negligible
fire risk, at most need only a portable extinguisher
within the cockpit. Fixed detection and extinguishing
systems only, require electiical power for their opera·
tion, and some typical examples are described in the
following paragraphs.
A fire detection system is installed mainly in engine
compartments, and consists of special detecting
elements strategically positioned within several fire
zones designated by the aircraft manufacturer. The
elements, ;,_,hich may be of the " unit" or ''spot" type
Fig 10.16
High-energy system (a.c. powered)
28- V d c:. busbor
L---- -- - - -- - ---()~o---- -+-,.
lgn,t,on Swllell
'Rel1CJht' sw,ICh
!gn,1,0t1 'on'
Fig 10.14
High-energy ignition system
the starter system, and also Independently through
the "relight" circuit. The coil, or generator, as approp·
riate, repeatedly charges a reservoir capacitor until its
voltage, usually of the order of 2,000 volts, is suffl·
cient to.break down the scaled discharge gap. The gap
is formed by two tun~ten electrodes within a chamber
exhausted of air, filled with an inert gas and scaled to
prevent oxidation whlch would otherwise occur with
the large current handled.
The discharge is conducted through a choke, which
extends the duration of the discharge, and through a
high tension lead to the igniter plug (see Fig. 10.15) at
which the energy is released. A pellet at the "firing"
end of the plug has a semi-conducting surface, and
during operation this permits a minute electrical
leakage from the centre electrode to the body, thereby
heating the surface. Due to the negative temperature/
resistance characteristics of the pellet a low resistance
path is provided for the energy, which discharges
across the surfac~ as a high intensity flashover as
opposed to a spark jumping an air gap. The capacitor
recharges and the cycle is repeated approximately
once every second. Once the fuel/air mixture has
been ignited, the flame spreads rapidly through balance
pipes which interconnect all the combustion chain·
hers; thus combustion is self.sustaining and the ignition system can be switched off. The energy stored in
the capacitors Is potentially lethal, and to 1:nsure their
discharge when the d.c. supply is disconnected, the
ou tput is connected to ground via a safety resistor.
The electrical energy supplied by the ignition unit
is measured in joules, and independent ignition systems
normally consist of two units rated at 12 joules each.
In the event that through adverse flight conditions
the flame is extinguished, the engine Is ''relit" by
switching on the ignition system until the engine runs
normally again. During relighting it is unnecessary
to use the starter motor since the engine continues to
rotate under the action of "windmilling". In some
Contact buttOn
Fig 10.15
High-energy igniter plug
or the "continuous" wire type, are connected lo
warning lights and/or bells, and either type may be
used separately, or together in a combined fire
warning and engine overheat system.
Unit type detectors are situated at points most
likely to be affected by fire, e.g. in an engine breather
outlet pipe, and the one most often used In engine compartments Is of the differential expansion switch type,
the principle of wh.ich was described on p. 106.
These detectors may also be used for sensing an overheat condition in areas of the airframe structure
adjacent to ducting supplying hot air for airconditioning or de-icing systems.
In order to provide maximum coverage of an engine
fire ione and to eliminate the use of a considerable
number of unit detectors, a continuous wire type
detector system (known as a "firewire'' system) is
nom1ally used. The elem en ts of a typical system
take the fom1 of various lengths of wire embedded
In a temperature sensitive material within a smaJl bore
stainless steel or Inconel tube, and joined together
by special coupling units to form a loop which may
be routed round the fire as required. The wire
and tube fonn centre and outer electrodes respectively and are connected to the aircraft's power
supply via a control unit. The power supply requirements are 28 volts d.c. and J 15 volts a.c. or, in some
systems, 28 volts d.c. only . Depending on the type of
control unit the method of operation may be based
on either variations in resistanc;e or variations in
capacitance with variations in temperature of the
element filling material.
The electrical interconnection of components
nom1ally comprising a system is shown in Fig. 10.17.
the control unit in this case is of the type employed
with a variable resistance system. The a.c. supply is fed
to a step-down transformer ; while d.c. is supplied to
the warning circuit via the contacts of a warning relay,
the coil of which may be energized by the rectified
output from the transformer secondary. With the test
switch in the normal position, the ends of the centre
wire electrode of the element are connected in parallel
to the rectifier and to one end of the transformer
secondary winding. The other end of the winding is
connected to the outer tube or electrode so that the
current path is always through the filling material, the
resistance of which will govern the strength of rectified current flowing tluough the relay coil. With this
arrangement the warning function is In no way affected
in the event that a break should occur in the loop.
Under normal ambient temperature conditions the
resistance of the filling material is such that only a
small standing current flows through the material ;
therefore, the current flowing through the warning
ll~·V OC
r--· - - · -·- - · - ·
~ t ·
warning l ight
and / or bell
Step• down
w,re element
Fig 10. 17
Fire detection system
relay coil is insufficient to energize it. In the event of
a temperature rise the resistance of the filling material
will fall since it has an inverse characteristic, hence
the rectified current through the relay coil will in·
crease, and when the fire lone temperature has risen
to such a value that the relay coil current is at a predetermined level, it will energize the relay thereby
completing the warning light or bell circuit. When the
temperature falls and the current drops to a pre·
determined level the relay de-energizes and the system
is automatically reset.
In a capacitance system the detector element is
similar in construction to that earlier described, but
in conjunction with a different type of control unit
it functions as a variable capacitance system, the
capacity of the element increasing as the ambient
temperature increases. The element is polarized by the
application of half-wave rectified a,c, from the control
unit, which it stores and then discharges as a feedback
current to the gate of a silicon.controlled rectifier
(SCR) in the control unit during the non-charging half
cycles. When the fire zone temperature rises the feedback current rises until at a pre-determined level the
SCR is triggered to energize a warning light, or
bell, relay. A principal advantage 0f this sytem is that
a short circuit grounding the element or system
wiring does not result in a false fire warning.
When the test switch is set to the "Test" position ,
the test relay is energized and its contacts change
over the supply from the rectifier so that the current
passes directly along the centre electrode. Thus, if
there is no break in the loop there is minimum resisttance and the warning relay circuit is actuated to
simulate a fire warning and so indicate continuity.
In sorne engine fire deteclion systems, detection is
effected by two distinct sensing element loops; an
"overheat" loop and a "fire'' loop. An example of
one such application based on the Lindberg Systron
Donner system, is shown schematically in Fig. 10.18.
This system, unlike that of the "firewire" system
described earlier, utllizes sensing elements which
trigger the warning circuits as a result of the temperature effects on the pressure of a gas.
An element consists of a stainless steel tube pro·
tected throughout its length by a teflon coating.
Inside the tube ls a metal hydride-coated element
surrounded by an inert gas (helium). One end of
the type is bifurcated and joined to two diaphragmoperated pressure switches, one in the open position
and the other always kept closed by the normal
pressure (20 psi) of the helium actihg on its
diaphragm. The power required for system operation
is 28 volts d.c. from the aircraft's battery busbar,
and is supplied to the open switch contacts via those
of the normally closed switch. Because the overall
system Is designed to sense two levels of tell'lperature, then there must be two detecting elements
each with a different temperature sensing level.
In the practical case, there are two pairs of
elements connected as shown in Fig. 10.18, one pair
forming the "overheat" loop, and the other pair the
"fiie'' loop . The "lements are located at the bottom
of an engine, and on the engine side of the firewall.
If the local temperature rises to 205 ± 30°C,
the coating of the element inside an ''overheat"
detector will release a gas (principally hydrogen).
This will increase the pressure inside the tube so
that the diaphragm of the nonnally-open switch will
be displaced and its contacts closed, The signal now
flowing from the contacts passes through an
operational amplifier the output of which biases a
transistor to allow the standing 28 volt d.c, to
energize a relay. The closed contacts of the relay
t\}en complete a circuit to an amber ''overheat"
light, and also a light on a master caution panel.
If the local temperature should continue to
increase and reach 3 I 5 ± 30 °C, the pressure of the
released hydrogen in a detecting element of the
"fire" loop will trigger a signal to pass through a
circuit similar to that of the overheat loop but, In
this case, to illuminate red warning lights, and to
set off an alarm bell .
In the event that the pressure of the helium
inside a detector element should decrease, the
normally-closed pressure switch would open to cause
a udetector inoperative" light (not shown) to illuminate. Test circuits are therefore provided in the
system so that the integrity of the normally-closed
pressure switches can be checked.
In many of the larger types of transport aircraft, the
freight holds, baggage compartments and equipment
bays are often fitted with equipment designed for the
de.tection of smoke . Detection equipment varies in
constructfon, but in most cases the operation is based
on the principle whereby air is sampled and any
smoke present, causes a change of electric current
within the detector circuit to trigger a warning system.
Figure 10.19 is a schematic arrangement of a
smoke detector in use in some types of transport
aircraft. The principal detecting elements are a
Metal hydride·
coated rod
N.O .
Fig 10.18
Overheat and fire detection system
2&-V d.c,
: > - - - - + To ralav and
[email protected]!) light
11s-v _
61owll)t moro,
Fig 10,19
Smoke detector operation
pilot light, a light trap and a photo diode, disposed
in a compartment or chamber as &hown. The pilot
light and photo diode are powered by 28-volt d.c.
Sampling tubes connect with the detector and
plenwn chambers and a blower motor powered by
115-volt a.c. When the system is armed, the blower
motor draws air through the detector from the
compartnent in which the detector is located. The
pilot light directs a beam to the light trap . If smoke
is present to a level of 10 per cent, the light reflected
from it as it passes through the beam will be detected
by the photo diode . The current generated by the
diode is then amplified to trigger a relay the contacts
of which complete a circuit lo the appropriate fire
wamLng light. The light emitting diode (LED) forms
part of a test circuit which activates the photo
diode to simulate a smoke condition.
Fixed fire extinguishing systems are used mainly for
the protection of engine installations. auxiliary power
units, landing gear wheel bays and baggage compart·
men ts, and are designed to dilute the atmosphere of
the appropriate compartments with an inert agent
that will not support combustion. Typical extinguishing agents are methyl bromide, bromochlorodifluoromelhane, freon, or halco, and these are contained metal cylinders or "bottles'' of a specified
capacity. The agen ts are pressurized by an inert gas,
usually dry nitrogen, the pressures varying between
types of ex tinguisher, e.g. 250 lbf/in 2 for l 2 pounds
of.methyl bromide, 600lbf/in 2 for 4 pounds-of freon.
Explosive cartridge units which are fired electrically,
are connected to distributor pipes and spray rings, or
nozzles, located in the potential f1re zones. Electrical
power for cartridge unit operation is 28 volts d.c. and
is supplied from an essential services busbar ; the
circuits controlled by switches located in the
cockpit. When the cartridge unit is fired a diaphragm
is ruptured and the appropriate extinguishing agent
is discharged through the distributor pipes and
spray rings.
ln the fire extinguisher systems of some types of
aircraft, electrical indicators are provided to show
when an extingui&her has been ftrcd. An indicator
consists of a special type of fuse and holder connccte1
in the extinguisher cartridge unit circuit . The fuse
takes the form of a small match-head type charge
covered by a red powder and sealed within the fuse
body by a disc. A tran~parent cover encloses the top
of the fuse body and is visible through another cover
screwed on to the fuse holder.
The fuse·is secured in the fuse holder by-a bayonet
type fixing, and electrical connection to the charge
is by way of terminals in the fuse holder, contact at
the base of the fuse and the metal disc.
When current flows in an extinguisher cartridge
circuit. the appropriate fuse charge is fired. thereby
displacing the disc and interrupting the circuit. Al the
same time red powder is spattered on to the inside of
the cover thus giving a positive visible indication of
the firing of the extinguisher cartridge.
Ice and Rain Protection Systems
king on aircraft is caused primarily by the presence
in the atmosphere of supercooled waler droplets, i.e.
droplets at a temperature below that at which water
normally freezes. In order to freeze. water must lose
heat to its surroundings, thus when It strikes, say, an
aircraft wing, an engine air intake or a propeller, ther,
is metal to conduct away the latent heat and the
water freezes instantly. The subsequent build-up of
ice can change the aerodynamic shape of the particul,
form causing such hazardous situations as decrease
of lift, changes of trim due to weight changes, loss of
engine power and damage to turbine engine blading.
In addition, loss of forward vision can occur due lo
ice forming on windshield panels, and on ex1ernally
mounted units such as pitot probes, obstruction of tr
pressure holes will result in false readings of airspe·ed
and altitude, Therefore, for aircraft which are in·
tended for flight in ice.forming conditions, protective
systems rnust be incorporated to ensure their safety
and that of the occupants. Fig\lre 10.20 indicates
the extent to which protection may be required
depending on the type and size of aircraft.
In addition to ice protection systems, some
Aroas & units
engine air
in ~k s
Wing IC~ding
ec!ge slats
'1ir $COOPS
Ar;gle o
Water &
toilet dr3ins
Fig 10.20
lee and rain protection systems
protection must also be affo rded when flying in
heavy rain conditions in order to improve visibility
through cockpit windshields. is accom plished
by windshield wiper and rain repellent systems.
There are three methods adopted in the syslems
in common use and these together with their applica.
lions and fundamental operating principles are set out
in Table l 0.1. They are all based on two techniques,
known respecti vely as de-icing a11d anti·icing. Ln deicing, ice is allowed to build up to an extent which
will not serit,usly affect the acrodyrtamic shape and
is then removed by operation of the system; this cycle
is then conti nuously repealed, usually by a ti ming
device. lri anti-icing, the system is in operation continuously so that Ice cannot be allowed to form.
Electrical power and certain electrical components
are required in varying degrees for all the systems listed
in Table 10.1. ln fluid, hot air bleed and combustion
heating systems the requirements are fairly simple
sface it is usually only necessary to operate an electrical
pump, air control valves and temperature-sensing sys.
terns as appropriate. The requirements for pneumatic
boot systems are also fairly simple, although the
number of air control valves Is increased proporllon·
ately to the number of boot sections necessary and an
electronic timer is used.
In what may be termed "pure electrical hea ting
systems", the application of electrical power and
components is much wider and as a result the systems
are of a more complex nature.
Details of the ice and protection systems applied
to some representative types of aircraft are tabulated
in Appendix 9.
The pneumatic method of de-icing the leading edges
of wings, horizontal and vertical stabilizers is used i.n
several types of small and medium-sized transport
aircraft (see Appendix 9). fn general, systems are
similar in respect of their principal components and
overall operation, and we may consider as an example
the one shown in Fig. l 0.21, which is applied to the
Piper "Navajo'' (PA 3 IP).
f'ig 10.21
Pneumatic dc·iccr boot wstem
Pressurized air to cabin
Deicer Ii
Pneymetic air to
instrument end cabin
mounted ejec1or
To deicer boots
: \:',/
1. Deicer boots
2. Timer
3. Circuit breaker panel
4, Overhead switch pan!!I
5. Flexible connec1ion
6, Pneumatic line
7, Pneumatic pressure switch
8. Solenoid valve (regulator)
9. Ejector valve
10. Pneumatic regulator
11. Solenoid valve (deicer)
12. Relay
Table IO.I
Wings, s1abilizers,
propeUers, windshields
A chemical which breaks down the bond
between ice and water and can be either
sprayed over the surface, e,g. a windshield,
or pumped through porous panels along
the leading edge of a surface, c,g. a wing.
Wings, stabilizers
Sections ~f rubber boot along \he leading
edges arc ln11atcd and deflated causing
ice to break up and, with aid of U1e air•
stream, crack off.
Wings, stabilizers, engine
Hot air from turbojet en1tino compressors
passed ~long inside of leading edge
Hot aiI from a separate combust ion he3ter
or from a heat exchanger associated with
(a) Hot bleed
nlr Intakes
Combustion heating
(c) Eloculcal healing
Wlni;s, stabilizers
Wings, stabilizers, engine
air intakes, propeUers, '
helicopter rotor blades,
a turbine engine exhaust fM system.
Hea ling effect of electric current passing
through wire, nat strip or film type
De-icing is effected by de-icer "boots" which are
cemented to the leading edges of the appropriate
surfaces, and which, at controlled time intervals,
inflate to break up ice which has formed on them.
In some systems the boots are inflated in a specific
sequence, but in the example shown, all the boots
inflate simultaneously for a period of 6 seconds.
The boots are of fabric-reinforced rubber and contain
built-in Inflation tubes arranged spanwise (see
Fig. 10.22) which are connected to an air supply
system via solenoid-operated valves. In some types
of aircraft, the tubes are arranged chordwise so
that they minimize interference with the airflow over
t_he relevant surfaces. A thin conductive coating is
provided over the surfaces of the boots to dissipate
static chBiges.
Fig 10.22
Dc·icer boot operation
The air supply in the case of the aircraft considered Is derived from engine-driven pumps and ls
regulated at 18 psi, but in aircraft powered by turbopropeller engines, the air is usually tapped from an
engine main compressor stage and then regulated to
the desired pressure . During the deflation period of
the operating cycle, and also during flight under
no-icing conditions, it is necessary for the boots to
be held Oat against the leading edges of the appropriate surfaces, and this is achieved by connecting
the boots, via their solenoid-operated valves, to a
vacuum source. This is derived by passing air continuously overboard from the engine-driven pumps
or, engine compressor as the case may be, through
an ejector/venturi.
A system is made up of three principal sections:
healing elements, control, protection and indicating.
The power supplies normally required are 115 volts
to 200 volts a,c, for heating (although the propellers
for some light aircraft types and some winds!Jeld
panels operate on 28 volts d.c.), 115 volts a.c. and 28
volts d.c. for control and for other sections of a sys·
tern. Depending on the application, heating current
may be controlled to permit de.icing, anti-icing or
The heating elements vary in design and construction depending on the application. For propellers they
are of the fme wire type sandwiched in insulating and
protective materials which form overshoes selected
for maximum resistance to environmental conditions
and bonded to the blade leading edges. For propeller.
turbine engine air intakes, leading edges of wings,
and helicopter rotor blades, the elements are of the
"sheared foil" type, i.e. they are cut from thin sheets
of high·grade metal to specified lengths and widths
and within very close tolerances, The final resistance
values of the elements which are selected from such
metals as nickel, copper-nickel and nickel-chrome, are
usually adjvsted by chemical etching. The elements
are also sandwiched between insulating and protective
layers to form overshoes or mats.
· Figure J0 .23 illustrates one example of a propeller
and air intake de-icing system. Electrical power, at
200 volts a,c and variable frequency, is supplied to
the propeller blades and spinner, via brushes and slip
rings and a cyclic time switch, so that during the
Propeller blades
H i~ beyond the scope of this book to go into the
construction and operating detail of any one specific
system, but the following details, although of
general nature only, may nevertheless, be considered
as typical.
Inner a,d ovt~ r ,urfoces
fig 10.23
Propeller and a.Ir in ts.kl} de·icing system
de-icing part of the cycle, heat is applied to all four
blades simultaneously. It is unnecessary to de-ice the
whole of each blade, as kinetic heating allied to
centrifugal force no1IDally keeps the outer halves
free from Ice.
The air intake elements are arranged so that those
positioned at the leading edges are continuously heated,
i.e. they perform an anti-icing function, while those
on the inner and outer surfaces are supplied via the
cyclic time switch and so perform a de-icing function.
In order that ice may be shed in reasonably.sized
sections, the leading edge heating elements are extended at intervAls to form "breaker" strips. The
resistance of the elements is graded to provide for
various heating intensities required at different parts
of the air Intake,
The heating element arrangements adopted in
another current type of turbopropeller engine, are
shown in Figs. 10.24 and 10.25. In this case the
power supply for heating is ·2s volts d .c.
Huter elament
Hooter alement
Fig 10.24
Engine air intake anti-icing
-- --------
Fig 10.25
Elcclrlcal healer elements - propeller de-icing
For windshields or other essential clear vision panet
in cockpits, a transparent metal mm type of element
is employed in the majority of applications, the
metal being either stannic oxide or gold. Panels are
of laminated construction, and in order to provide
rapid heat transfer the metal furn is electrically
deposited on the in~ide of the outer glass layer. It
is protected from damage and completely Insulated
by further layers of polyvinyl butyral, glass and/or
acrylic. Heating current, normally from an a.c. source,
is supplied to the film by metal busbars at opposite
edges of the glass layer. The power necessary to deal
with the most severe icing conditions is in the order
of 5- 6 watts/in 1 of windshield area.
In systems applied to the windshields of some
small types of aircraft, heating elements made up of
fine resistance wire are used and are connected to a
28.volts d.c. power source.
Windshield systems are essentially anti,icing sys.
tems for, in addition to the protective function, the
temperature of the panels must be higher than ambien
during take-off, flight at low altitudes and landing,
thus 111aking them "pliable" and thereby improving
their impact strength against possible collision with
Temperature Control Methods
In view of the high amounts of power required for
the foregoing electrical heating rnethods, it is essential
lo provide each system with apµropriate controlling
circuits and devices. Although there are a number of
variations between systems and between designs
adopted by different manufacturers. from the poin l
of view of primary functions they are more or less
the same, i.e. to cycle the power automatically, lo
detect any overloading and to isolate power supplies
under specific conditions.
Figure 10.26 represents the supply and control
circuit for the engine air intake and propeller shown
in Fig. 10.23.
When the system is switched on, direct cunent
energizes the power relay via closed contacts in the
overload sensing device, thus allowing the 200 volts
a.c. to flow directly through to the continuously
heated elements and up to the time switch (see also
p. 104). This unit is energized to run either "fast" or
"slow'' by a selector switch, the settings being
governed by outside air temperature and severity of
icing. [n this case, ''fast" is selected at temperatures
between +10°C and -6°C and the dura lion of the
"heat on" and "heat ofr' periods of the cyclic heated
elements is short compared with '!slow'', which is
selected at temperatures below -6°C. The cycling is
usually controlled by cam-operated microswitches.
An indication of lime switch operation is provided by
a flashing blue or green light on the control panel,
while a general indication that the correct power is
being applied to the whole system is provided by an
ammeter connected to a current transformer (see
also p. 61) across the generator busbar.
In the event of an a.c. overload, the heater clements
are protected by the sensing device which is actuated
in such a manner that it Interrupts the d.c. supply to
the power relay, this in turn interrupting the supply
of heating current. The current balance relay fulfils
a similar function and is actuated whenever there is
an unbalance between phases beyond a predetermined
For ground operation of the system described ,
It is usual for the applied voltage to be reduced i11
order to prevent overheating, This is effected by the
automatic closing of a microswitch fitted to a landing
gear shock-strut, the switch permitting direct current
to flow to a reduced voltage control section within
the generator voltage regulator.
The circuit of a d.c. powered system as applied
to the air intake illustrated in Fig. 10.24, is shown
in Fig. 10.27. When the system is switched on, the
control relay is energized by the 28-volt supply
having to pass to ground through the closed contacts
of a thermostat and through the contacts of the
engine oil pressure switch which closes when the
pressure reaches SO psi. The supply for the heater
elements which is rated at 500 watts, then passes
through the energized relay contacts.
The thennostat prevents overheating of the
heater element by opening the circuit when the
element temperature reaches 49 ± 3 °C (120 ± S °F).
The oil pressure switch opens the circuit when as a
result of engine shutdown the oil pressure falls below
50 :I: 2 psi. Functioning of the heating circuit is
Indicated by an annunciator light on the system
control panel. The light is illuminated by a currentsensing relay which is in series with the heater
element, and energizes when heater current is above
IS amperes.
200· Vo.c.
28-v d.c.
L.ond,ng gear
D,C lo power reJoyond current bolonee ,cloy
to •educed
volloge sect.on of 011e,no1or
cont rQl unll
r- ·
- - · -Air-tnroke
- -elements
- · - - -~
Fig 10.26
Engine itlr' intake and propoUor dB-icing and an ti-icing
control citcult
Propeller elements
Conlrol ,witch
' : i-1''
He!lter olement
Control relay
1---.,I - ~
Fig 10.27
,,..-t,,- - -- - - - - '
6 'J
l_ ___
Engine afr int.tke 11nU·icing system
An example of a propeller de.icing system utilizing
28-volt d.c. power for heating and control is shown
in Fig. lO .28, and is one which is applied to several
types of small twin-engined aircraft.
The propeller blades each have two heater
elements bonded to them; one at the outboard
section of a blade and the other at the inboard
soclion. The elements are connected to the power
supply via slip rings, brushes and an electrically.
operated timer which is common to both propellers.
The cycling sequence of the timer is set so that (i) the outboard elements of each propeller are simultaneously heated before the inboard elements, and
(ii) only one propeller is de-iced at a time. The
sequence for the right-hand propeller is shown at
(a) and (b) of Fig. 10.28 respectively , The segments
3 and 4 respectively cotU1ect the supply lo the out,
board and inboard elements of the left-hand
propeller. The timer energizes the elements for
approximately 34 seconds and repeats the cycle as
long as the control switch is in the "on" position.
Operation uf the system is indicated by the
ammeter, the pointer of which registers within a
shaded portion of the ammeter scale corresponding
to current consumed (typically between 14 and
18 amperes) at the normal system voltage.
The control methods adopted for windshield antiicing systems are nonnal1y thermostatic, and a typical
Fig 10,28
Propeller de-icing system
system (Fig. 10.29) consists of a temperature-sensin1
element and a control unit. The element is embeddec
within the panel in such a way that it is electrically
insulated from the main heating film and yet is
capable of responding to its temperature changes
without any serious· lag. A control unit comprises
mainly a bridge circuit, of which the sensing element
forms part, an amplifier and a relay. When all the
required power is switched on initially, the control
unil relay is energized by an unbalanced bridge signal
and the power control relay is energized to supply the
windshield panel. ~ the· panel temperature begins to
increase, the sensing element resistance also increases
until at a predetermined controlling temperature (a
typical val~ is 40°C) the current flowing through the
sensing element halances the bridge circuit, and the
control unit and k.ower control relays are de-energized,
thereby interrupting the heating current supply. As
the temperature cools the sensing element resistance
decreases so as to unbalance the bridge circuit and
thereby restore the heating current supply. In a num·
ber of aircraft types the windshields are each fitted
with an additional overheat sensing element which in
the event of failure of the normal sensing element
takes over its function and controls at a suitably higher
temperature; 55°C is a typical value.
Despite aceurate control during manufacture slight
variations in healer film Tesistance, and consequently
glass temperature,.can occur. Sensing clements are,
therefore, individually embedded ln each panel al one
of the hotter spots but where it least affects visibility,
In some types of aircraft, windshields are heated
by resistance elements of fine wire supplied with
28 volts d.c, Temperature sensing and control of heating current is carried out by a control unit operating
on a similar principle lo that already described ,
Hot-Air Bleed Anti-Icing Systems
Systems of this type are standard principally on the
larger typ~s of public transport aircraft , for the antiicing of engine air intake nose cowlings, wing leading
edges and leading edge devices such as slats and flaps
(see Appendix 9).
The hot air is bled from certain stages of main
engine compressors and is then ducted through metal
ducting to the air intakes and leading edges. As far :lli
the use of electrical power is concerned, this Is
requirad solely for the.operation of motorized
control valves in the ducting, valve position indicating
lights , and duct temperature sensing devices. The
motors arc limit switch controlled at the full open
and closed positions, and in most applications they
200•V o,c. tieot,ng
s.--r-- - 0~=---·1-b-i~
- ~o'.~
Tempe,ot v,u con trol ~nil
5ens1ng elefT\ent
w,ndsNeld poeel
Fig 10.29
Schematic arrangement of windshield lillti•icing control system
are of the 115-volts single-phase a.c. type. A power
supply of 28 volts· d.c. is used for valve control relay
switching and position indicating light circuits.
The d.c. supply for valve control relay switching
passes through a landing gear shock-strut micro.
switch so that when an aircraft is on the ground the
anti-icing system cannot be operated as in the normal
in-flight situation. A ground test switch circuit is
therefore provided to check the operation of valves
and position indicating lights.
Ice Detection Systems
These systems consist mainly of a sensing probe
located at a strategic point on an aircraft (usually the
front fuselage section) and a warning light, thei r
purpose being to give adequate warning, and an
assessment of the likely severity of an impending
icing hazard In sufficient lime for the ice protection
systems to be brought Into operation . Detectors
are made· in a variety of forms, and In those most
commonly used actuation of the warning circuit ls
triggered off by ice accretion at the sensing probe.
In one type of system ice accretion causes a
drop in pressure sensed by the probe and a diaphragm,
the deflections of which make a circuit to the warning-light and to a heater the probe. When
the ice has melted the warning light and heater
circuits are interrupted and the system is reset for
further lee detection .
A second type of system is designed to give a
warning and also automatically switch on airframe
and engine de-icing systems. It consists of an a.c.
motor-drivan rotor which rotates in close proximit y
to a knife.edge cutter, a time delay unit and a warn28VI .C
ing lamp. Under icing conditions ice builds up on the
rotor and closes the gap between it and the culter.
This results in a substantial increase in the torque.
loading on the detector motor, causing it to rotate
slightly in its mounting and to trip a microswitch
inside the detector, Tripping of the microswitch
completes the circuit to the warning light and time
delay unit wllich initiates operation of the de-icing
systems. These conditions arc maintained until the
icing dimini.shes to the point whe reby the knife.
edge cutter ceases to "shave" ice, and the rnicroswitch
ls returned to the open circuit condition. The deteclor
unit Is designed to provide a two minute interval be.
tween the cessation of an ice warning and shut down of
a de-icing system, to prevent continuous interruption
of the system during intermittent icin& conditions.
In a third type of system, the fundamentals of
operation are dependent on the phenomenon of
magnetostriclion, i.e . its sensing probe is caused
to vibrate axially when subjected to a magnetic field
at specific frequencies. The function of the system
is shown in Fig. 10.30,
The sensing probe is a ~-inch dlamcter nickel
alloy (Ni-Span C) tube mounted at its mechanical
cenlrc. The inherent resonant frequency of the probe
is inversely proportional to its length, the simplified
relationship being expressed as f =
f = frequency in Hz, S "' I ·88 x I0 inches per
scconct"(the speed of sound in the tube material) and
L .::: length of the probe in in ches. Based on this
expression , the tube may be cut to a specific length to
ach.ieve a desired frequency; in this particular system
the designed tube length is 2 ·3 inches, resulting in a
11rt>Y I .C
Functional diagram of ultrasonic pwbc ws tcm
resonant ultrasonic frequency of 41 kHz. This
frequency , however, is reduced to a nominal 40 kHz
by the brazing of heating elements within the tube
and also by capping the tip of the tube. The probe
is maintained in its axial vibration by the ultrasonic
frequency excitation current produced by an
oscillator and passed through a drive coil wound
around the probe . The frequency is controlled by
a feedback coil circuit such that the drive coil will
excite the probe at whatever the natural frequency
of the probe might be at the time. When Ice fonns
on the probe th'e natural frequency is reduced, and
the output frequency of the oscillator drive coil is
in turn reduced to match the probe frequency . By
means of a comparator circuit, the lower output
frequency Is compared with a f1Xed frequency output from a reference oscillator. The frequency
difference between the two osciliators relates to the
Ice fonnation on the probe, and when the difference
ha_s reached a preset level (1 SO Hz or less) determined
by a band pass mter and a limiting amplifier, a signal
is sent to a switch and delay circuit. When this occurs,
two timer circuits are triggered: one controlling the
a.c. supply via a logic AND gate, to the probe heater,
and the other controlling the duration an Icing signal
Is available to an annunciator light for warning the
flight crew. Thus, as will be noted from Fig. 10.30,
there is a standing logic 1 input to the AND gate
from the 11 S-volt bus, so when timer "A" -is triggered
it will supply a second logic I input to the gate
causing it to switch on the heate.r for a period of
4.5 seconds. The signal from timer "B" is 28 volt
d.c. an9 keeps the annunciator Ught illuminated for
a period of 60 seconds. Melting of the Ice from the
probe increases the frequencies of the probe, and If
no other icing signal is detected within 60 seconds,
timer "A" auto11Jatlcally resets to Isolate the heater
from the a,c. supply. This cycle of operation ls
repeated while icing conditions prevail.
Failure monitoring of the detector is accomplished
with unijunction oscillators which are set at both
ends of the ma.xirnum difference frequency band. Jf
the probe becomes severely damaged causing a
significant change in the resonant frequency, or lf an
electronic component failure causes a malfunction
in the reference frequency circuit, the annunciator
tight will be continuously illuminated .
Landing Gear Control
In a number of the smaller types of aircraft having a
retractable landing gear system, the extension and
retraction of the main wheels and nose wheel, is
accomplished by means of electrical power. Fig. 10.31
is a simplified circuit diagram of a representative
control system.
The motor is of the series-wound split-field type
(see also pp. 137 and 138) which is mechanically
coupled to the three "leg" units, usually by a gearbox,
torque shafts, cables, and screw jacks, The 28 volts d.c.
supply to the motor is controlled by 3'Selector switch,
relay, and switches in the "down-lock" and ''up-lock"
circuits. A safety switch is also included in the circuit
to prevent accidental retraction of the gear while the
aircraft is on the ground. The switch is fitted to the
shock-strut of one of the main wheel gear units, such
that the compression of the strut keeps the switch
contacts in the open position as shown in the diagram.
~'-'"N:1,11u lcGNtciqr,1
•~;;:::::,::: 1·101'
Fig 10.31
Landing gear control system
After take-off, the weight of the aircraft comes off
the landing geRr shock-struts, and because they have
a limited amount of telescopic movement, the strut
controlling the safety switch causes it to close the
switch contacts. Thus, when the pilot selects "gear up",
a circuit is completed via the selector switch, and
closed contacts of the up-Jock switch, to the coil of
the relay which then completes the supply circuit to
the "up" winding of the motor, When the landing gear
units commence retracting, the down-lock switch Is
automatically actuated such that its contacts will
also close, and will remain so up to and in the fully
retracted position. As soon as this position i8 reached.
the up-lock switch is also actuated so as to open its
contacts, thereby Interrupting the supply to the
motor. and the ''down" winding circuit of the motor
is held in readiness for extending the landing gear. As
and when the appropriate selection is made, and the
landing gear units commence extending, the up-lock
switch contacts now close and when the landing gear
is down and locked, and the aircraft has landed, the
circuit is again restored to the condition shown in
Fig. 10.31.
To prevent over-run of the motor, and hence overtravel of the landing gear units, some form of braking
is necessary. This is accomplished in some cases, by
incorporating a dynamic brake relay in the circuit.
The relay operates in such a manner that during
over-run, the motor is caused to function as a
generator, the resulting electrical load on the
armature stopping the motor and gear instantly.
Landing Gear Position Indication
In retractable landing gear systems, it is, of course,
necessary to provide some indication that the main
and nose landing gear units are locked in their retracte<
positions during flight, and in their extended positions
safe for lanwng. The indication method most widely
adopted is based on a system of indicating lights which
are connected to microswitches actuated by the up·
lock and down-lock mechanisms of each landing gear
unit. To guard against landing with the landing gear
retracted or unlocked, a warning horn is also incor·
porated in the indication system. Toe horn circuit is
activated by a microswitch the contacts of which are
made or broken by the engine throttle. Fig. 10.32
illustrates a typical circuit arrangement.
The system operates from a 28 volts d.c. power
supply which is connected to lamps the indicator case, and also to the up-lock and down·lock micro·
switches of the main and nose landing gear units. Thre1
of the lamps are positioned behind red screens, and
three behind green screeens; thus, when illuminated
they indicate respectively, ''ge ar up and locked" and
''gear down and locked". In the ''gear up and locked''
position a!J lights are extinguished. In the event of
failure of a green lamp ntament, provision is made for
switching-in a standby set of lamps.
The circuit as drawn, represen ts the conditions
when U1e aircraft is on the grow1d in a completely
static condition. As soon as power goes onto the bus,
bar, the three green lamps will illuminate because
lndi~Q tor
Tu1 swlleh
Fig 10.32
Landing ge:u position indicating system
, ·-·-·-·- ·-·-·-·-·-- ·-·-·-·-·1
dcloelion and
~l• ·S~!dconlrol unrt
. · ·-
_._ ,_J
·-·~·-·,-- ---.J- ·-·-··-·1
- - - ------t- --·1
- - - _ __J
Syslem controllinQ wheel oner broke on the olher moon 1ondin9
goar unol
Fig 10.33
Anti·skid control systern
their circuits are completed to ground via the left-hand
In the static condition shown in Fig. 10.32, the
set of contacts of the correspondfog down-lock micro•
throttle microswitch is closed, but the warning horn
switches, The engine throttle is closed, and although
will not sound since the circuit Is interrupted by all
its microswitch is also closed, the warning'horn circuit
three down.Jock microswitches. Similarly, the circuit
is isolated since there is no path to ground for current
will be interrupted by the throttle microswitch which
from the busbar. Assume now that the aircraft has
is opened when the throttle is set for take-off and
taken off and the pilot has selected "landing gear up";
nonnal cruise power. In the case of an approach to
the down-lock mechanisms of the gear units aie disland, the engine power is reduced by closing the
engaged and they cause their mlcroswitches to change
throttle to a particular approach power setting and
contact positions, thus interrupting the circuits to the
this action closes the throttle microswitch. If, in this
green lamps. At the same time, the red lamps are
flight condition, the landing gear has not been selected
illuminated to indicate that the gear units are unlocked, down in readiness for landing, then the warning horn
the power supply for the circuit passing to ground via
will sound since the circuit to ground is then com•
the up-lock switches, and the right-hand contacts of
pleted via the right-hand con tacts of the down-look
the down-lock switches. When the landing gear urtits
mlcroswitches. After selecting "down'', the horn con·
·each their retracted positions, the up-Jock mechanisms tinues to sound, but it may be silenced by operating
ire engaged'and cause their microswitches to interrupt
a push switch which, as will be noted from the diagram,
the circuits to the red lamps; thus, all lamps are exenergizes a relay lo interrupt the horn circuit. 111e relay
tinguished. When the pilot select$ "landing gear down", incorporates a hold-in circuit so that it will remain
the up-lock mechanisms now dJsengage and the microenergized until the d.c. power supply is finally switched
:wi tches again complete the circuit to the red lamps
off. Functional testing of the horn circuit on the
to indicate an unlocked condition. As soon as the gear
ground, and under engine static conditions, may be
mits reach the fully extended position, the down-lock
carried out by closing the throttle and its microswitch,
nechanisms engage and their microswitches revert to
and then operating a test switch.
the original position shown in Fig. 10.32 i.e., red
Anti -Sk id Control Systems
.amps exlingulshed, and green lamps illuminated to
ndicate "down and locked".
The braking systems of many large transport aircraft
As noted earlier, a warning hom ls included in tlw provided with a means of preven ling thn main
:ystem, the making and breaking of the horn circuit
landing gear wheels from skidding on wet or icy
>eing controlled by·a throttle·operated microswltch.
surfaces, and of ensuring that optimum braking effect
can be obtained under all conditions, by modulating
the hydraulic pressure applied to the brakes. Funda·
mentally, an anti·skid system senses the rate of change
of wheel deceleration, decreases the hydraulic pressure
applied to the brakes when there is an impending skid
condition, and restores the pressure as the wt.eel
accelerates again.
A number of current anti•skld systems utilize
electrical power, and typically a complete system
consists of a number of transducers (one for each
wheel of each main landing gear unit), an anti.skid
control unit containing the requisite number of
individual circuits, and electro-hydraulic anti·skid
connol valves corresponding to the number of trans·
ducers, and con trol circuits. A block diagram of one
such system is illustrated in Fig. I0.33; for simplicity
of explanation the diagram and description that
follows, relate to single wheel operation.
The transducer is a speed sensing device and consists
of a stator which is firmly attached to the wheel axle,
and a rotor which is attached to, and rotates with, the
wheel. The stator contains a permanent magnet, and
when the wheel and rotor are rotated, the magnetic
coupling, or magnetic reluctance, between the rotor
and stator,is varied. The variation generates within
the stator, an a.c. voltage signal which is directly
proportional to the ,rotational speed of the wheel. The
signal is fed to the converter in the control unit, and
is converted to a d.c. voltage signal which serves as a
measure of the rate of wheel deceleration. The signal
is then applied to a skid control circuit and is com•
pared with a reference velocity signal which has been
predetermined from a known deceleration rate of the
aircraft. Any differences between the two signals produce error voltages which are processed to determine
whether or not a correction signal is to be applied to
the electro-hydraulic control valve. If wheel deceleration rates are below the reference velocity no conection signal is produced. If, however, the rates are above
the reference velocity, they are then treated as skids or
approaching skids, and correction signals are applied
to the control valve which reduces the hydraulic
pressure applied to the wheel brakes. When the wheel
speed falls below the reference deceleration rate, the
skid control unit transmits a release signal to the
control valve. Subsequent wheel "spin.up" causes the
brakes to be re•applied, but at a lower pressure deter·
mined by the length of time required for the wheel to
spin-up, Sensing circuits are also provided , and in con.
junction with the systems of the other wheels of the
landing gear, they detect and compare ''locked wheel"
conditions. In the event of such conditlons occurring
the circuits will cause signals to be applied to the
relevant control valves such that they will fully rclcas
brake pressure.
Windshield Wiper Systems
The circuit arrangement shown in Fig. 10.34 is typic.
of many of the windshield wiper systems currently
in use. The wiper anns and blades for each wind·
shield are actuated by their own 28,volt d.c. variable,
speed motors coupled to converters. Each motor is
supplied from different busbars and is controlled
by a four-position selector switch (in some cases the
switch may have six positions) and the Sf.leed
variation ~ccording to selection, is accomplished
by voltage dividing resistances.
[n the "low" position of the switch, voltage is
applied to the field and armature circuits of the
motor, and then to ground via a second con tact
of the switch and two resistors. The voltage is therefore reduced and the motor runs at a low speed, and
by means of its converter sweeps the wiper arm back
and forth. When the "high'' position of the switch
is selected, the supply passes to ground through only
one resistor and so the motor and wiper will operate
at a faster speed.
When the use of the wipers is no longer required,
the control switch is turned back through the "off'
position to a ''park'' position. There is no detent in
this position, and so the switch is manually held
there momentarily. As will be noted from Fig. 10.34
the supply voltage is iniUally applied to the motor in
the nonna1 way, but as the connection to ground is
now directly through the nonnally,-0losed contacts
of a brake switch within the motor, then it will run
at its fastest speed. As the wiper blade reaches Its
parked position, the motor operates a cam to change
over the brake switch contacts which then short out
the armature to stop the motor. The switch is then
released to spring back to the "off' position.
The purpose of the thermal switch is lo open the
motor circuit if the field winding temperature or
field current should exceed pre-determined values.
Typical values are 150 ° C (300 °F) and 8 to 10
amperes respectively.
Rain Repellent Systems
The purpose of these systems is to maintain a clear
area on the windshields of an aircraft during take-ofl
approach and landing in rain conditions. A system
consists of a pressurized container of repellent
28-V d.c.
r- Off
i-~ -------
Motor unit
~--------i------,--+-1~ I
I High
e_r_ak~ _ _.__ __,
Resistor unit
1 Park
Control Switch
Fig 10.34
Wlndshleld wiper system
fluid , control switch, a solenoid valve controlling
the supply of fluid to a spray nozzle mounted in the
fuselage skin in front of each windshield. The fluid
container is common to each,]d system and
ls located in the cockpit.
The operation of the system is illustrated in
Fig. 10.35. When the conlrol switch is pushed in,
a 28-volt d.c. supply is fed to the solenoid valve via
the closed contacts "B" of the control relay. The
spray nozzle solenoid is therefore energized to open
the valve and allow fluid to flow under pressure
through the spray nozzle and onto the windshield.
The fluid is of a type which causes the surface tension
in water to change so that the water is formed into
globules whlch are blown off the windshield by the
airstream. Through the action of a time delay circuit,
approximately 5 c.c. of fluid flows through the
nozzle for approximately 0.25 seconds.
At the end of this period, the time delay circuit
applies power to the gate of an SCR (see also p. S5)
which then energizes the control relay and In tum
de-energizes the spray nozzle solenoid valve. If the
control switch remains pushed in, the time delay
circuit will keep the control relay energized via a
hold-in circuit across the closed contacts "A", When
the switch is released, the Lime delay circuit and
SCR are returned to their original state.
The fluid is contained in a can which when
screwed onto the mounting bracket opens a valve
to allow fluid to drain into a reservoir and the system
tubing. The reservoir is a clear plastrc cyllnder containing a float-type contents indicator. A manually-
Spray noule
Tirne delay
\._ ___
- --· _I
Fig 10.35
Rain repellent system
operated shut-0ff valve is provided between the
reservoir and can and is used during can replacement.
Airconditioning Systems
These systems are designed to maintain selected air
temperature conditions within flight crew, passenger
and other compartments, and in general, they are
comprised of five principal sections : air supply,
heating, cooling, temperature control, and distribution. The operation of systems varies depending
on the size and type of aircraft for which they are
designed, and space does not allow for them all to
be described. However, if we take the case of most
of the large transport aircraft, we find that there
are a number of common features which may be
represented as shown in Fig. 10.36.
As in the case of hot air bleed anti-icing systems,
air is supplied from stages of the main engine compressors and serves not only to provide air conditioning but also pressurization of the cabin. Since
the air from the compressor stages is too warm for
direct admission to the cabin, it has to be mixed
with some cold air in order to attain preselected
temperature conditions. This is effected by directing
some of the bleed air through a cooling pack con sisting of a heal exchanger system and a cooling
tuibine or air cycle machine. The control of the
bleed air flow is accomplished by nn electrically.
controlled pack valve, which Is energized by a switch
on the system control panel in the cockpit. Down•
stream of the pack valve is a mix valve which has
the function of proportionately dividing the hot
air flow from the pack valve, and the cold air flow
from the afr cycle machine, into a mixing chamber.
The mix valve is of the dual type; both valves being
positioned by a common 11 S-volt a,c, actuator
motor. The valves are monitored by signals from
the temperature control system such that as one
valve moves towards its close position, the other
valve moves towards its open position.
The temperature control system is comprised
principally of a selector switch, regulator, and
temperature sensors located at selected points In
the system. The whole system operates automatically and continuously monitors the mix valve
position, but in the event of failure of the regulator,
mix·va1ve position may be carried out manually
from. the selector switch.
When the selector switch is in the ''auto" position
and at a desired cabin temperature, a potentiometer
within the switch establishes a reference resistance
value in an ann of a control bridge circuit of the
regulator. A cabin temperature sensor is in the
other arm of the control bridge circuit so that if the
cabin temperature is at a level other than that
selected, then the bridge will be unbalanced. As a
result, a signal is developed in the circuit of the mix
valve motor so that it will drive the valves to either
a hot or cold position, as required, to attain the
selected cabin temperature. At the ~ame time,
conditioned air is sensed by an anticipator sensor,
and a limit sensor both of which are located in the
ducting to the cabin, and are connected in an
electrical bridge configuration. The purpose of
the sensors is to modulate any rapjd changes
demanded by an unbalanced control bridge so that
when the actuator control moves the mix valve it
will produce cabin temperature changes without
sudden blasts of hot or cold air, and without raising
duct temperatures above limits,
To prevent the mix valve staying at a "too hot"
position, a thennal switch which ls set at a particular
level (e.g. 90 °C (195 °F)) is located in the ducting
to complete a circuit to the mix valve so that lts
motor will run the valve to the full cold position.
At the same time a "duct overheat" light is
r.,_==--"""'."9 from
r - - - -- - -- --
- --28·V d.c.
Anticipator sensor
..... Hot air m:a::.;,. Cooled air
==:>Cold a i r ~ Conditioned air
Fig 10.36
Aircondltloning syst~m
illuminated. After the overheat condition has been
corrected, the system may be returned to nonnal
by means of a reset switch. Another thermal switch
set to close at a rug.her level (e.g. 120 °C (250 °F))
protects against duct overheat should power control
be lost. It completes a circuit which closes the pack
valve and illuminates a "pack trip off' light. The
system may be returned to normal after the.trip
condition has been corrected, by operating the reset
switch referred to above.
Manual control of the system is effected by
moving the selector switch to ''cool" or "warm" to
directly actuate the mix valve as appropriate.
Propeller Synchronizer Systems
These systems are used in some types of twin-engined
aircraft, their purpose being to automatically
synchronize the r.p.m. of the propellers. This is
accomplished by utilizing the speed governor of
one propeller as a master wlit, and the governor of
the second propeller as the slave unit. Both governors
have magnetic pick-ups which supply electrical pulses
to a control unit which detects any difference in the
frequency of the pulses. The resulting output from
the control unifis fed to a stepping type motor
actuator mounted on the slave governor which is
then "trimmed" to maintain its propeller
at the same value as the master governor unit, and
within a limited range. The limiting range of operation
is built into the synchronizer system to prevent the
slave governor unit from losing more than a fixed
amount of propeller r.p.m. in the event of the master
engine and propeller being "feathered" when the
system is in operation,
Before the system is activated, the r.p.m. of each
propeller is manually synchronized as close as
possible. When this has been done and the system
is then activated, a maximum synchronizing r.p.m.
range (typically± 67) is effective.
Passenger Cabin Services
In passenger transport aircraft electrical power is
required within the main cabin compartments for the
service and convenience of the passengers, the extent
of power utilization being governed of course, by the
aircraft size and number of passengers it is designed to
carry. Apart from the main cabin lighting referred to
on page 152 it is necessary to provide such additional
services as individual reading lights at each seat posi,
tion, a cabin attendant call system, public address
system and a galley for the preparation and serving
of anything from light refreshments to several fullcourse meals. In.flight cinema entertainment also
accounts for the utilization of electrical power In
many types of aircraft.
Reading lights may be of the incandescent or
fluorescent type, and are located on passenger
service panels on the underside of hat racks, or in each
seat headrest and are controlled individually. Cabin
attendant call systems are interlinked systems com·
prising switches at each passenger service panel con•
nected to an electrical and indicator light at
the cabin attendant's panel station. The service panel
switches are of the illuminating type to visually indi·
cate to the cabin attendant the seat location from
which a call has been made. In addition the system
provides an interconnection between the flight crew
compartment and cabin attendant's station.
A public address system is provided for giving
passengers instructions and route information, and
usually comprises a central amplifier unit and a number of loudspeakers concealed at various points
throughout the cabin, and In toilet compartments.
lnformation is given. as appropriate, by the aircraft's
captain or cabin attendant by means of separate
telephone type handsets connected to the loud·
speakers. Tape-recorded music may also be relayed
through the system during passenger embarkation and
Galley equipment has a considerable teclmical
Influence on the design of an aircraft's electrical
system, in that it represents a very high percentage
of the total system power requirement, and once
installed it usually becomes a hard-worked section of
an aircraft. The type of equipment and power loading:
are governed by such factors as route distances to be
flown, number of passengers to be carried and the
class configurations, i.e. ''economy", "11rst,class" or
"mixed". For aircraft in the "jumbo" and "wide·
bodied" categories, galley requirements are, as may
be imagined, fairly extensive. In the Boeing 747 for
example, three galley complexes are installed in the
cabin utilizing both 28 volts d.c. and 115 volts a.c.
power and having a total power output of 140 kV A;
thus, assuming that the generator output is rated with
a power fact or of unit y, the equivalent d.c. output is
140 kilowatts or in terms of horsepower approxi·
mately 187 ! The galley unit of the wide-bodied Lock·
heed "Tristar" is also a complex unit but is located as
a central underfloor unit. It also utilizes d,c. and a.c.
power not only for heating purposes but also for the
operation of lifts which transport service trolleys to
cabin floor level.
The equipment varies, some typical units being
containers and hot cups for heating of beverages, hot
cupboards for the heating of pre-cooked meals and ovens
for heating of cold pre-cooked meals, a number of
which may have to be served, e.g. on long-distance
flights. Other appliances required are water heaters
for galley washing-up and toilet washbasins, and refrigerators. In most cases, the equipment is assembled
as a self-contained galley unit which can be "plugged
in" at the desired location within the aircraft. ·
It is usual for the electrical power to be supplied
from the main distribution systems, via a subsidiary
busbar and protection system, and also for certain
galley equipment to be off-loaded in the event of
failure of a generating system. The load-shedding
circuit Is automatic in operation and any override
system provided is under the pilot's control; on some
aircraft load.shedding is also controlled via a landing
gear shock-strut microswitch thereby conserving
electrical power on the ground. The control panel or
panels, which may be mounted on or adjacent to the
galley unit, incorporates the control switches, indi,
cater Ughts and circuit breakers associated with each
item of galley equipment, and also the indicator
lights of the cabin attendant call system.
Electrical Diagrams and
Identification Schemes
As in all cases involving an assembly, interconnection
or maintenance, of a number of components forming
a specific system, a diagram is required to provide the
practical guide to the system . Alrcraf~ electrical instal.
lations are, of course, no exception to this requirement
and the relevant drawing practices are specialized subjects necessitating separate standardization of detail to
ensure unifonnity in preparation and presentation.
The standards to whlch all diagrams are nonnally
drawn are those down by appropriate national
organizations, e.g. the British Standards Institution,
Society of British Aerospace Constructors (S. B.A.C.)
and in Specification I 00 of the Air Transport Association (A.T.A.) of America. The ATA IOO system has
a much greater application internationally than any
other. There are usually three types of diagram
produced for aircraft namely, circuit diagrams, wiring
diagrams and routing charts.
Circuit Diagran'/$. These are of a theoretical nature
and show the internal circuit arrangements of electrical
and electronic components both individually and
collectively, as a complete distribution or power con•
sumer system, in the detail necessary to understand
the operating principle of the components and system.
Circuits are normally drawn in the "aircraft-on-the.
ground" condition with the main power supply off, In
general, switches are drawn in the "orr' position, and
aU components such as relays and contactors are
sl~own i-n their demagnetized state. Circuit breakers
are drawn in the closed condition. In the event that
it is necessary to deviate frorn these standard conditions,
a note is added to the diagram to clearly define the con·
ditions selected.
Wiring Diagrams. These are of a more practical
nature in that they show how all components and
cables of each individual system making up the whole
installation, are to be connected to each other, their
locations within the aircraft and groups of figures an
letters to indicate how all components can be identi·
fled directly on the aircraft.
Routing Charts. These charts have a similar functi
lo wiring diagrams, but are set ou t in such a manner
that components and cables arc drawn under
"location" hcadin~ so that the route of distribution
can be readily traced out on the aircraft. ln some
cases, both functions may be combined in one diagr:
(see Fig. l l.l ).
Wiring diagrams and routing charts are provided fc
the use of maintenance engineers to assist them in
their practical tasks of testing circuits, fault finding
and installation procedures. The nu mber of diagram~
or charts required for a particular aircraft, obviously
depends on the size of the aircraft and its electrical
installation, and can vary from a few pages at the en,
of a maintenance manual for a small light aircraft, tc
several massive volumes for large transport aircraft.
Coding Schemes
As an aid to the correlation of the details illustrated
In any particular diagram with the actual physical
conditions, i.e. where items are located, siies of
cables used, etc., aircraft manufacturers also adopt a
identification coding scheme apart from those adopt
by cable manufacturers. Such a scheme may either b
to the manufacturer's own specification, or to one
devised as a standard coding scheme. In order to
illustrate the principles of schemes generally, some
example appl:ications of one of the more widely ado
ted coding standards will be described.
In this scheme, devised by the Alr Transport Ass~
ation of America under Specification No. l 00, the
N~2 d.C.~
f.l);r2WG2C 22 .
l•)Otol 2WGIA22t-L+D-:l--',-D-+'- - -- ~ ~
2WG3A22N ~ >8'-.~=;;..___ , . - - - · -- --==-2W(;2622----r-+c:;,h - ·- -t--~,q
intake overneot
warning lamp
Temnerolv re
sens,t1vc switch
i......-----.---- - - -
L------'--- -- --
2WG1822---,----,---tt----.- -'
2WG2A22- --,--.......H-- . - - - --"
Fig 11.1
Routing chart
coding for cable Installations consists of a six·position
combination of letters and numbers which is quoted
on all relevant wiring diagrams and routing charts and
is imprinted on the outer covering of cables. In cases
where the code cannQt be affixed to a cable it Is
printed on non-metallic sleeves placed over lhe ends
of the cable. The code is printed at specified Intervals
along the length of a cable by feeding it through a
special printing machine. The following example serves
to illustrate the significance of each position of the
Position J. The number in this position is called the
unit number and is only used where components have
identical circuits, e.g., the components of a twin
generator system. In this case number I refers to the
cables interconnecting the components of the first
system. The number is omitted from cables used
Position 2. In this position, a letter is used to indicate
the function of the circuit i.e., it designates the circuit
or system with which the cable is connected. Each
system has its own letter. When the ci rcuit is part of
radar, radio, special electronic equipment, a second
letter is used to further define the circuit.
Position 3. The number In this position is that of U1e
cable and is used to differentiate between cables wh.ic~
do not have a common terminal in the same circuit.
In this respect, contacts of switches, relays, etc. arc
not classified as common terminals. Beginning with
the lowest number and progressing in numerical order
as far as is practicable, a different number is given to
each cable.
Position 4. The letter used in titls position, signifies the
segment of cable (i.e., that portion of cable between
two terminals or connections) and differentiates
between segments in a particular circuit when the
same cable number is used throughout. When practicable, segments are lettered In alphabetical sequence
(excluding the letter ''I'' and "O") the letter" A''
identifying the first segment of each cable, beginning
at the power source. A different letter is used for
each of the cable segments having a common terminal
or connection.
Position 5. In this pdsltion, the number used indicates
the cable size and corresponds to the· American Wire
Gauge (AWG) range of sizes. This do~s not. apply to
coaxial cables for which the number 1s om1ltcd, or to
thermocouple cables for which a dash ( - ) is used as
a substitute.
Position 6, In position, a letter indicates whether
a cable is used as a connection to a neutral or earth
point, an a.c. phase cable, or as a thermocouple cable.
TI1e letter "N'' indicates an earth-connected cable, the
letter "V" indicates a supply cable in a single-phase
circull, while in three-phase circuits the cables arc
identified by the letters "A", ''B" and "C". Thermocouple cables are identified by letters which indicate
the type of conductor material, thus: AL (Alumcl);
CH (Chrome!) ; CU (Copper): CN (Constantan).
TI,e practical application 01 the coding scheme may
be understood from Fig. 11.1 which shows the
wiring of a very simple temperature sensing switch
and warning lamp system .
The system is related ~o the No. 2 engine ai~.inta,~e.
its circuit function is designated by the lell ers WG ,
and it uses cables of wire size 22 throughout. Starting
from the power source i.e., from the No. 2 d.c. busbar,
the first cable is run from the fuse connection 2,
through a pressure bung to terminal I of the switch.;
thus the code for this cable ls 2 WG l A 22. Terminal
J als'o serves as a common power supply connection to
the contact 2 of the press-to.test facility in the warn·
ing lamp; therefore, the interconnecting cable which
also passes through a pressure bung, ls a second seg•
rnent cable and so its code becomes 2 WG I B 22.
Terminal 2 of the switch serves as a common con·
necti~~ for the d.c. output from both contact 1 of
the facility, and the sensing switch contacts, and as the cables are the second pair in the circuit and respectively f1rst and second segments, their
code numbers are 2 WG 2 A 22 and 2 WO 2 B 22. The
cable shown going away from the B+ terminal of the
lamp, is a third segment connecting a supply lo a
lamp in a centralized warning system arJd so accordingly carries the code 2 WG 2 C 22, The circuit is completed via cable number 3 and since it connects to
earth it carries the full six-posifion code ; thus, 2 WG
2 A 22 N.
The coding schemes adopted for items o[ electrical
equipment, control panels, connector groups, junction
boxes, etc, are related to physical locations within the
aircraft and for this purpose aircraft are divided into
electrical zones. A reference Jetter and number are
allocated to each zone and also to equipment,
connectors, panels etc., so that they can be identified
within the zones. The reference letters and numbers
are given in the approprla Le wiring diagrams and are
correlated to the diagrammatic representations of all
items. In the aircraft itself, references are marked
on or near the related items.
Logic Circuits and Diagrams
The operation of the majority of units comprising
electrical systems is largely based on the application
of solid.stale circuit technology i.e. components
such as resistors, capacitors and rectifiers that are
nom1aJly interconnected as separate discrete
components, are all "embedded" In micro.size
sections of semiconductor material. Apart from
the vast reduction in dimensions, this form of
integration also makes possible the production of
circuit "packs'' capable-of performing a vast number
of Individually dedicated functions . Thus, in know·
ing the operating parameters of a system overall,
and the functions constituent units are required to
perfom1, the complete circuit of a system is built
up by interconnecting selected functional packs.
The packs consist of basic decision-making elements
referred to as logic gates, each performing combinational operations on their inputs and so determining the state of their outputs.
As far as the diagrammatic presentation of the
foregoing circuits is concerned, greater use is made
of a schematic form depicting interconnected blocks
and a variety of special logic symbols, ead1 representing a specific circuit network "hidden away'' in
the semiconductor material. The study of a system 's
operation is therefore based more on the interpre·
talion of symbols and the logic state of signal
functions at the various interconnections of the
circuit rather than tracing through diagrams that
. al
depict 'all internal circuit de tails in more theoretic
form .
Logic gates are of a binary nature i.e. the Inputs and
the outputs are in one of two states expressed by
the digital notation 1 or O. Other corresponding
expressions are also frequently used as follows :
1 - on; true; high (H) ; dosed ; engaged
0 - off; faJse: low (L); open : disengaged
The 1 and Ostate designations are arbitrary. For
exati:iple, If the states are represented by voltage
levels, one may be positiv(? and the other 0-volts,
one may be negative and the other 0-volts, one may
be positive and the other negative, both may be
positive , or both may be negative. The applications
of logic to a system or a device may therefore, be
further defined as follows:
1. Positive logic when the more positive potential
(high) is consistently selected as the 1 state.
2. Negative logic when the less positive potential
(low) is consistently selected as the 1 state.
3. Hybrid or mixed logic when both positive or
both negative logic is used.
The inherent function of a logic gate is equivalent
to that of a conventional switch which can be referred
to as a "two-state'' device and this may be illustrated
by considering the theoretical circuit of a simple
motor control system shown in Fig. 11 ;2(a). 1n the
''ofr' position of the switch, the whole circuit is
open, and is in an inactive or logic Ostate. In the
on" position, the switch closes the relay coil
circuit causing positive d.c. to pass through the coil.
Since the Input voltage at A is at a high level with
respect to ground then the input to the relay coil
is of a logic 1 state and so the coil is energised . The
input voltage at B Is also high and so operates the
motor from the logic l state existing at point C by
closing the relay contacts. The circuit may therefo re
be considered as a positive logic function circuit.
As a further example of logic switching, let us
consider the motor control circuit.shown in
Fig. l l.2(b). In this case, control is effected by
selecting either of two parallel.connected switches
located remotely from each other. If it is required to
ope~ate the motor from, say, the switch 1 location,
the circuit from input A to the felay coil is closed by
placing the switch at the "on" position, thereby
producing an active logic 1 stale In the coil circuit
and at the output C. Switch 2 remains open so that
the circuit from input B is in the logic Ostate. The
converse would be true were the motor to be
operated from the switch 2 location and with
switch I off. The circuit is also a positive logic
The circuits to which digital logic is applied are
combinations of three basic gates performing
functions referred to as "AND" , ''OR" and "NOT'' ;
the last being an inverting function, and giving rise
to two other gates referred to as "NAND" and
Gate circuits are designed so that switching is
carried out by either junction diodes, transistors
or by a combinati.on of both. In ord·er to simplify
diagrams as much as possible, the Internal circuit
arrangements are omitted, and the gates are rep resented by corresponding distinctively-shaped
symbols which confom1 to accep ted standards.
28V{~ O
~r.o_,_,- •
Fig 11.2
Logic switching functions
The three basic gate symbols are shown in Fig, 11.3.
Variations in the symbol shapes adopted will be
found in some literature, but those shown are used
in the majority of manuals related to aircraft
All possible combinations of Input logic states
and their corresponding output states, expressed
Ln terms of the binary digits (bits) 0 and l, can be
displayed by means of truth tables. The tables
appropriate to the gates referred to thus far are
given in Appendix 11, and to illustrate how they
are constructed, let us consider the one shown in
Fig. 11.4. This corresponds to an AND gate, and
as will be noted the table is a rectangular co-
Fig 11,3
Basic gB.te symbols
Fig 11.4
table construction
ordinate presentation, the columns representing
the inputs and output, and the rows representing
the logic combinations.
The number of different pos.5ible combinations
is expressed by 2n, where n is the number of inputs.
Thus, for a basic 2-input gate the possible combinations are 2" - 21 .. 4, and so the table has two
input columns and four rows. For a 3-input gate
there would be eight possible combinations, and
so on . The sequencing of the Os and the l s which
make up the logic combinations, is based on identify.
ing the inputs with 2s which are raised to a power
based on the input positions in a table. The table
in Fig. 11.4 has two input columns, and working
from right to left (this sequence always applies)
column B Is identified with 2° and column A with
21 • Since 2 raised to zero power equals l , one 0
and one 1 are alternately placed in each row of
column B. Two raised to the power of one equals
2; therefore, two Os and two ls are alternately
placed in column A. From column C it will be
noted that an AND gate can only produce an output
when the input combination is in the logic 1 state;
for this reason, the gate is often refened to as an
''all or nothing'' gate.
The same input combinations apply to an OR
gate, but as will be noted from Its truth table in
Appendix 11, it will produce an output when the
inputs either singly, or in combination, are in the
logic 1 state; the gate is therefore re ferred to as an
"any or all" gate,
As an illustration of how gate functions may be
related to theoretical circuits, let us again refer to
Fig. 11.2. ln order for the motor shown in diagram
(a) to operate It must have a logic l input supplied
A Contcol switch 1 on D
A Control switch on
Relay contacts
Power to
C drive motor
C dnve motor
Control switch 2 off
Control switch 1 o f f D C Power to
B Control sw1tc
. h 2 on
Fig 11.5
Logic gate/theoretical circuit relationship
drive motor
to it, and since this can only be obtained when
both the switch and relay contacts are closed, the
circuit corresponds to an AND function and may
be represented as in Fig. 11.5 . For the motor in
diagram (b) to operate, the logic 1 input can be
supplied to it when either switch 1 or switch 2 Is
closed; thus, the circuit corresponds to an OR
function and may be represented by the appropriate
The NOT logic gate Is used in circuits that require
the state of a signal to be changed without having
a voltage at the output every time there ls one at
the input, or vice versa, In other words, the function
of its circuit is to invert the input signal such that
the output ls always of the opposite state. The
symbol for an inverter is the same as that adopted
for an amplifier but with the addition of a small
circle (called a "state indicator'') drawn at either
Fig 11.6
Inhibited gates
the input or output sides. When the circle Is at the
input side, it means the input signal must be "low''
for it to be an activating signal; when at the output
side, an activated output functibn ls "low". In many
cases, the NOT function is used in conjunction with
the input to an AND or OR gate, as in Fig. 11.6 ; the
gate Is then said to be inhibited or negated. In order
to emphasise the inversion, a line is drawn over the
letter designating the Inverted input. The truth
tables are also given in Appendix 11, and should
be compared with those of the AND and OR gates.
The addition of an inverter at the output of an
AND gate and of an OR gate changes their function
and they are then known respectively as NAND
(a contraction of Not AND) and NOR (a contraction
of Not OR) gates. They are Identified by the symbols
shown in Fig. 11.7.
Fig 11.7
NANO and NOR gates
Figure 11 .8 illustrates the symbols of two logic
gates called exclusive OR and exclusive NOR, each
being a combination of two inhibited AND gates
and an OR gate. In some cases, the inputs to AND
and OR gates may be connected together in the
configuration known as "wired AND" and "wired
OR". They are symbolised as shown in Appendix 11.
As an aid to the interpretation of schematic
diagrams that depict the operation of systems in
logic form, let us now consider some representative
Figure 11.9 relates to the operation of a twin a.c.
generator system, and in particular it shows the
logic states required to connect one of the
generators to its respective load busbar, via the
control relay (GCR) and circuit breaker (GCB),
Control is effected through an OR gate, a multiinput AND gate and an inverter, and all the inputs
are processed by the appropriate circuit modules
of the generator control unit (GCU).
1t will be noted that three of the inputs to the
AND gate are negated; therefore, if the relevant
ci1cuit conditions - no internal faults the
Fig 11,8
Exclusive gates
GCU, no faults in the bus·tie breaker (BTB) and
no command signal from the extemal power
contactor (EPC) - are satisfied, each input will be
in the logic l state, and together with the other
inputs a logic 1 output will be supplied from the
AND gale to energise the "close" section of the
GCB control relay. The output is also supplied
to the inverter which maintains the "trip" section
of the control relay de-energised under nonnal
operating conditions.
The contacts in this section of the relay thereby
complete the circuit to the GCB, which on being
energised by d,c. power from the battery busbar,
closes the niain contacts between the generator
and its load busbar. The GCB utilises magnetic
latching and operates in a similar manner to that
described on p. 110. With a.c. power now on the
busbar, the input to the OR gate changes to logic 0,
but since the output still remains logic 1 the AND
gate output will be unaffected and so the GCB
remains closed,
The system on which Fig. 11.9 is based is one
which permits tbe application of power to a load
busbar from only one source at a time; in other
words, sources cannot be paralleled. lf, for
example, external power is selected, the input F
lo the AND gate changes to logic 1; howe,ver,
since it is Inhibited the gate output will be logic 0
and so the GCB control relay and GCB itself cannot
be energised. Thus, the output from an operating
generator cannot be connected to the load busbar
while it is being supplied from the external power
If any fauJts occur while a generator is supplying
its load busbar they will be detected by the CCU,
and the logic states of the appropriate Inputs to
the AND gate will be changed, thereby causing It
to switch off outputs to the GCB control.relay and
to the GCB. At the same time, there will be a logic 0
input to the inverter which will energise the "trip"
coil section of the relay to provide a d.c. supply to
the coil of the GCB, the magnetic latch of which
releases all contactsto the open position .
Figure 11.10 is an example of a logic diagram
related to a system designed to give warning of low
pressure In the pressurised cabin of an aircraft. ln
No fa_ults on system "' Logic 1
Generator speed
= Logic 1
> 4300 rpm
Generator control switch on
.. Logic 1
No fault~ in GCU
= Logic O
BTB protection: no faults
= Logic O
No EPC command signal
= Logic O
I ••I .
No a.c power on
load bus =Logic 1
I Trip
28-V d.c.
batt. bus
Fig 11 ,9
Logic diagram - generator circuit breaker control
Ir=- _ --
Horn cut-out
....:c.... 1(1 )!OJ
Logic 0
Cabin pressure
Logic states :
normal pressure range
(1 )(O) pressure below normal {
(1 ][OJ horn cut-oul closed
Fig fl.10
Logic diagram of a low-pressure warning system
this case, Lhc logic elemenls used in the circuit are
an inverter, a flip.flop, and a driver. A 11ip-flop
is a bi-stable multi-vibrator device that has the basic
function of storing a single bit of binary data ; In
this application It is comprised of three NAND
gates. It is so called because the application of a
suitable pulse at one input causes it to "flip" into
one of its two stable states and to remain in that
state, until a pulse at a second input causes it to
"flop" into the other stato . The driver may be
considered as a form of amplifying device. Cabin
pressure sensing is effected by a pressure switch
whfoh is adjusted to close under a pre-set lowpressure condition, and so provide a ground (logic 0)
connection to operate the warning horn.
With tho cabin pressure fn its normal (N) range,
the pressure switch is open, and a logic I is applied
as input to the inverter; the inverted output Is applied
to NAND gate 2. The hom cut-out switch is in the
open position and a logic 1 state is applied to NAND
gate 1. The characteristics of the type of flip-flop
used is such that with logic O at its reset (R) input,
and logic 1 at its set (S) input, NAND gate 1 will
provide a logic O output and apply this as the second
input to gate 2. Thus, from the NANO gate truth
table, gate 2 will provide a logic 1 output as a second
Input to gate 1 so that it can maintain its logic 0
output. The logic l is also applied to gate 3, and
since its second input Is logic 0, it will provide a
logic 1 input to the driver, the output of which is
also logic I to maintain the warning hom in a deactivated state.
If the cabin pressure should go below normal
(N) it will be sensed by the pressure switch whose
contacts will change over to provide a ground
(logic 0) connection to the inverter. An inverted
output is now applied to the R input of the flip-flop
together with the logic 1 input at S; the output
logic states of gates 1 and 2 therefore remain
unchanged. It will, however, be noted from the
diagram that the logic l output from the inverter
is also applied as an input to NANO gate 3, so that
the output to the driver now provides the logic 0
causing it to activate the horn and thereby give
warning of the low cabin pressure condition.
De-activation of the horn is carried out by
depressing the cut-out switch. This changes the
logic input at S from I to 0, and since the Input
at R remains unchanged, the output from gate 1 is
now logic 1. In tracing this through gates 2 and 3
it can be seen that a logic 1 is produced at the output
of gate 3, and of the driver, so when the cabin
u o+
Fig 11.11
Logic diagram of a landing-gear aural warning system
pressure is below its normal range, an open•circuit
condltion prevails at the horn. When the cut-out
switch is released a logic I is agaln applied to input S
at gate 1, and because there is still a logic 1 at R,
the output logic state of the flip-flop remains
unchanged (or "set") until tho cabin pressure switch
detects that normal pressure conditions have again
been attained.
Figure 11.11 illustrates the logic diagram
appropriate to a system which operates a horn to
warn the flight crew that. the landing gear is not
down and locked when the trailing edge flaps aro
set to the landing configuration, or when any
engine thrust lever is set to the idle position. The
switches A to F represent input sensors which are
activated by _the naps, landing gear and engine
thrust levers.
When the flaps are in the landing configuration
or range (R), switches A and B are closed and so
produce ground potential (logic 0) inputs to NAND
gate l; this in turn presents a logic 1 input to pte 2.
If the landing gear is not down and locked (D & L)
a logic 1 from switch C will also be applied as a
second input to gate 2, resulting in a logic O output
to the horn thereby causing it to sound. The logic l
from switch C is also applied as an input to gate 4.
Since the flaps not fully up (U) switches D and
E will apply logic O inputs to gate 3 and thls also
produces a logic l input to gate 4. The thlrd input
to gate 4 Is also logic 1 and is derived from gate 5
by inversion of the inputs produced when the
corresponding thrust levers are in the idle position.
The output of gate 4 remains logic O and the horn
continues to sound.
When the landing gear is fully extended to the
down and locked position, the inputs to gates 2
and 4 from the switch C will be changed to logic 0,
and the horn will therefore be silenced. The horn
may also be silenced by depressing the cut-0ut
switch, thereby resetting the flip.flop in the thrust
idle logic circuit,
Electrical and Magnetic Quantities,
Definitions and Units
Electric potential
That measured by the sn1ngy of a unit
positive charge at a point, ex.pressed
rclaUve to zero potential, or earth.
That between two points when main·
tained by an e.m.f., or by a current
flowing tluough a resis ta_nce.
Difference of potential p1oduced by
sources of electrical energy which
can be used to drlve currents through
ex. temal circuits.
difference (p. d.)
fo1ce (e.m.f.)
The rate of now of electric charge at a
point in a circuit
Name of unit
Millbmpere (AX l o·J)
Microampere (A x Io· 6)
The tendency of a conductor to
oppose the now of cunen t and to con- Megohm
vert clccttical energy into hea L It,
magnitude depends on such factors as:
natutc of conductor material, its
physical state, d!Jncrulons, temperalure !IDd thermal properties; fte·
qucncy of current !ll'ld its magnitude,
The rate of doing work or transform·
Ing energy,
The number of cycles in unit time.
en x 10
KilOW!iU (W
10 1)
Unlt definition
Difference of electric
potential between two points
or a conductor carrying conSl;rnl current of l ampere,
when the power dissipated
between thssG poin IS i_s equal
to 1 watt.
l11e ampere is that constll-llt
current which, if mi!.lrttained
In two straight parallel conductors of infinite length, of
ne~ble cross section, and
placed 1 m ctre apart in
vacuum, would produce
between the conductors a
force equal to 2 x 10- 1 newto1
per metre oflength.
The ohm is the electtical
resistance between two points
of 3 conductor when a con·
stant p.d. of 1 volt, applied to
these points, produces In the
conductor a current of 1
ampere, the conductor not
bBing the SC!il of any e.m.f.
Is the power which In 1
second gives rise to energy
of 1 joule.
The dcfiruUon of frequ ency
aho applies with the unit of
time boing taken as 1 second.
Name of unit
Unit definition
The property of an element or
circuit which, when carryins a
current, is characterized by the
formation of a magnetic field and
the storage of magnetic energy.
The inductance of a clo&ed
circuit in which an e.m.f. of
I volt is produced when t,he
current In the circuit varies
at the rate of I ampere per
The property of a sYstem of conductors and insulators (a system known
as a capacitor) which allows the
storage of an electric charge when a
p.d. exist! betwccm tho conductors. 1n
a capacitor, the conductors a1e known
as electrodes or plates, and the in·
sulator, which may be solid, liquid or
ga,cous, known as the dlelectrlc.
Microfarad (F x 10-1)
Picofarad {F x JO".,)
Th!il capacitance of a capaci·
tor between the plates of
which there app~ars a p.d. of
1 volt when it is charged by a
qua.ntit}' of electricity of l
Electric chnrge
The quantity of electrici!Y on on
electrically charged body, or passing
at a point in an electric cl,rcuit during
a given tin1e.
The quantilY of electricity
carried in l second by a
current of I ampere.
The capaci!Y for doing work.
The, work done when the
point of application of a
force of l newton is displaced
through a distance of 1 me trc
in the direction of the force.
The ex tent to which the flow of alter· Ohm
natlng current pt a givon frequency is
restricted, a;nd represented by the
ratio of r.m.s. v;uues of voltage anrt
cummt. Combines resistance, capacitive
and inductive reactance.
That part of the inlpedance which is
due to Inductance or capacitance, or
both, and which stores energy rather
than dissipates it,
Magnetic flux
A phenomenon produced in the
Weber {Volt-second)
medium surrounding electric currents
or magnets. The amount of flux
through llllY atea Is meamred by the
quantity of electricity caused by flow
in a circuit of given resistance bounding
the area when this circuit Is removed
from the magnetic field.
The magnetic flux which,
linking a circuit of I turn,
would p1oduce In It an c.mJ.
of l volt If it were reduced
to zero at a unlform rate in
1 second.
Magnetic flux
donsitY (Magnetic
The amount of magnetic flux per
square centimetre, over ll small
at a point In a magnelic: field. Tho
direction of the ma;gnetic flux is at
right angles to the area.
Equal to 1 weber per square
metre of circuit area.
Magnetic field
strength (Mag•
netizlng force)
The strength or force which produce$
or is associated with magnetic flux
density, It is equal to the magnetomotive force per ctin timetre measured
along the linB of forco.
Ampere per metre
force (m.m.f.)
Tho magnetic annlogue of e.m.f. It
represents the summatcd current or
equivalent current, including any dis·
placement current, which threads a
closed line in a magnetic field and
product1s a m11gnetic flux along It.
C:in also be stated as th111 wod1. dono
in moving a unit magnetic polt1
around a closed magnetic circuit.
Name of unit
The iatio of magnetic force to
magnetic nu.x. May be considered
as the opposition to the flux cst11bllshed by the force. It is lhe recipro~
of permeance.
Permeability (µ)
The ratio of the magnetic flux density
in a medium to the magnetizing force
producing it.
The cnpability or a magnetic circuit to
produce a magnetic flux under the
influence of an m.m.f., and which l.s
represented as \he quotient of a given
magnetic flux in the magnetic circuit
and the m.m,f. required to produce It.
Unit definition
The product of current and
the number of turns or 11
Derived SI Units with Special Names
Physical quantfly
Definition of unit
kg m 1 s· 1
kg m s·2 = J m·1
kgm 1 s·) A· 1 •
kg m I s ·• A •1 = VA· 1
A Is' kg l m~1 =CV· 1
Electric charge
Electric p.d.
Electric resistance
Electric capacitance
Magnetic flux
Magnetic flux ·density
Lwninous flux
Viscosity, dynamic
kg m 1 s· • =J s·1
kg m I s -, A•1 ... V s
kgs· 1 A· h• Wb m·2
kgm 1 s· 1 A- 1 ... VsA- 1
cd Sr
cd srm· 1
kg m- 1 s~•= Nm·l
A Is• kg· 1 m· 1 .-n-1
kg m· 1 s-1 =N s m· 2
Other Derived SI Units
Electric field strength volt per metre V/m
Electric charge density coulomb per C/m 3
cubic metre
Electric flux density coulomb per C/mt
square metre
farad per
Current density
ampere per
square metre
Magnetic neJd strength ampere per
henry per
m kg s· 3 A· 1
sAm· 3
SAm· 1
s 4 A I m· 1 kg·1
Am· 1
Am· 1
m kg s- 1 A· 1
Decimal Prefixes
1011 = l 000 000 000 000
10' = l 000 000 000
10• - 1 000 000
103 = 1 000
10• = 100
10- 1
10· 1
10· 3
• 0·001
= 0·000 001
10-• =0·000 000 001
10-u =0·000 000 000 001
10·1S =0·000 000 000 000 001
10·11 • 0·000 000 000 000 000 001
Ohm's Law
Titis law is fundamental to all direct current circuits,
and can in a modified form also be applied to alterna·
ting current circuits.
The law may be stated as follows: When current
[lows in a conductor, che difference in potential
between the ends of the conductor, divided by the
current flowing, is a constant provided there is no
change in the physical condition of .the conductor.
The constant is called the resistance (R) of the con.
ductor, and is measured in oluns (0). In symbols,
Thus, if~ is substituted for I in the power formula,
it becomes
P- V xRor P "' R
Similarly, if IR is substituted for Vin the power
formula, then
P"' Ix l x R or P = 12R
By transposing the formula Pc 1iR to solve for the
current l, we obtain
r1 =R
from which
V"' potential difference in volts
l "' current in amperes.
Calculations involving most conductors, either
singly or in a variety of combinations (see ·p. 199), are
easily solved by this law, for if any two of the three
quantities 0/1 I and R) are known 1 the third can always
be found by simple transposition. Thus, from (I)
V" IR volts
I = R amperes
Since some of the values used to determine the
power delivered to a circuit are the same as those used
in Ohm's law, it ls possible to substitute Olun's law
values for equivalents in the fundamental formula for
power (P) which is:
p = V x l watts
Other transpositions of the foregoing formulae are
as follows :
V =f_
V"' ../PR
Total resist(lnce
RT= R1 + R; + Ra+. , . ohms
Yr = 01R 1)+ (1 2.R,) + Cl,R,}+
... volts
or VT • IRT volts
or RT=-ohms
total current
! ,~. }
- =-+-+- + . ..
or RT • - - - - -
- + - + - + .. .
R1 R, R3
or RT • -
If the 1esistanccs a:ro of equal
v~Jue R, th11n:
When only two resistances in
parallel, the total resistance is:
- RI ii R,
R T - - -R1 + R,
i .. .. 'f
Series- p.arollel
IT= 11 = 12 = lJ "'
... amps
or lT=-
If lhe resistances are of equal
value R then:
RT " nR ohms
where n • nu mbr.r of resistors
Total voltage
RT, VT ancl IT are fou nd by first reducing the p(lrallel circuit to a
single resistance, and then solvins the whole as a simple serios circuit.
Power in A.C. Circuits
Real (or Average) Power
The power dissipated is P -- VI cos 8 where
V - r.m.s. voltage across circuit
I "" r.m .s. current flowing in circuit
8 - phase angle between V and I
cos 8 "'the power factor (P.F.) of the circuit
(i) For inductors, capacitors, or circuits containing
only inductors and capacitors, P. F: = 0 i.e., no power
is dissipated,
(ii) For resistors and resistive circuits, P.F. - I
i.e., power is dissipated.
(iii) For circuits containing resistance and reactance,
phase angle Bvaries between 0° and 90°.
Real power dissipated in an a.c. circuit is also equal to
I1 R and R where
I -= r.m.s. current flowing in R
V - r.m.s. voltage across R.
Reactive Power
Reactive power Pq"' VJ sin 8 where V, I and 8 are the
same as for real power,
Also Pq "' l 1X andxwhere
I "' r.m.s. current in the reactance
V "" r.m.s. voltage across reactance
X = net reactance.
Pq is measured in volt-amperes reactive (V Ar)
Apparent Power
Apparent Power Pa - VI where V and I are the same
as for real power.
Pa is measured in volt-amperes (VA).
Connection of Capacitors and Inductors
Capacitors in series may be considered as increasing
the separation of the outer plates of the combination.
TilUs, the total capacitance Cr is less than the smallest
capacitance of the lndividuaJ capacitors, and so the
relationship for Cr is similar to that for resistors in
parallel, i.e.,
TI1e energy (ec) stored in a capacitor Is ec =
1 1 1
CT=c;+c;+c3 + ... orCT"'1,_
C, + C2 +C3 + ...
When only two capacitors are in serles, then
C ,,,
c, X C2
+ C2 .
If the capacitors
arc of equal value C, then CT.:::.£
where n - the number of capacitors in series.
The total working voltage rating of capacitors in
series is equal to the sum of the ratings of the
The total charge is Or"" 01 "' 02"' Q3 "' . , ,
Capacitors in parallel may be considered as effectively
increasing the area of the plates; therefore, since
capacitance increases with plate area, the total capac[.
tance Cr is equal to the sum of the individual capaci,
lances. Thus, the relati onship for CT is similar to that
for resistors in parallel, i.e.
Cr "' C1 + C2 + C3 + . ..
The working voltage of a parallel combination is
limited by the smallest working voltage of the
Individual capacitors.
TI1e total charge is Qr=: Q 1 + 02 + Q3 + .. .
C "' capacitance in farads
V"' voltage Impressed across capacitor.
Inductors in series, parallel or in combination circµits
act similarly to resistors. Thus:
in series, the total Inductance Lr - L 1 + ~
+L 3
in parallel,
L-r "'
r:;+r:;+c;+ .. .
The energy (er.) stored in an inductor is eL"'
L"' inductance in henrys
I "' current flow through inrluctor
Fundamental A .C. Circuits and Formulae
Inductive reactance (XL)
G :?J
XL= 2rrfL
where f = frequency (Hz)
L = Inductance In henrys
XLto tal = XL 1 + XL,+···
XLtotal •
7 L,~
As for Inductive
Reac tance (Xi)
++ ...
XL , XL;
CaJ!acitive reactanro (Xe)
Xc 11 211fC
Applied voltage
Current OT)
J ;: VT
. XL
Yr= IXL,+ IXL,
Ir= IL,= IL,
Yr= IX1.. 1 = IXL,
IT"' It ,+ IL ,
where f = frequency (Hz)
C = capacitance
Xetotal "'
Xe,+ Xe: + ···
~ fc, ·fc2j
Xctotal •
- + - + ...
Xe , Xc 1
Impedance (Z)
A~ for Capacitance
Reactance (Xe)
YT • !Xe,+ [ Xe,
IT =
ie, = ic,
Yr ..
IT =lc,+ Icl
IT = ..:.I
Z(R) • I
V ,. lz
l = y'R'+ XL'
Yr " .JffR"52+
(IX ,}'
{ m~•-
ixc, = !Xe,
7.=y'R' +Xc'
Z 11 ~
- Xe)'
VT " v'(IR)' ~
(IXL - JXc)'
I '" Vr.
Resonant Circuits
From the reactance formulae given above, ii will be
evident that changes in frequency will change the
ohmic values of reactance, e.g. a decrease in frequency
decreases inductive reactance but increases capacitive
reactance. At some particular frequency, known as
the resonant frequency (Fn), these reactive effects
will be equal, and since in a circuit containing a
capacitor and inductor in series they will cancel each
other, then only the ohmic value of circuit resistance
would remain to oppose current flow in the circuit.
Such a circuit is said to be ''in resonance'' and is
referred to as a series resonant circuit. If the value
of circuit resistance is small or consists only of the
resistance In the conductors, the value of current
flow can become very high. In such cases, the voltage
drop across the inductor or capacitor will often be
higher than the applied voltage.
rnsonant circuit
Resonant frequency is determined from the
l ln a paralleI resonant c1rcu1
· 't ,
fom1ula: Fn "' 2rr,J[c'
the reactances are equal and equal currents will flow
through the inductor and the capacitor. Since the
inductive reactance causes the current through the
inductor to lag the voltage by 90°, and the capacitive
reactance causes the current through the capacitor
to lead the voltage by 90°, the two currents are 180°
out of phase. Thus, no current would flow from the
a.c. supply and the parallel combination of the
inductor and the capacitor would be an infinite
impedance. In practice this would not be achieved
since some resistance is always present and so the
parallel circuit acts as a very high impedance. The
circuit is sometimes referred to as a tank circuit, or
an anti-resonant circuit because of its effect being
opposite to that of a series.resonant circuit.
Conversion Factors
Amperes/square metre
Amperes/square inch
Ampere tum$/crn
Am pert turM/lrtch
Ampere turns/inch
To obtain
X }Q"l
Amperes/square metre
Ampere turns/inch
Ampere turns/cm
Ampere tums/meue
39 .37
x 10-•
0.293 l
Amperes/square inch
x 10-•
Square centimetres
Circular mils
Circular m!Is
SquaJe mils
Circular mUs
7.854 x 10-,
Square inches
Coulombs/square metre
6.452 x 10-·
Coulombs/square inch
In ches
Kilowat ts
2.260 )(
10· 1
745 .7
Square centimetres
Btu/m in
Circular mUs
Square inch
Square mils
Circular mils
Circular mils
Power Generat ion Syst em Applications
Power generation systems may be classified as being
primarily either direct current or alternating current ,
and from this, aircraft are generally and loosely
referred to as "d.c. aircraft" or ''a.c. aircraft'' depending on the power utilization requirements of components and systems. Inevitably, some systems
require power differing from that of the primary
generation systc...n and for tlus purpose secondary
power conversion equipment (e.g. inverters and
TRU's) is also employed. In some isolated cases
of "d.c. aircraft", frequency-wild a.c. power is
also utilized in addition to primary d.c. generators
and powi!r conversion equipment.
The application of these power requirements to
a representative selection of aircraft are tabulated
for reference in this Appendix on pages 206- 11 .
Power Generation Systems of some
representative types of Aircraft
A irbus
A .C.
Aerospat iole
ATA 42
Duchess 76
K ing Air
Queen Air
St arship
Boeing 670 7
B Ao
1.- 11
Jetstreem 31
Conco rde
Sel frectifying
Soe Note 1
Seo Note i
See Noto 5
See No!i: 5
See Note 4
Seo Notes 1 and 3
150, 170, 180
404 , 421
42S, 441
Do rn ier
Oou(llas DC-9
6mbraer Bal'ldlerante
Fok ker F-27
F-1 00
Grumman Agcat
Gulfstream I
Gulfstream II
Gulfstre11m 111
Leerjet 35A
Learjet 55
l.1 01 1
roc1i lying
Seu Norn 5
Cherokee Arrow
Chieft ain
Twin Com or\che
114 Commandor
Aero Commander
Ra llvo
Pll11tus B-N
Saab Pairch ild S F340
Shorts Skvvan
Swear ingen
Merlin III B
NOTE 1 Main ~.c. wst ems non,pgr3l!eled,
NOTE 2 Alternator fo r heating o f wind$hlelds, fligh t deck side w indows. pitot probes.
NOTE 3 One st a t ic Inverter for bnck•!.!P single-phase a.c. powor.
TRU 's
Seo Noto 6
NOTE 4 Alternators for windshield ~nti•icing.
NOTE 5 Alternator for wlnd$hic!d and power unit anti-Icing.
NOTE 6 A.c. t apped fr o m J,phase windings of storter/generator for wiridshield anti-icing.
Electrical Diagram Symbols
....!.j ,I ,:_
- - - - 111 lntvrnal
~ Crosaing
8 8 8
SUP RING----()-
Roceptoc10· Plug
-,'"'-, -
, - shield
•r:;ae ahlh
S1ep•up •
lit IIE
Tharmal 1eal1101
Sansing alamanl
-#~ N.O.
- - - N.C.
, ... .;,
Thorrnal relay
wilh limo delav
Conlfnuoua loop
Con1ac11 N.o .
ContA1ct1 N.C.
Thermal overload
Thormol swilch
off-Momentary ON Pv,h ON
S.P.0 .T.
0-,-,::, NC
E3 Pressure
O.P.O.T. with
e.e~ excilation
Pu,h with
hold, fn
Representative Aircraft Ice and
Rain Protection Systems
8.A. 81oed Ai r; El. El&e1rieal; FL. Fllli<!; P,B, Pneumotic 80011;COMB. CombuJtlon;
1--..-- -i-- ....---+--r--t--,--..--"T'"--r--+-....L-'W:~--r--"Oll,-~-QN-T'l'A---,-l.
Alrbua AJ1 0
Boeing 707
·Queen Air'
BAil 146
8eec~ 8110n
Fokker F-27
Falcon $0
Pi~tPA31P .
I/ /
f t'M,.
[l ,
,·-·otl; OECT'N
~.IT.~ s
• :/
CSO 011 cooler coops
Outboord wing L.E. 1111, only hee1ed
• • V • • lL"' •
• • • •
• • • • • • l/ •
/ / /
I/ • • / / • / •
/ ./ lL I / • • / • / / •
/ / lL V • • /
•/ •
/ / L
V • •/ • •/ •
I/ • • / / lL • I /
• I/ /
I/ ,./ V /"
I/ I/ / I/
~c6:lr. ~~!~
V /
1/ V
• i/ V
V V V lL
V V V lL
• I/
V V lL lL I/
• V I/ V lL lL /
Ptopellors de-ieed oilhor olectrlc1lly or by lluid
I/ • • V V • L V
V •
• ;/
• / I/
• ///
• I/ V
• I/ V
V /
I / / L I/
l/ / L I/
V • / I/
V • •
Ho! rod 1vpe ol detector
V • V I/ 0 i / /
I/ • V I / • I/ I/
• I/ i / L V • V / / / V /
!/ • V / / lL • /
/ V / •
• V / / / • V V I/ V ~V
I/ • I/ V V V / I/
/ I / / V -- I/ • • • I/ I/ e •
I/ I/ / /
• • •
I/ • • • •
I/ I/ I/ l/
/ •
I/ V V IL • V e I/ I/ / 1. / / /
• V V lL V • V V V V V I/v/
• V V lL I/ 0 V V I/ V •
Stall Wirning 1y11em
r . D on oulooaro w,ng L,0'1: o~~-C ·on lnlioara wing
L.e·,. Ra in ropolianr by B.A. FL. de,ici"ll 11;0 UH,;!
H back-up fo r windshield do• lclng.
0.C. power for wlndshio ld healing
Stall w11rn1ng sys tem.
Sta ll wernlng system. Prenuro typo o f ica do!octor
FL. systam es sta ndcy ,or w ind1~iold
do-Icing. Ro11ry 'cuner' type lea delector.
Vibra1t:1g tvpe lee de1ec1or.
Radome al, o dt-iCl!d.
E~houJt g~••• through heat B•ch,nger1 to,
wino do-1c1ng
Stoll werriing 1y1tem. Propeller de-icing syste m
utllisn5 d.c4powar.
Abbreviations and Acronyms associated
with Electrical Systems
AC Motor Pump
Anticvcling Rolay
Autom11t ic Fire/Overheat Logic Test
Auxlllery Power
Auxillory Power Breaker
Aul<illary Power Control Relay
Aux iliary Power Control Unit
Auxiliary Power Generator CQntrol
Auxiliary Power Relay
Auxiliary Power Slave Relay
Auxiliary Power Unit
Annunciator Relay
An nunciator Re, et Re1!1y
Autom et lc Test Equ ipment
Bus Control
Battery Charger
Bus Cont rol Penel
811Uery Ctiarger Tron~fer Relay
Bullt-ln Test Equipment
Bus Powor Co111rol Unit
Bus Protection Ponel
Battery Relay
Battery Switch
Bus Sensing Reloy
Bus Tio Breaker
Caution Advisory Computer
Circuit Breaker
Contact or
Control Relay
CB (C/B)
Constant Speed Drive
Current Transformer
Cross Tie Lockout Rolay
Cross Tic Lockout Reloy S leve
Cross Tio Relay
Cross Tie Time Delay
Driw Annunciator Relay
Dead Bus Relay
Dead Bus Slave Relay
Drive Disconnect Relay
Differentiel Current
Dlfferentiol Protection
Differential Protect ion Control Tronsfor,
Different ial Protection Relay
Drive Running S!evo Roley
Distribution System Annunc!etor Relay
Dead Tis Bus
Electronic Centralized Aircraft Mon itor
Emergency D,c. Bus Sen,ing Ael11y
Electrlcal /E lectron lcs
Electrical Load Control Unit
Elcctroma9netic Inter-face
E;Kterna l P.ower
External Power Available
external Power CQnteetor
External Power Control Rel~y
External Power Monito r
Extornal Powor Roley
Emergency Power Transfer Relay
Engine Runn ing Signal Relay
Feodor Fault
Reeder Fau lt Warning
Fault Selector Time Deley
Generator Annunciator Relay
Generator Breaker
Generator Control
Genorator Control Annunciator Relay
Generator Control Brea ker
Generator Cont rol Pe11el
Generator Control Relay
Generator Control Rolay Auxiliary
Gonerator Control Unit
Ground Handllng Relay
Galley Load Relay
Ground Power Control Unit
Generator Ptiase Sequence Re lay
Generator Relay
Ground Refuelling Relay
Gener11tor Relay Slave
Ground Service Auxiliary Power Reh1y
Ground Survice External Power Relay
Ground Service Raley
Ground Service Select Relay
High Frequency
High Voltage
Integrated Drive Generator
Intor Sys:tem Bus
LiQuid Cryual Display
Llgh1 Emitti119 Diode
Line Replaceable Unit
Ligh t
Low Voltaoo
Linear Varieble Displacement TrBnsformer
Maintenance Control & Display Pe11el
Master Caution & Warning
Manual Trip
Master Warr, ing Panel
Master Warning System
Over Current
Over Froouoncy
Open Phase
Over Voltage
Overvoltego Relay
Power Cont rol Actuator
Printed Circuit Board
Phase Sequence
Permanent Magnet Genorator
Perma11cnt Magnet Gonorator Relay
Poton tiomoter
Power Reedy Relay
Phase Soquenco
Power Supply Modula
Phase Sequence Relay
Press To Test
Pulse Width Modulator
Ram Air Turbine
Reverse Curreni Ach1y
Acsot Relay
Reset Switch
Rotl!ry Variable Oisl)lacoment Trensformer
Si llcon Control Rectifier
Symbo l Generator Unit
Solenoid Valve
Starter Relay
Solid Starn Relay
Undor Frequency
Synchron iso
Under Frequency Relay
Under Frequency Time Delay
Tio Bu$
Tie Bu$ Ditterentie! Protection
Undor Volta90
Under Voltage Relay
Tie BV$ Feult
Transfer Bus Sonsing Relay
Time Dulay
Teu Point
Transformer Rectifier
Transfer (Bus) Control Relay
Tren$former Rectifier Unit
UWitY Bus Relay
Under Voltage Time Deley
Voltege Regulator
Voltage Reguletor Annunciator Relay
Warn ing Light Display Panol
Logic Gates and Truth Tables
B ,
~ ~
~ Js
Chapter 1
I. Describe how direct current is produced by a
2. Describe how generators are classified, naming
the three classes recognized and the class
normally employed in aircraft systems.
3. (a) Briefly describe armature reaction and the
effects it has on generator operation.
(b) How is armature reaction corrected in aircraft
4. What is meant by reactance sparking? Explain
how it is counteracted .
5. In connection with generator brushes, state:
(a) the materials from which they are made;
(b) why several pairs or brushes are used .
6. Briefly describe the causes of brush wear under
high altitude fiight conditions and the methods
adopted for reducing wear.
7. Which of the factors affecting the output voltage
of a generator is normally con trolled?
8. With the aid of a circuit diagram, describe the
fundamental principle of the carbon pile method
of voltage regulation ,
9. Describe how the voltage output of a generator is
controlled by a vibrating contact type of regulator.
10. How is the generator shunt-field resistance con trolled by a vibrating contact type of regulator
under heaVy external load conditions?
11. What additions must be made to voltage regula tion circuits of a multi-generator system?
I 2. With the aid of a circuit diagram describe how
parallel operation of generators can be obtained.
13. Describe a means for cooling aircraft generators.
14. Briefly describe how the d,c. power is derived in
aircraft utilizing a frequency -wild alterna tor
15. What principal methods are used for the driving
of generators?
16. What are the typical contact arrangements of
transistors? Describe how current is made to
flow in one of these arrangements.
17. State the functions of a Zener diode in the
circuit of a solid-state type of voltage regulator.
18. What are the principal functions of batteries in
19. Describe the construction ofa lead-acid battery
and the chemical changes which occur during
20. Describe the construction of a nickel-<:admium
battery and the chemical changes which occur
during charging.
21. Th.e capacity of a battery is measured in :
(a) volts.
(b) cubic centimetres ,
(c) ampere-hours.
22. What indications would be displayed by a leadacid battery of the free electrolyte type , and a
nickekadmium battery, which would serve as
a guide to their state of charge?
23. Describe a typical method of extracting fumes
and gases from the battery compartment of an
24. What do you understand by the term " thermal
25. What is the purpose of using a parallel-to-series
configuration of batteries in some ty pes of
26. With the aid of a circuit diagram, describe how
in some types of aircraft the battery may be
charged from an external power unit.
27. How are batteries which are installed in most
types of large transport aircraft maintained in
a stale of charge?
Chapter 2
1. The frequency of an alternator may be deter-
mined by :
(a) dividing the number of phases by the voltage.
(b) multiplying the number of poles by 60 and
dividing by the rev/min.
(c) multiplying the rev/min by the number of
pairs of poles and dividing by 60.
2. Explain the tertn r.m.s. value.
3. The current in a purely capacitive circuit will:
(a) lead the applied voltage.
(b) lag the applied voltage.
(c) be in phase with the applied voltage.
4. (a) With the aid of circuit diagrams briefly
describe the two methods of interconnecting
(b) State the mathematical expressions for
calculating line voltage and line current in
each case.
5. The phase voltage of a three-phase star-connected
a.c. generator is:
(a) equal to line voltage.
(b) greater than line voltage.
(c) less than line voltage .
6, The ratio of true power to apparent power of
an a.c. circuit is known as:
(a) reactive power.
(b) power factor.
(c) real power.
7. The impedance of an a.c. circuit Is measured in:
(a) kilovolt-amperes.
(b) amperes.
(c) ohms.
8. How is Power Factor affected by a circuit
containing inductance and capacitance?
9. What do you understand by the term "frequency.
wild system"?
10. State the factors upon which the frequency
output of an a.c. generator depend.
11. For what type of load is a frequency.wild supply
most suitable?
12. With the aid of a schematic diagram, describe
how a frequency-wild generator can be excited
and how its output voltage can be controlled.
13. In a CSD unit, the control cylinder is mechanically coupled to the :
(a) variable displacement unit .
(b) fixed displacement unit.
(c) governor.
l 4. The gove rnor of a CSD unit is driven by the.
(a) input gear.
(b) input ring gear.
(c) output gear shaft.
15. In which phase is a CSD unit said to be operating
when the governor causes charge oil to flow into
the control valve?
16. Ex.plain the purpose of the ''fine" adjustment of
the governor, and how It is accomplished.
17. A CSD unit which has been disconnected duriJ1g
flight :
(a) may only he reconnected on the ground.
(b) may be reconnected In light by re-set
(c) automatically resets at engine shutdown.
18. What is the purpose of the a.c. exciter and
rotating rectifier assemblies of a constantfrequency generator?
19. Explain how temperature effects on an a.c.
exciter are compensated.
20. What factors must be controUed when constant.
frequency a.c. generators are operated in
21. What is the meaning of kV AR and to which of
the factors does it refer?
22. When constant-frequency generators are in parallel, the sharing of real load is
controlled by :
(a) varying the excitation current in each
(b) varying the output speed of the CSD units.
(c) shedding certain loads,
23 . State the functions which current transformers
can perform in controlling load sharing between
constant.frequency generators.
24. What is a mutual reactor and in which section of
a load-sharing circuit is it used?
Chapter 3
l. Rectification is the process of converting :
(a) a high value of a.c. into a lower value.
(b) d.c. into a.c.
(c) a.c. into d.c.
2. Describe the fundamental principle on which
rectification is based .
3. An "n-type" semiconductor element ls one
11 .
l S.
(a) an excess of"holes".
(b) a deficiency of "holes''.
(c) an excess of electrons.
What semi-conductor elements are usually
employed in rectil1ers used In aircraft? Describe
the construction of one of these rectifiers.
What is meant by the term Zener voltage?
ls the Zener voltage of any practical value in
rectificatiort equipment?
Explain the operating principle of a siliconcontrolled rectifier (S.C.R.).
With the aid of a circuit diagram explain how
full-wave rectification of a three-phase input
takes place.
Describe the basic construction and principle of
the device used for converting alternating current
from one value to another.
What is meant by transformation ratio and how
is it applied to "step-up" and ''step-down"
tram formers?
Draw a circuit diagram to illustrate a star·
connected three-phase transfom1er,
Describe the operation of a current transformer.
For what purpose Is such a device used?
What effects do changes In frequency have ot1 the
operation of a transfonner?
With the aid of a circuit diagram, describe the
operating principle of a typical transformerre ctifier unit .
For what purpose are the power converting
machines of the rotary type utilized in aircraft?
Describe a method of regulating the voltage and
frequency of a rotary inverter.
Describe how transistors are utilized for the
conversion of electrical power supplies and
regulat ion of voltage and frequency levels.
Chapter 4
I. Explain why il is necessary for an external
power supply circu it lo fo rm part of an aircraft's electrical system.
2. Draw a diagram of a basic external d.c. power
supply circuit and explain its operation.
3. In a multi-pin plug how is it ensured that the
breaking of the ground power supply circuit
takes place without arcing?
4. Draw a diagram of an external power supply
circuit of a typical ''all-a.c.'' aircraft and explain
its operation.
S. What principal items are located on a typical
control panel as provided on some types of
6. State the purpose of an APU and the services
usually provided by it.
Chapter 5
I. What is the function of busbars and what form
do they normally talce?
2. What is mean t by a split busbar system?
3. Define the three groups which usually categoriie
the importance of consumer services.
4. State the function of a bus-tie breaker and the
type ofbusbar arrangement to which it would
be applied .
S. Describe three different types of electrical cable
commonly used in aircraft, stating their properties, limitations and identifications. State a
typical application of each type.
6. Whal principal methods are adopted for routing
cables through an aircraft?
7. Describe a method of routing wires and cables
from a pressurized IQ a non-pressurized area of
an aircraft .
8. Name some of the materials used for thermocouple cables and state their appllcations.
9. What is meanl by earthing or grounding?
IO. How is a ground system formed in an aircraft the
primary structure or which is non -me,tallic?
l J. What is a crimped terminal?
12. What is the function of an in-line connector?
13. What precautions must be taken when making
aluminium cable connections?
14. How is it ensured thal a plug mates correctly
with its socket?
IS. State how plug pins and socket cavities are
identified and aiso how their sequencing is
16. Discuss briefly the process of"polting" a cab le
lo a plug or socket.
17. What are the principal functions of a bonding
18. State some of the applications ol'primary and
secondary honding.
19. Briefly describe the methods generally adopted
for the discharging of static.
20. What is the purpose or screening'?
Chapter 6
I. The number of circuits which can be completed
through the poles of a switch is indicated by the
(a) pole.
(b) position.
(c) throw.
2. What do you understand by the term "position"
in relation to toggle switches?
3. To which circuits are (a) push-switches and
(b) rotary switches normally applied?
4. Describe the construction and operation of a
5. What are the three main stages of movement of a
micro-switch operating plunger?
6. Describe the construction and operation of a
mercury switch arranged to "break" a circuit.
7. In a thermal switch employing steel and invar
elements, actuation of the contacts under
increasing temperature conditions is caused by :
(a) expansion of the steel element only.
(b) contraction of the invar element only.
(c) expansion of the steel element causing dis·
placement of the invar element.
8. What are the principal ways in which relays
may be classified?
9. What do you understand by the terms "pull-in''
voltage and "drop-0ut voltage?
10. Sketch a cross-section of a typical pressure
switch ; explain its operation.
11 . What type ·of relay Is required for a circuit In
which control circuit current Is of a very low
value? Briefly describe the relay and its
operation .
12. (a) For what purpose are ''slugged'' relays used?
(b) Describe the methods usually adopted for
obtaining the slugging effects.
13. Explain how the contacts of a typical bus-tie
breaker remain in the latched position .
Chapter 7
1. What are the principal differe nces between a fuse
and a current limiter as far as functions and
applications are concerned?
2. State the function of a limiting resistor, and
with the aid of a circuit cliagram describe a
typical application.
3. A circuit breaker is a device for:
(a) protecting an electrical circuit from current
(b) collapsing the primary circuit of a magneto ;
(c) completing a circuit without being affected
by current flow.
4. With the aid of a sketch, describe the construction
and explain the principle of operation, and
characteristics, of a thennal circuit breaker.
5. What is meant by the term "trip free" when
applied to a thennal circuit breaker?
6. Under what conditions would you say that it is
pennisslble for a circuit breaker to be used as a
7. What do you understand by the term "reverse
8. Describe the operation of a reverse current cut.
out relay .
9. What is the function of a reverse current circuit
10. Briefly describe the operating principle of a
reverse current circuit breaker.
11. Describe a typical method of protecting a d.c.
generating system against overvoltage.
12. What is the purpose of incorporating lime
delays in the undervoltage and overvoltage
protection circuits of constant frequency
generating systems?
13. What are the overall effects of over .excitation
and under-excitation on a.c. busbar voltage,
and how is protection provided?
14. What is meant by a differential or feeder fault
and how Is it caused?
15, Briefly describe the operation of a differential
current protection system.
Chapter 8
1. Describe the principle of a moving coil
2. Can moving coil instruments be directly connected
in the circuits of a.c. systems for measurement
of voltage and current, or is it necessary for them
lo be used with certain other components?
3. A soft-iron core is placed within the coil of a
moving coil instrument because:
(a) it provides a solid spindle about which the
coil can rotate.
(b) this ensures an even, radial and Intensified
magnetic field for the coll to move in.
(c) the inertia of the core will damp out oscillations of the coil and pointer.
Describe how ammeters can measure very high
current values without actually carrying full
load current.
How are moving coil instruments protected
against the effects of external magnetic fields?
What is the purpose of central warning systems?
Briefly describe a typical system.
Define the acronyms ECAM and ElCAS.
What is the foml8t of the displays presented by
the display units of the ECAM system7
11 . In terms of the amou nt of field rotation relative
to one cycle of the power supply, wh11t are the
differences between 2-pole, 4-pole and 6-pole
12. Describe how a rotating magnetic field is pro,
duced in a single-phase induction motor. ·
13. Describe the operation of a hysteresis motor
and state one of its applications.
Chapter 10
1. A typical frequency of anti-collision light beam
Chapter 9
1. Define the characteristics which govern the
application of a d.c. motor to a particular
2. What are the principal characteristics of a shuntwound and series-wound motor?
3. When the r.p.m. of a shunt-wound motor
increases the current drawn by It :
(a) decreases.
(b) remains the same.
(c) increases.
4. Draw a circuit diagram of the motor to be
applied to a system where high starting torque
and steady "off-load" running is required.
5. What is meant by the term "shunt limiting"?
6. With the aid of a circuit diagram explain the
operation of a motor required for simple
reversing functions.
7. Actuator mo tors are prevented from over.
running their limits of travel by means of:
(a) manually controlled switches .
(b) electromagnetic brakes.
(c) cam-operated limit switches.
8. (a) Explain how a three-phase rotating magnetic
fie ld is produced in an induction motor.
(b) Why does the rotor run at a speed slightly
less than tha t of the rotating field?
9. In an a.c. molor, the diffe rence between synchronous speed and the speed of the rotor is
termed :
(a) the motor loss speed.
(b) the brake speed,
(c) the slip speed.
10. Wha t is the formula for determining the synchronous speed of at1 induction motor?
rotation is :
(a) 40-45 cycles per minute,
(b)80-90 cycles per second .
(c) 80-90 cycles per minute.
Why are the two surfaces of a V-shaped reflector
arranged differently from each other?What are the principal functions of a strobe
lighting system?
Describe the operating principle of a strobe
lighting system.
Give a brief description of the construction of a
typical landing light unit.
At which stage of landing light operation is the
light nonnally UJuminated?
How is it ensured that the beam of an extending/
retracting type of landing Ught located In a flap
track, remains parallel to a fore and aft datum
during flap extension?
By means of a diagram show the interconnection
of the components of a simple engine starting
system .
Jn terms of cranking speeds what arc the differences between starter motor requirements for
reciprocating and lurbine engines?
The self-sustaining speed is the:
(a) maximum speed at which lhe starter motor
runs to maintain rotation of an engine.
(b) speed at which the engine is capable of maintaining rotation.
(c) speed at which current to the motor is
What type of motor is used for engine starting
What is the function ofan overspeed relay
fitted in some turbine engine starting systems?
Describe how it fulfils this function.
13. The purpose of a "blow-0ut'' cycle is to :
(a) remove excess air from an engine during
(b) blow cooling air through the starter motor
after starting.
(c) remove the unburnt fuel from an engine in
the event of an unsuccessful start.
14, In systems itlcorporatlng a "blow-out" facility,
why Is it necessary for the motor running time
to be limHed?
15. Describe the operation of a typical startergenerator system.
16. The contact breaker of a magneto is connected
in the:
(a) primary winding circuit.
(b) secondary winding circuit.
(c) circuit between dist ributor and spark plugs.
17. Explain how the rate of collapsing of the primary
winding Oux is increased ,
18. What is the formula for calculating the speed of a
19. A rotating armature magneto to be fitted to a
6-cylinder engine must be driven at:
(a) the same speed as the engine .
(b) half the speed of the engine,
(c) one and a half times the engine speed.
20. Why is it necessary for the output of a magneto
to be boosted during starting? Describe a method
of achieving this.
21. What are the essential difference.s between low
and high tension magneto systems?
22. What are the materials generally used for the
insulators and elect rodes of spark plugs?
23. What are the essential differences between a
turbine engine ignition system and the system
used for a reciprocating engine?
24. With the aid of a circuit cilagram, explain the
operation of a high energy ignition unit.
25. What is the purpose of a ''relight" circuit and
what methods are adopted?
26. Describe the construction of a " fuewire" type of
detecting element, and state the effects that
temperature changes have on it.
27. In what way does a detecting element of the
Systron Donner System differ from a "firewire"
28. Describe the operation of a typical smoke
29. Describe the operation of an electrically-operated
fire extinguisher.
30. Describe the two techniques "de-icing" and
''anti-icing" .
31. What types of electrical healing elements are
used for the de-icing of propellers and engine
air intakes?
32. How is electrical power transmitted to propeller
blade heating elements?
33. What types of heating elements and power
supplies are used for the anti-icing of windshields?
34. In a propeller and air intake system using power
cycling, to what .ur temperatures do " fast" and
"slow" selections correspond?
35. ln what sequence is current supplied to the
heater elements of a propeller de-icing system
which is d.c. operated?
36. Describe the operation of a control method
adopted In a typical wlndshleld anti-lciJ1g
37. For what purpose Is a.c. and d.c. power utilized
in hot-air bleed anti-Icing systems?
38. State t11e purpose of the anticlpator and limit
sensors in the ducting of an airconditioning
39. How is a desired cabin temperature signal
established when an airconditioning system is
operating in ''auto''?
40. How is a mix valve prevented from staying at a
''too hot" position?
41. 1n the event of power control being lost , how is
the airconditioning ducting protected against
42. Describe the operation of a propeller synchronizer
Ch apter 11
I. Name the functions performed by the three
basic logic gates.
2. The logic symbol shown in Fig. E 11.1 represents :
(a) an AND gate.
(b) a NOR gate.
Fig. El 1.1
(c) an OR gate.
3. In order to energise the relay in the circuit shown
in Fig. El 1.2, the logic stale at the inputs must
-----o--i--0 - - -
7. The circuit shown in Fig. EI J.4 performs a logic:
Fig. El 1.2
(a) !ogle O ai points A and B.
(b) 0 at point A and I at point B.
(c) I at bOLh points.
4 . What is the significance of the small circle drawn
at the inputs or output of some types of logic
5. What is the purpose oflogic gate 'truth tables?
6. The truth table shown in Fig. E11.3 corresponds
(a) an OR gate ,
(b) an AND gate.
(c) a NOR gate.
Fig. El 1.3
Fig. El 1.4
(a) AND function.
(b) OR function.
(c) NAND function.
8. What is the significance of the line drawn over a
let ter or signal function when related to the
input or output of a logic gate?
Solutions to Exercises
Chapter 1
Chapter 2
Chapter 3
Chapter 6
Chapter 7
Chapter 8
Chapter 9
Chapter 10
(a l
Chapter 11
A.C. exciter, 4 I, 44
A.C. g1merators constant frequency , 41 , 44
frequency-wild , 36
A.C. motors, 142
A.C. power supplies compounding transformer, 43
constant-frequency, 37 , 47
constant-speed drive, 3 7
field exdtation, 43
freq ueni;y-wi)d, 36
kilovolt-amperes reactive
(KVAR), 35
load controller, 48
load sharing, 4 7
Meri-Price system. I 22
mutual reactor. 50
voltage regulation, 45
A.C. principles active component, 35
ampli tud e value, 32
apparent power. 35
cycle, 32
'delta' connection, 35
effeclive power, 35
effective value, 33
frequen cy, 32
in -phase, 33
instantaneous value, 3 2
interconnection of phases, 34
kilovolt-amperes, 35
line voltage, 35, 58
neutral point, 34
out-of phase, 33
peak value, 32
phase angle, 33
phase relationships, 33
phase voltage-, 3 5 ·
phasing, 33
power factor, 35
quadrature, 33
qua<Jruture component, 35
reactive component, 35, 36
root mean square value, 32
s.ine wave, 32
single-phase, 33
'star; connection , 35
three-phase, 33
true power, 3 5
volt-amperes reactive, 129
wattful component, 35
wattless component, 35
working component, 35
Advisory lig.h ts, 130
Airconditionjng system, 180
Air-driven generators, 52
Air intake anti-icing, 170
Air turbine , 52
Alternator, 7
Aluminium cable connections , 89
Ammeters, 123
Amortisseur windings , 42
AND gate, 188
Annunciator panel, 13 2
Anti -collision Jigh!s, 146
Anti·icins systems , 167
Anti-skid control systems, 177
Apparent power, 35
Armature. 6
Arniat\lre reaction, 4
A.T.A. Specification, 184
Allto-transformer, 61
Auxiliary interpoles, 4
Auxiliary power units , 73
Baretter, 4 5
Base , 14
Batteries ca pacity, 21
charging methods , 27
chemical re11ctions, 19, 20, 22
connect ions, 25
discharge rate, 23
functions , 18
lead-acid, 20
location , 23
nickel-cadmium, 20 , 2 I
state of charge, 23
thermal runaway , 23
ventilation, 24
Battery charging, 27
Battery systems, 25
'Blow out' position , 156
Bonding system, 92
Booster coils, 159
Breakers, 110
Bridge lighting, 151
'Bridge' rectifier connections ,
Brushes, 6, I 0
Brushless gen erators, 41
Brush wear, 10, 76
Bus tie breaker, 110
Cables co-axial , 86
coding schemes, 82, 184
connections, 88
ignition, 86
routing, 84
seals, 85
terminations , 88
thermocouple, 86
types, 82
Cable seals, 85
Capacitive circuit, 33
Capacitive load, 62
Carbon pile regulator, I 2, 67
Caution lights, 129
Central warning systems, 131
Circuit breakers, 113
Circuit controlling devices relays, 106
switches, 99
Circuit diagrams, 184
Circuit protection devices, 111
Circular-scale indicator, 123
Coaxial cables, 86
Cockpit lighting, I 50
Coding schemes, 82, 184
Collector, 14
Com mutator, I
Compensa ting windi ngs, 4
Compound motor, 138 , 140
Conduits, 85
Const an t·frequency generators, 41
Constant .frequency systems, 7,
37, 46,47
Constont,speed drive , 37
Constant-speed drive disconnect , 39
Contactors, I I 8
Control panels, 80, 96, 123
Conversion factor, 204
Cooling of generators, 8, 36, 42, 43
Corona discharge, 94
Crimped terminal s, 88
Current lim_ilers, I 12
Current regulator, I 2
Current transfonner, 48, 61
Cycle, 32
Damper windingS, 42
D.C. generators characteristics, 3
classifications, 2
cooling, 9
coup\ingS, 8
load-sharing, 16
principles, l
shunt -woun d, 3
spark suppression, 7
voltage regulation, 10
windings, 4
D.C. motors, 1".36
'Dead beat' indications, 123
De-icing systems, 167
'Delta' connection, 35
Differential current protection, I 21
Dimming facility, 153
'Drop-out' voltage, l 07
Dual Ignition, 160
Ducted loom , 85
Full-wave rectlfication, 57
Fuses, 111
Galley equipment, 182
'Ganging', I 00
'Gate', 55
Generators a.c., 36, 41
air-driven, 52
d.c., 1
Generator circuit breaker, 110
Half-wave rectification, 57
Heat sinks, 7
'Holes', 14, 53
Hot air bleed systems, 173
Hysteresis motor, 144
Ice and rain protection, 166
lce' detectlon, 174
Ignition systems cables, 86
distributor, 158
Earthing, 87
dual, 160
Earth stations, 87
high energy, 161
Earth return system , 87
magneto, 1S7
ECAM system , 133
spark plugs, 160
Eddy current damping, 123
switches, 159
Effective power, 35
Indicating fuse, 112
Electrical bonding, 92
Indicating lights, 129
Electroluminescent Ughting, 152
I11duction motors, 142
Electronic display systems, 133
Inductive circuit, 33
Emergency circuits, 77
Inductive load , 62
Emergency Ughting, 154
Inhibited gates, 189
Emitter, 14
In-line connectors, 89
Equalizing coils, 18
In-phase, 33
Essential service~. 77
Instrument lighting, 151
Exclusive gates, J89
External characteristics (generator), Integrated drive generators
(IDG's), 43
Internal c haracteristics (generator),
External lighting, 145
External power supplies Internal lighting, 150
a.c. systems, 71
Interpole windings, 4
auxiliary power units, 73
Inverter, 66
d.c. systems, 69
'Jumpers', 94
Field excitat ion a.c. generators, 43
d.c, generators, 3, IO
Kilovolt-amperes (kV A), 35
Kilovolt-amperes reactive (kV AR),
Field 'flashing', 4
Fire detection, 162
Fire extinguishins, 166
Landin-g'gear control, 175
Frequency, 32
Frequency meters, 128
Landing gear position, 176
Frequency regulation (inverters}, 66 Lead-acld battery, 19, 20
Frequency-wild generators, ·3 6
Lighting Frequency-wild systems, 36
advisory, 130
anti-collision, 146
cockpit, 150
em ergen cy, 154
ice Inspection, 150
instrument, 151
integral, I 51
intensity control, 153
landing lamps, 147
master warning and caution, 132
navigation, 14 5
passenger cabin, 152
strobe, 147
taxi lamps, 147
Limiting resistors, 11 2
Limit switches, 141
Linear actuators, 140
Line voltage, 34, 58
toad controller, 48
Load equalizing circuit, 16
Load-sharing a.c. generators, 47
d. c. generators, I 6
reactive load , 49
real load , 48
Load-ehedding circuit, 183
Logic circuits, 186
diagrams, 186, 189
gates, 186, 219
switching functions, 187
symbols , 187
Low tension magneto, 160
Magnetic amplifiers, 40, 48
Magnetic indicators, 130
Magnetic neutral axis, 4
Magnetic screen, 123
Magnetos, 157
Measuring instruments ammeters, 123
frequency meters, 128
moving coil, 123
power meters, 129
shunts, 126
vanneter, 129
voltmeters, 123
wattmeter, 129
watt/var meter, I 29
Mercury switches, 104
Merz-Price system , 122
Micro-5witches, l 03
Motors actuator, 140
capacitor, 143
characteristics, 137
compound, 138
hysteresis, 144
induction , 14 2
instrument, 142
series , 13 7
shunt, 138
split-field, 138
starter, 138
Moving coil, 123
Mutual reactors, 50
NAND gate , 189
Navigation lights, 145
Negated, 189
Negative logic, 187
Neutral point, 34
Nickel-cadmium battery, 20
Non-essential services, 77
Non-parallel a.c. srstems, 77
NOR gate , 189
NOT function, 189
'Notch time', 68
N-P-N transistor, 14
N·type semiconductor, 14, 53
On-board battery charge r. 27
Open loo m, 84
OR gate, 188
Out--0f-phasc, 33
Overfrequency protection, 121
Overheat detection, 164
Overvoltage protection , J 17
Paralleling a.c, generators, 4 7
d. c. generators, 16
Passenger cabin services , J 52, 182
'Pigtails', 7
Pillar lighting, 151
Plugs, 90
P-N·P transistor, 14
Pneumatic de·lcing, 167
Polar inductor magneto, 158
Polarized armature relay, 108
Polarizing keys, 91
Positive logi c, 187
'Potting', 92
Power conversion equipment rotary, 66
static, 53
Power distribution systems busbars, 76
cables, 82
coding schemes, 82
earthing, 87
electrical bonding, 92
electrical diagrams, 184
services, 76
'standardising', 95
wires, 82
Power factor, 35
Powe r meters, 12 9
Power utilization a.c. motors, 14 2
actuators , 140
d.c. motors, 136
de-icins, 167
external lighting, 145
engine starting, 154
fire detection, 162
fire extinguishing, 166
ignition, 15 7
internal lighting, 150
passenger cabin , 152, 182
smoke det ectors, 164
'Press-to-test', 130
Primary bonding conductors, 94
Propeller de,icing, 170
synchronizing, I 82
Proximity switches, I 06
P·type semiconu uctor, 14, 5 3
'Pull-in' voltage , 107
Pulse shaper circuit, 67
Quadrature, 33
Quadraturu c;omponent, 35
Quill-drive, 8
Rain repellent system, 178
Reactance sparking, 4
Reactive component, 35 , 48
Reactive load, 48
Real load, 48
Rectified power supplies, 7
Re ctifiers circuit connections, 57
' holes', 14, 53
rotating, 41
selenium, 54
silicon, 54
silicon-controlled, 55
silicon junction diode, 5S
Zener diode, 15 , 55
Relays attrpcted-armature, I 08
attracted -core , 107
overvol tage, I 18
polari zed armature, 108,
slugged, 108
' Relight ' i.:ircuH, 161
Residual magn etism, 3
Resistive circuit, 33
Reverse current circuit breaker, 11 7
Reverse current cut-out, 115
Reverse current protection, 115
Reversible compound motor, 139
Ripple frequency, 5 7
Root mean value, 3 2
Rotary actuators, 140
Rotary converting equi pment frequency regulation, 66
inverter, 66
static inverter, 67
voltage regulation, 66
Rotating armatu re magneto. 159
Rotating magnet magn eto, I S8
Rotating rcctlficr, 41
Routing charts, 184
Screening, 7, 9 5
Secondary bonding conduc1o rs, 94
Self.sustaining speed, 161
Semiconductor, 14
Series-wound motor, 13 7
Shunts, 126
Shunt motors, 138
Silicon-controlled rectifii:r (S.C.R.),
Silicon junction diode, 55
Single-phase system , 33
Single-phase transform er, 60
Slip speed, 143
Slugged relay, I 08
Smoke detectors, 164
Sockets, 90
Spark suppression, 7
Speed characteristic (motors), 137
Split busbar system, 77
Split-field motor, 138
Squirrel-cage, 14 2
' St ar' con ne ction, 34
Starter·.(ll!nCrntor, 157
Starter motor systems, 154
State o f charge, 20
Sta lie c;harger, 93
Static converting eq ui'pment rectifiers, 4 7
static inverter, 67
transformer~. 59
transformer-reclifler units, 63
Static discharge wi cks , 9 5
Static inverter, 67
Step-down transformer, 59
Step-up transformer , 59
Strobe lighting, 147
Switches - .
double-pole , 100
ignition, 159
limit, 141
mercury, l 04
micro, 103
position, 100
pressure, I 04
proximity , I 06
push, 101
rheostats, l 03
rocker-button, J 02
rotary, I 02
single-pole, I 00
thermal, 106,163 , 164
time , 103 , 170
toggle, I 00
Switched reverse current relay, 116
Synchronizing lights, SI
Synchronous speed, 14 3
Symbols, 21 2
Systron Donn er detection system,
Temperature control (de-icing and
anti-icing), 170
Thermal runaway, 23
Thermistor, 44
Thermocouple cables, 86
Three-phase system, 34
Three-phase t ramformer, 6 2
Thyristor, 55
Torque characteristics (motor), 137
Transformation ratio, 59
Transformers auto, 61
circuit connections, 60
compounding, 36
current, 40, 6 I, 126
instrument, 126
ratings, 62
single-phase, 60
step'1own, 59
step-up, 59
three.phase, 62
turns ratio, 59
voltage, 59
Transformer-rectifier units
(T.R.U's), 63
Transistors , 14
Transistorized voltage regulator, I 4
Trlp·frce 1 circuit breaker, 113
Truth tables, 187,219
Turbine engine ignition, 160
Turbine engine starting, 154
Turbo-starter systems, 157
Underfrequen cy pr_otection, 121
Undervoltage protection, 120
Varmeter, 129
Vibrating contact regulator, 11
Vital services, 77
Volt·amperes reactive, 48, 129
Voltage drop, 3
Voltage regulation -
constant-frequency generatorS,
d.c. generators, I 0
frequency-wild generators, 43
inverters, 66
Voltage transformer, 59
Voltmeters, 123
Wammg lights, 129
Wattmeter, I 29
Watt/VaI meter, 129
Windshield anti-icing, J73
wipers, 178
Wires, 82
Wiring diagrams, 184
Yoke, 4
Zener diode, 15, 55
Zener voltage, 55
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