This month we begin a new series on aircraft wiring....

This month we begin a new series on aircraft wiring....
This month we begin a new series on aircraft wiring. A preceding Tech Time series detailed the proper
use of wire using the FAA’s Advisory Circular AC 43.13-1B Acceptable Methods, Techniques and
Practices for Aircraft Inspection and Repair. We are now going to expand our coverage by examining
electrical wiring from a broader perspective.
The topic of aircraft wiring has come to our attention due to several high profile airline accidents. The
airline industry is addressing the issue through the establishment of FAA/industry working groups. The
typical pattern of action is for Part 25 aircraft (and the airlines) to meet new mandates, then work down to
Part 25 business and commercial aircraft, and finally to Part 23 aircraft. There is currently a debate about
what wiring deficiencies exist in our fleet. What is not questioned is the age of the fleet and the condition
of the wiring. We still have aircraft flying with aluminum conductors and most aircraft manufactured
through the 1970s have wiring with PVC insulation. There is a growing consensus that we must begin to
treat the wiring in an aircraft as an independent system, just like communications, navigation, fuel,
pneumatic, hydraulic, etc. This is going to be the premise of this series.
How do you verify the serviceability of a wiring system? We currently use the data found in AC 43.131B. This information, due to its generalization, leaves much interpretation to the technician. This also
leaves much interpretation to FAA Principal Inspectors. The result is a lack of standardization and
industry wide compliance. We are going to review and explore one subset of wiring systems, that is,
electrical load capacity and the corresponding determination of electrical load. Guidelines for technicians
and real world examples will be included to make it easier to perform this service.
Let us begin by reviewing the FAA’s AC 43.13-1B CHG 1 dated 9/27/01, found at
Paragraphs 11-35 and 11-36 provide the current guidance for the Repair Station.
11-35 a. OUTPUT RATING. The generator or alternator output ratings and limits prescribed by the
manufacturer must be checked against the electrical loads that can be imposed on the affected generator
or alternator by installed equipment. When electrical load calculations show that the total continuous
electrical load can exceed 80 percent output load limits of the generator or alternator, and where special
placards or monitoring devices are not installed, the electrical load must be reduced or the generating
capacity of the charging system must be increased. (This is strictly a “rule of thumb” method and should
not be confused with an electrical load analysis, which is a complete and accurate analysis, which is a
complete and accurate (sic) of the composite aircraft power sources and all electrical loads) When a
storage battery is part of the electrical power system, the battery will be continuously charged in flight.
The devil is in the details. Guidance is lacking to tell the Repair Station how to perform electrical load
calculations. Do you calculate with all lighting, anti-ice, electrical equipment on, gear and flaps down,
transmitting with all transmitters i.e. communications, in-flight weather, sat-com, etc., auxiliary power
jacks drawing rated current, and more? Or do you suppose a typical scenario (with perhaps 99%
probability) with reduced loading? Where do you measure the electrical load being drawn? Do you
measure from the generator/alternator source or at the storage battery that adds capacity for short
durations? What special placards and/or monitoring devices specifically allow you to exceed the 80%
output load limits prescribed? By how much?
11-35 b. describes the installation of placards to inform pilots of acceptable load/power source
combinations and the use of low voltage warning lights.
11-35 c. describes installations where the ammeter is in-line with the battery lead. If such a system is
regulated to limit the maximum current and a voltmeter is installed to read the aircraft buss voltage, the
system is not considered to be overloaded if the ammeter never reads “discharge” (except for short
periods of time) and the voltmeter remains at the system voltage. This paragraph helps establish how to
determine acceptable loading but still leaves the question of how much you turn on for the test, how long
is a “short period of time” and “system voltage”.
11-35 d. describes another method of monitoring loading. In installations where the ammeter is in the
generator or alternator lead and the regulator system does not limit the maximum current that the
generator or alternator can deliver, the ammeter can be redlined at 100 percent of the generator or
alternator rating. If the ammeter reading is never allowed to exceed the red line, except for short
intermittent loads, the generator or alternator will not be overloaded. Again we do not know how much
equipment to turn on for the test and further, when was the last time these current measuring devices were
checked for accuracy? Should these load tests be performed with a calibrated device?
11-35 e. Where the use of placards or monitoring devices is not practical or desired, and where
assurance is needed that the battery will be charged in flight, the total continuous connected electrical
load should be held to approximately 80 percent of the total generator output capacity. When more than
one generator is used in parallel, the total rated output is the combined output of the installed generators.
This is self-explanatory and in line with other guidance. It is left to the technician to quantify
“approximately 80%”.
11-35 f. When two or more generators and alternators are operated in parallel and the total connected
system load can exceed the rated output of a single generator, a method should be provided for quickly
coping with a sudden overload that can be caused by generator or engine failure. A quick load reduction
system or procedure should be identified whereby the total load can be reduced by the pilot to a quantity
within the rated capacity of the remaining operable generator or generators.
This is self-explanatory and in line with other guidance.
The connected load of an aircraft’s electrical system may be determined by any one or a combination of
several acceptable methods, techniques, or practices. However, those with a need to know the status of a
particular aircraft’s electrical system should have accurate and up-to-date data concerning the capacity
of the installed electrical power source(s) and the load(s) imposed by installed electrical powerconsuming devices. Such data should provide a true picture of the status of the electrical system. New or
additional electrical devices should not be installed in an aircraft, nor the capacity changed of any power
source, until the status of the electrical system in the aircraft has been determined accurately and found
not to adversely affect the integrity of the electrical system.
Few acceptable methods, techniques and practices to determine electrical load exist that are universally
recognized and within the scope of most Repair Station’s capabilities. What is needed is a clarification of
concepts and acceptance of specific actions and measurements within the industry.
Next Month: A look at electrical loads.
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