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he annual calibration bill for the
typical avionics shop can make
anyone blue. Not only do we
calibrate the precision test equipment
required to do bench repairs and infield adjustments of avionics, but also
the crimping tools required for installations and wiring repairs, tensiometers used for autopilot cable checks,
and gauges used for pressure testing.
In addition, some of the rack mounted
test panels and perhaps even some of
the bench harnessing and break-out
boxes may require calibration.
Device used for tension meter calibrations
What are the basics of calibration
and why do we need to do it? What
can be done to control these costs?
In order to answer these questions,
we must delve into the world of
metrology and calibration. Metrology
is the science of weights and measures. It also encompasses a system of
weights and measures.
The reason we calibrate is to ensure
accuracy and what is accuracy? For
the purposes of metrology, it is the
conformity to fact or exactness. What
is calibration? The act of calibration of
a measurement device is to measure,
adjust and compare the measurement
device results to a standard. What is
defined as the act of measuring or
being measured.
What do we measure to? The
International System of Units, SI, is a
coherent system of metric units
defined by international consensus.
These units define the standards we
use. There are seven SI base units and
they are the meter (length), kilogram
(mass), second (time), ampere (electric current), kelvin (temperature),
mole (amount of substance), and candela (luminous intensity).
What are standards? Primary standards are defined as standards to
which comparisons can be made that
are designated or recognized as having
the highest metrological qualities or
precision in their field.
What is precision? Precision is the
quality of being exact or precise.
Primary Laboratories are metrology
laboratories that maintain an SI unit or
units through the use of primary standards and are capable of carrying out
calibrations with reference to precise
primary standards.
Other basic terms to know are; resolution, which is the amount of detail
Measuring the calibration of a torque wrench
using two standards
Calibrating a cable tensiometer
that can be measured; tolerance, which
is the allowable variation from a particular standard; traceability, which is
the ability to trace a calibration back to
a primary standard; and uncertainty,
which is the estimated amount that a
measured or calculated result may be
off from the true result.
So why calibrate? It is amazing how
much calibration affects our lives. The
next time you are at the gas pump,
look for the certificate of calibration.
In my locale, it is a sticker with the
smiling face of the county auditor.
When you set your watch, you are
actually calibrating it to a higher standard (hopefully). The scale at the grocery will have a sticker as well.
Measured mile markers on the interstates are used to check the calibration
of your speedometer.
In our industry, the test equipment
we use undergoes mechanical stress,
changes in temperature and the natural
process of aging. All of these factors
contribute to drift. We control the
menace of drift with the process of
In researching this article, I visited a
local calibration lab and looked at several different calibrations. The shop I
visited calibrates a variety of items
related to aviation and general industry. Many of you probably think I am
about to bore you with the description
of a Fluke Voltage Standard used to
calibrate a digital multimeter, but fear
not! How about the calibration of a
cable tensiometer? What about a
torque wrench calibration?
The bench used to calibrate cable
tensiometers is located in a temperature and humidity controlled environment. The setup employs a rack device
to apply controlled amounts of tension
to short cable samples. The tension is
measured by an electronic transducer
in-line with the cable itself. The tensiometer is then tested in the same
Digital Pressure Standard
manner in which we use it in the field
by actually taking readings with it on
the cable sample.
One caveat though. When using a
tensiometer such as this to measure the
cable tension of steel cable, it is necessary to take three measurements in different positions on the cable and average them together, a procedure commonly missed in the field. This is necessary because the strands of the steel
cable create an uneven surface with
peaks and valleys.
The calibration bench used for
torque wrenches is also located in the
controlled environment and during the
demonstration, an analog standard as
well as an electronic standard were
used to check the torque wrench. See
the picture for the set-up. Dual standards ensure the reliability of the
measurement. Here is a caution about
torque wrenches and tools in general:
During normal use, torque wrenches
and any gage in general, should only
be exercised to two-thirds of the fullscale value.
Other common tools we use that
require calibration checks are crimping tools. The frequency of calibration
for crimpers depends upon the frequency of use. At the cal shop I visited, they perform the calibrations on
these tools by actually crimping the
pins for which the tool was designed
and performing a pull test on the wire.
In addition, they inspect the crimp for
evenness, quality and height.
Many crimpers come with a go/nogo gauge. The gauge will have two
machined ends. One end should fit
through the closed jaws of the tool and
the other end should not. Depending
upon your FAA inspector, this may be
good enough to satisfy the calibration
check requirement of the tool. In a
production environment, the go/no-go
gauge should be used at the start of
every shift and whenever the tool is
dropped to verify continued quality of
crimps. Some may consider the go/nogo gau ge as a tool for confidence
checks which are a form of self-checking between regular, more exacting
The typical avionics shop has
numerous test panels that employ analog meters. These meters should be
calibrated on an annual basis. At my
shop, we calibrate these meters ourselves in order to control costs. We use
our 4-1/2 digit Fluke as a secondary
standard when doing these calibrations. The Fluke meter we use is only
considered a secondary standard right
after a good traceable calibration and
before it is put into service in any
other capacity. This ensures a high
level of confidence in the results.
When sending out equipment for
calibration, we are required by the
FAA to get Certificates of Calibration
with traceability to primary standards.
When you receive a piece of avionics
from a vendor, you always examine
the paperwork for completeness and
accuracy. But what do you look for
when you receive a piece of test equipment from calibration?
The Certificate of Calibration
should contain the following elements
Continued on page 59
Continued from page 57
as a minimum. There should be a
unique tracking or certificate number
on the paperwork. This will allow
trace back to the specific records for
the calibration. The equipment calibrated should be clearly identified
with the correct part number and serial number. The testing date and test
conditions should be specified. The
confidence level of the testing should
be stated. A performance summary
should state whether the equipment
was found in-cal or out-of-cal and
whether any adjustments were
required. The final state of the equipment after calibration should also be
indicated. Details of the traceable
standards should be supplied. This
commonly means listing the test
equipment used by model and serial
number, including a statement that the
standards used in the calibration are
traceable back to primary standards.
Measured values should be provided
especially for non-conforming tests.
The performance summary should
be examined to determine if the equipment was found in an out-of-tolerance
condition. If so, you need to ask several questions. Was the equipment used
while it was out-of-tolerance? Did the
out-of-tolerance condition cause any
problems during use? Will a recall of
work be necessary? Unfortunately,
some cal shops will adjust as they calibrate. This may influence the measurements downstream of the adjustment, which will make the measured
values less reflective of “as-found”
readings. This makes the question,
“Will a recall of the work be necessary?” much harder to answer.
When the performance summary
indicates that a piece of equipment
holds its calibration, you can petition
your local FAA avionics inspector to
extend the cal interval beyond the normal calibration interval. In my shop, I
save all of the calibration certificates
for just this purpose. Our calibration
records book contains clear page protectors and I stuff the certificates in the
protector so they are readily available
for review by the FAA.
In many ways the typical avionics
shop “calibrates” aircraft systems.
During pitot-static and altimeter tests,
we are comparing (calibrating) the
ship’s instruments to known standards.
As we move toward domestic RVSM,
the standards we use must be upgraded to exceed the accuracy of the aircraft instruments. Even in the nonRVSM world, you must know the limitations of your equipment. For
instance, is it proper to use a standard
master altimeter to calibrate a servo
counter-drum altimeter in a jet?
The rule of thumb for metrology is
that the calibration standard should be
at least four times the accuracy of the
unit being calibrated. The answer to
the above question is probably no. It
may be allowed by your local FAA,
but is not a good practice. The standard analog master altimeter has
numerous sources of error including
hysteresis and after-effect. Digital
altimeter standards do not suffer from
these limitations and will deliver reliable test results time after time.
I wish I had more time to devote to
this subject and I realize that we have
barely scratched the surface. The parting thought I want to leave you with is
that you must treat your calibrated test
equipment with great care. You must
teach all users of the equipment to
handle it carefully and be mindful of
their part in the unbroken chain of
traceability back to primary standards.
There is a corollary to the phrase “If
you drop it, recheck it.” That corollary
Special thanks goes to Har ry Harris of
Cleveland Instrument Corp.
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