Avionics News April 2022 - 54

Can you find this error with glideslope frequencies and localizer frequencies? (KX 155A Maintenance Manual)
THEORY & PRACTICE
Continued from page 53
frequencies using their phone. I liken this information to
knowledge of the local landscape. For most avionics, the
landscape is radio. It's important to know the avionics
radio landscape just like it's important to know the
locations of the nearest grocery store and gas station.
After moving to a new community, a person would be
expected to find the local store using the navigation
system in the phone or car, but that same person would
be expected to have the ability to find the store on their
own after the first trip.
The next error is a chance to walk a student through
an investigation since it does not involve any knowledge
they were expected to commit to memory. We use a
transponder manual that asks the technician to activate
certain pulses to test the Gilham Gray Code altitude
reporting (Mode C) of an air traffic control transponder.
I have to admit, I'm not quite sure what's going
on with this error. We have checked multiple units,
and all behave the same. I have chosen not to trouble
Honeywell/BendixKing technical support with the
problem, and since it provides a teachable moment, I
haven't checked for updates on the manual.
The students will work their way through the return to
service tests and check seven of the eight altitudes called
out in the manual. For those unfamiliar with transponder
specifics, a device within the aircraft converts air
pressure to a code, which the transponder will send to
54 avionics news * april 2022
air traffic control or to aircraft equipped with collision
avoidance equipment. Each pulse has a name. For
example, if the transponder sends pulses C1 and B2, then
the altitude is reported as 300 feet.
The seventh altitude test calls for the students
to imitate the altitude encoding device and tell the
transponder to send pulses C1 and D4, which translates
to 62,300 feet. The final test is to add the D2 pulse. C1,
D2 and D4 translates to 63,200 feet. The transponders
won't do this. The students add the D4 pulse, and
nothing changes, so we get out some more manuals.
In one manual, we look up the first occurrence of D2
in the Gray Code. The D2 pulse is not used until 62,750
feet. Next, we discuss the type of aircraft in which this
unit would be installed. It's possible, but highly unlikely,
this type of unit would be mounted in any aircraft capable
of reaching 40,000 feet, let alone 62,750 feet. Finally,
we get out the installation manual and discover the unit
is certified for 50,000 feet. maximum. As a result, we
conclude this must be a mistake, and in the field, we
would look for a manual update or call tech support.
Usually, none of us like to find errors in a manual.
At worst, the error could cause us to damage an
expensive piece of avionics gear. We often spend some
unbillable time making sure the error gets corrected.
A knowledgeable technician might even gloss over
some errors like the kHz problem (an obvious copy and
paste mistake) and keep going. For those of us teaching
avionics, the errors can be great opportunities for
students to learn. q

Avionics News April 2022

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