Avionics News July 2019 - 35

Part 25
"The best troubleshooting problem I ever solved?" asked
Kevin Hufford, avionics manager at Chicago Jet Group
in Sugar Grove, Illinois. "I'm in the throes of a real big
problem on a (privately owned) Boeing 757. We did a
FANS and ATN upgrade with ADS-B STC and, oh, man,
we're really out in the weeds on it right now. We're trying
to figure it out with the operator, but it is not resolved yet,
so it might not be a good example."
Hufford explained that ATN (aeronautical
telecommunications network) is Europe's version of
CPDLC (controller pilot data link communications), which
is one component of FANS (the Future Air Navigation
System). "The install went great, and everything
works well," he said. The problem is that the dual
transponders, dual DME systems, and TCAS are all L band
communicators, which means only one of them can talk at
a time.
"Right now, when Transponder 2 is selected, it steps on
DME 2, which dashes its display, meaning it is no longer
computing its data," he said. That means the suppression
circuit that connects the five components and silences the
others while one box is talking is working correctly. "This
happens in milliseconds; it's like a multiplex behavior."
A third-party shop did the install work, and Hufford at
first suspected an easily fixable wiring switch. But Boeing
designed a modular avionics shelf system with quick
disconnects "that Murphy-proofed everything so that
couldn't happen," Hufford explained.
"So something is not right. I'm looking at the 429
data outputs to the DMEs, seeing if something is being
corrupted when the No. 2 transponder is selected, which is

Chicago Jet Group
Avionics Manager
Kevin Hufford.

Photo by Scott M. Spangler

short, that autopilot works on AC signals and phasing,"
he said. "I found that the two lines that carry the
excitation signals looked like they were right, but they
were reversed."
When comparing the letters stamped in the connecter
that identified the pin positions, all seemed correct. But
it turned out that the letters were in the reverse direction,
confirmed by comparing it to the connector on another
autopilot.
"It was a really weird problem, one you normally
wouldn't look for, but it slipped through a number of
levels of quality control," Karpowitz said. "That's why
attention to detail is so critical in this profession. If
something doesn't look right, smell right, or feel right,
you have to look into it no matter how much work it
takes. It is always better to send out a product that is safe
and reliable."

affecting the DME. All my expert friends seem to think it
is a suppression issue, but all the harnessing is ringing out
true. It's frustrating; it is really frustrating."
Adding to the frustration is troubleshooting the airplane
at its home base. "That's tricky because we're limited
in what we could dismantle to get access to things," he
explained. "The airplane is airworthy; it's flying to Europe.
Both transponders work, both DMEs work; they just have
to fly with the avionics configured where nothing hurts the
other signals.
"We know this problem is embedded, and
troubleshooting problems like this is what we do. We know
that it all works in certain configurations, and we know the
configuration where it doesn't work. And we will ferret
out the source of the problem using all available sources of
technical data."
If there is a troubleshooting mantra, this persistent,
determined dedication to making it work right, no matter
what, seems universal. q

avionics news

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july

2019

35



Avionics News July 2019

Table of Contents for the Digital Edition of Avionics News July 2019

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Avionics News July 2019 - Intro
Avionics News July 2019 - No label
Avionics News July 2019 - Cover2
Avionics News July 2019 - 1
Avionics News July 2019 - 2
Avionics News July 2019 - 3
Avionics News July 2019 - 4
Avionics News July 2019 - 5
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Avionics News July 2019 - Cover3
Avionics News July 2019 - Cover4
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