Avionics News March 2013 - 71

tighter tolerances and shorter inspection intervals.
43-203B offers a good starting point, though it does not
Requirements for carrying out the tests are spelled out in
spell out procedures in exacting detail. Ochs warned
FAR 43, Appendices E and F.
that even piston singles and light twins can have unique
While the FARs give the requirements, they don’t spell
requirements, particularly if they were built or retrofitted
out procedures to meet those requirements. For many
with a glass panel including one or more air-data
years, avionics technicians performing altimeter systems
computers.
checks relied on FAA advisory circular 43-203B for
“The key is to use the right maintenance instructions
guidance. While it’s still a useful document, times have
– check to see if there are specific instructions either
changed.
from the aircraft manufacturer or the STC holder,” Ochs
According to Rick Ochs of Spirit Avionics, first check
said. “It’s also a good idea to check the aircraft logs and
to see if there’s an instruction for continued airworthiness
any 337 identifying an upgrade to the air-data system
specified either by the airframe manufacturer or in a
for an ICA.”
supplemental type certificate. This will always be the
In a conversation with Garmin’s avionics product
case for an RVSM aircraft and is
support manager, Randy Stewart,
increasingly likely even for light
and Tim Larberg, field service
aircraft using a glass (electronic)
engineer, they learned from
The requirement for
primary flight display. If an ICA
Garmin’s G-1000 and G-600
applies, it must be followed
glass panels. They noted, “Our
periodic altimeter system
exactly. Ochs noted the best time
ICA says to perform (altimetry)
to discover this is before you
tests per FAR 91.411.” They
testing every 24 months
begin work – preferably before
also said modern solid-state
a price is quoted. Altimetry tests
transducers aren’t subject to some
is established by Federal
for some RVSM aircraft require
problems that were common
special tooling and/or test
in the steam-gauge era of
Aviation Regulation 91.411
equipment that may have to be
mechanical indicators: “When an
rented. Ochs suggested a check
inaccuracy turns up, it tends to
for aircraft flown on
of the logs to identify what
be consistently off by the same
procedure was used for the last
amount from sea level to 30,000
instrument flight rules.
altimeter system test.
feet – where the regs give you
Former avionics technician
different tolerances at different
Brad Primm, now president of
altitudes. If an offset shows
Lane Aviation, wrote an excellent article on this topic
up, it can be adjusted in software during bench test,
published by Avionics News nine years ago. He noted
which requires pulling the air-data computer out of the
that since then, proliferation of RVSM aircraft has made
airplane.”
altimetry checks more complex. He also noted FAA
Typically, transponders are tested at the same time as
concern about failure to follow detailed maintenance
the pitot-static/altimetry system, per FAR 91 Appendix F.
instructions.
According to Garmin’s Stewart and Larberg, transponder
“Just because an aircraft is capable of RVSM doesn’t
test procedures haven’t changed dramatically, though the
make it legal to operate there,” Primm said. “Operators
test equipment has gotten more complex.
must have an operational plan, and the manual must be
Barry Beasley of Aeroflex, a manufacturer of
carried on the airplane. You need to pull that manual,
transponder test equipment, said modern test sets offer
look at it and reference it in your paperwork. Many
a wide range of options. “We can do an over-the-air
operators may not be fully aware of the requirements and
radiating test, which tests end-to-end, and you also can
assume a previous operator’s manual still applies.”
For aircraft that do not have an altimetry ICA, AC
Continued on following page
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Avionics News March 2013

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