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| Subject: | encoder approval |
--> Avionics-List message posted by: <bakerocb@cox.net>
Responding to a posting from Skip Simpson:
8/10/2006
Hello Skip, The issue on the use of non TSO'd altitude encoders is currently
under review (again) at FAA headquarters. I have been involved in this issue
for some time, but have refrained from posting any information on this
unresolved issue because of the potentially huge adverse impact upon our
amateur built community. I wanted to avoid much controversial and
distracting communications pending the, hopefully favorable, eventual ruling
by FAA on this subject. Here in a fairly brief summary form is the
situation:
1) FAR 91.217 Reads as follows: "Data correspondence between automatically
reported pressure altitude data and the pilot's altitude reference.
No person may operate any automatic pressure altitude reporting equipment
associated with a radar beacon transponder-
(a) When deactivation of that equipment is directed by ATC;
(b) Unless, as installed, that equipment was tested and calibrated to
transmit altitude data corresponding within 125 feet (on a 95 percent
probability basis) of the indicated or calibrated datum of the altimeter
normally used to maintain flight altitude, with that altimeter referenced to
29.92 inches of mercury for altitudes from sea level to the maximum
operating altitude of the aircraft; or
(c) Unless the altimeters and digitizers in that equipment meet the
standards of TSO-C10b and TSO-C88, respectively."
2) It would appear that any aircraft, standard type certificated or
experimentally certificated, whether flying IFR or VFR, and replying with a
mode C transponder altitude read out to ATC, either must comply with 91.217
(b) or be using a TSO-C88 approved altitude encoder.
3) Some companies providing altitude encoders to the amateur built
experimental aircraft community, some of which are incorporated into EFIS,
have been providing non TSO'd altitude encoders. It is not always made clear
by the manufacturing companies whether the altitude encoders within their
EFIS are TSO'd or not.
4) Some of these non TSO'd altitude encoders have better performance than
the TSO calls for both in terms of altitude granularity output and in output
format (serial instead of gray code).
5) There are many of these non TSO'd encoders in aircraft that are currently
flying and many in aircraft under construction.
6) A general presumption in the community was made (at least by those that
thought about it) that if an altimeter - altitude encoder - transponder
installation passed the FAR Part 43 Appendix E and F tests which are
required by FAR 91.411 and 91.413 every two years, that FAR 91.217 (b) was
being complied with.
7) A ruling from FAA headquarters in response to a letter from me said "not
so" to such compliance interpretation in the following fashion:
"Your letter posed the following questions:
1. If an amateur built experimental aircraft has an installed TSO'd ATC
transponder as required by Title 14 Code of Federal Regulations (14 CFR)
section 91.215, but a non-TSO'd altitude encoder and the installation has
passed the test and inspection requirements of 14 CFR sections 91.411 and
91.413 within the preceding 24 calendar months, does the installation meet
the requirements of 14 CFR section 91.217(b), and therefore make that
installation acceptable for IFR operations?
2. If the answer to question one is No, can you please tell me
why?
The answer to question one is "No." The testing required to show the
transmitted altitude data corresponds within 125 feet (on a 95 percent
probability basis) is more rigorous than the requirements referenced in 14
CFR sections 91.411, 91.413, and 14 CFR, part 43 appendices E and F. The
tests required by 14 CFR part 43 appendix E(c) measure the automatic
pressure altitude at a sufficient number of test points to ensure the
altitude reporting equipment performs its intended function.
Title 14 CFR section 91.217 paragraphs (b) and (c), state that pressure
altitude reporting equipment must be tested and calibrated to transmit
altitude data correspondence within stated specifications; or, the
altimeters and digitizers must meet the standards in TSO-C10B and TSO-C88,
respectively.
Should the owner/operator elect to exhibit compliance with tests and
calibration provided in 14 CFR section 91.217(b), a test method would need
to be developed that
ensures the transmitted data corresponds within 125 feet of the indicated
altitudes from sea level to the maximum operating altitude of the aircraft
on a 95 percent probability basis. This testing also needs to ensure the
performance characteristics of the equipment are not impacted when
subjected to environmental conditions (voltage fluctuations temperature,
vibration, etc.) which may be encountered in airborne operations.
Completed tests and calibration results should be maintained in the
aircraft records.
Thank you for your interest in aviation safety."
8) You can see the tremendous impact that enforcement of such a position
would have on the companies making and selling non TSO'd encoders or EFIS
containing non TSO'd encoders, the airplanes under construction planning to
incorporate those EFIS, and all of those airplanes currently flying with non
TSO'd altitude encoders.
9) I did not accept the FAA's position in 7) above as the final word and am
working through a cooperating local FAA FSDO employee to both educate FAA
headquarters and to get them to adopt a more reasonable position on the use
of non TSO'd altitude encoders.
10) I would encouage our community to not react in an adverse manner to the
FAA's current position and to continue to work the issue on a cooperative
basis. I will post additional information as it becomes available and
attempt to answer any questions that you may have.
OC -- The best investment we will ever make is in gathering knowledge.
<<Time: 09:12:25 PM PST US
From: CardinalNSB@aol.com
Subject: AeroElectric-List: Re: encoder approval
Is the Rocky Mountain encoder approved for certificated aircraft, the
factory says that "it conforms to c88a", is that enough, or is there more
needed.
Any opinions on the unit. Thanks, Skip Simpson>>
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