Hi David,
I propose to share with you my personal experience on that subject :
My aircraft was originally fitted with a Garmin GTX320 and a Terra AT3000
encoder.
After a few years flying, I started to experience altitude reporting problems.
ATC was occasionally seeing me at 6000 ft or below ground level while I was
flying at 2000 ft AMSL. I was once wrongly accused of having busted the class
A London TMA while I was flying well below the max allowed altitude. Then
I reworked the wiring without success, then I had the encoder tested by an
avionic
shop and it was found to be ok. Then I borowed another GTX 320 from a frend
for a few flights and the system was working ok. So the culprit was clearly
the transponder which was intermittently reporting an altitude different from
the one transmitted by the encoder.
I have since installed a mode S GTX328, keeping the Terra encoder and it has
been
working fine for the last five years.
Conclusion : in most of the cases the culprit is the altitude encoder but it
could
also be the transponder itself.
I hope that helps.
Regards
Remi
F-PGKL
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