If a transponder has a blind altitude encoder how does ATC or flight following know the actual altitude?
Asked by: Alex Censor 11239 views General Aviation
It`s my understanding that many GA aircraft`s transponders have blind altitude encoders (set to a barometer of 29.92) so they can of course be way off (not uncommon to be as much as 500 feet off).
That would be not a huge problem in separating and advising if everyone was flying with blind encoders, but (again IIRR) some aircraft have thier regular panel adjustable altimeters (that they periodically reset to reported current barometer) that transmitt that more realistic altitude into the transponder, and in turn to ATC radar, etc. So they`re, so to speak, on a different page it would seem.
(And let`s not complicate the issue further with the pilot that might not have a transponder and reads/reports his altitude off the GPS because GPS altitude is frequently more accurate than reading off a altimeter corrected to a barometer reading from a distance station.)
So my question is along these lines: Does ATC (or its presumably sophisticated radar system) know if a transponder has a blind encoder sending very likely inaccurate altitude reports (unless pressure happens to be close to 29.92 that moment)?
I`m imagining that if the transponder is known to have a blind encoder that it would be easy to take the reported altitude from it and have the system at ATC (which knows the actual barometric pressure ... at least close to the airspace) and automatically make a correction for the blind encoded pressure report.
Is that so? Or is there something else mitigating this potential SNAFU?
Or are aircraft at the same altitude flying around with the two different types of transponder altitude encoders (blind vs non-blind) often substantually in disagreement about what altitude they are on?
Alex
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