My old way to test a transponder versus using a portable tester

For years, I'd just check the transponder reply on the aircraft's own test function and call it good. About 6 months ago, a plane I signed off had a squawk for intermittent altitude reporting, and I felt like an idiot. Now I always hook up my portable tester, run the full interrogation cycle, and verify the Mode S data block. It adds maybe 15 minutes, but it catches stuff the built-in test misses. Anyone else made that switch after getting burned?

3 comments

3 Comments

parker_hall51mo ago

Yeah, that "intermittent altitude reporting" thing is a killer. My buddy got bit by that last year on a 172, the built-in test showed green but his portable found a bad pressure sensor.

wade_anderson1mo ago

That exact scenario is why I don't trust a single green light anymore. It reminds me of my old car's check engine light that would only come on after driving for an hour. Modern systems are so complex that a simple pass/fail test can miss the real problem. We're putting too much faith in a single indicator that says everything is fine. It creates this false sense of security right up until the moment something quietly fails.

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hayes.wade1mo ago

But doesn't the built-in test still catch most major faults? Adding 15 minutes for every check seems like a lot of extra time.

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