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1. 06:40 AM - Weird Transponder Problem - Fixed (Galin Hernandez)
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| Subject: | Weird Transponder Problem - Fixed |
The guys that are partners in a Grumman Cheetah in the hangar behind mine
told me they had been fighting an issue with their transponder (King KT76)
for over 3 months. Their transponder worked fine when squawking a code
(like 1200) but when they went to squawk altitude (Mode 3C) the code would
instantly change to 7770 with no altitude being displayed. Worst was that
their Tail Beacon ADS-B would act the exact same way. The squawk was
correct until the transponder altitude was turned on. Then even the ADS-B
would display 7770 with no altitude being reported.
This happened at different airport towers, different approach control and
even different center control so they knew it wasn't an FAA ground system
error. A local avionics repair station swapped their transponder with a
known good one and it did the exact same thing. They checked all the
gray-code wiring and it checked out perfect. No matter what code was dialed
up, as soon as the altitude reporting was turned on the, squawk went to
7770 with no altitude displayed. It was baffling.
I told them I had never seen or heard anything like this but would try to
help out. With a person from a very well known large aviation company here
at our airport I spent several days discussing transponder operation theory
trying to make sense of this. We eventually theorized that somehow the
altitude encoder wasn't sending the encoded altitude pulses at the correct
point within the data stream to the transponder. If the encoded pulses were
at a specific wrong point in the stream, 7770 would be the code transmitted
with no altitude reported no matter what was actually dialed in. The Tail
Beacon ADS-B would then report the erroneous data exactly as it was
received, GIGO. With this recommendation they changed the encoder and the
problem went away.
So if your transponder squawks 7770 when Mode C is activated no matter what
you have dialed in, replace your altitude encoder. Hopefully you will never
encounter this but I wanted to share it so you don't wind up with days of
headaches and ATC getting angry at you as they did.
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