GalinHdz
Joined: 08 Nov 2017
Posts: 150
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 Posted: Fri Mar 26, 2021 5:40 am Post subject: Weird Transponder Problem - Fixed |
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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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