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Mode C troubleshooting


OR75

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while in flight, ATC started getting erratic altitude reading from my mode C transponder (seeing me at 14000 when I was at 8000 or 1400ft when I was at 5000, etc ...). They eventually asked me to turn my mode C veil off and leave it on mode A which worked fine.


I was wondering if anyone experienced that  and whether the error is coming from encoder or from the transponder.


the Garmin 430 on occasion did display that it was not receiving altitude info. 


 

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Sounds like a loose connection on or related to the encoder. 


When you first start-up your Garmin 430, do you get an altitude failure indication when you enter the altimeter setting? (I am using KNL90B experience here)


Encoder data is supplied to the GPS and to the Transponder.  If both are getting an unusable signal look for bare wire or disconnected/blocked static line at or near the encoder.


Then seek help from your electronics guy.  It seems your system is sending a signal, just a faulty one.  It will be hard to diagnose further without proper tools.


Best regards,


-a-

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There is no failure indication at the start. Altitude error seems to be erratic and could well be a loose connection. I will make sure to tighten those and test fly. If not, likely a visit to the shop soon.  My pitot-static is due soon anyway. 

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Quote: OR75

while in flight, ATC started getting erratic altitude reading from my mode C transponder (seeing me at 14000 when I was at 8000 or 1400ft when I was at 5000, etc ...). They eventually asked me to turn my mode C veil off and leave it on mode A which worked fine.

I was wondering if anyone experienced that  and whether the error is coming from encoder or from the transponder.

the Garmin 430 on occasion did display that it was not receiving altitude info. 

 

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