Transponder reading 500 feet off...Intermittent

[MyNameIsNobody]

If a transponder is reading 500' off on an intermittent basis what is likely cause?

1. Encoder 2. Transponder 3. Could be either? Thanks for input. Last cert was: 2/14/14

[Alan Fox]

Could be water in the static line , could be the static line is off , does it have a digital display of pressure altitude??

[MyNameIsNobody]

Pressure altitude. Not a lot of ramp time/rain exposure and we hand wash. Thanks for input. Will check the lines.

[Shadrach]

Mine did the same thing last week. Off by 1000ft. Recycled it and it was spot on. Tower said they did not think it was their equipment, but I'm not so sure.

[yvesg]

One suspect would be your C1 wire... maybe intermittent connection. See this:

http://www.mglavionics.com/kb/article/AA-00208/0/Transponder-Gray-Code-Gillham-Code-Reference.html

This bit could cause a 500 feet difference, could also be 400 or 600.

Yves

[takair]

Is it at a specific altitude? Could be a grey code wire or encoder connector. I assume this is ATC reported altitude?

[MyNameIsNobody]

Yes ATC.

[1964-M20E]

Not same issue but I had a broken wire on the gray code wiring and above 7000 it would report me descending.

 

Interesting when ATC calls to inquire about your altitude while you are still climbing.

[Piloto]

Must likely is a contact problem with the wires coming from the encoder. Try contact cleaner on the transponder tray connector and on the encoder connector end. Best way to have it check is doing a transponder biennial check on the ground. Or with an ATD-300 while in flight

 

José