Encoder reads several hundred feet high when in flight.
This is typical of an installation with a leaky or incorrect static line system. The encoder may be sensing cabin pressure which is often a slight vacuum compared to atmospheric pressure.
The altitude displayed on the transponder does not agree with the altimeter.
The displayed altitude is, by default, uncompensated for barometric pressure (PA mode) so it will be different. The transponder can be set to display the corrected altitude via the QNH setting. The transponder will always transmit the uncorrected (PA) altitude.
No Alt displayed on transponder.
- The encoder will not output until it has reached operating temperature. In very cold conditions, this may take longer. In extreme cold conditions and where the encoder is mounted exposed to cold air flow, operating temperature may not be reached or maintained.
- The encoder will not output if it is uncalibrated. This can happen if calibration via the switches has been attempted and not completed correctly.
- The encoder has been reverse polaritied. It will be damaged beyond repair.
- The transponder has not been upgraded to Rev 7.
- There is a problem with the harness between transponder and encoder.