TDR-94/94D Common Failures: Transponder Off Frequency

April 2017

ATC (Air Traffic Control) monitors our skies, sending out a beacon signal to locate and identify all the aircraft in the area. This signal is transmitted at 1030 MHz. Aircraft transponders reply in kind with a signal containing all of the aircraft’s information.

At the same time, on another frequency (1150 MHz?), aircraft in the area are sending out DME (Distance Measuring Equipment) signals to radio towers to calculate distance.

If the transponder on your aircraft is off frequency, it could begin to respond to the DME signal from other aircraft and reject the beacon from ATC.

What happens when your transponder rejects the ATC signal?

The ATC will not have any information about your aircraft if your transponder is responding to the wrong frequency and rejecting the ATC signal. You will show up as an intermittent blip on radar. They will know something is there, but won’t have accurate information. They will let you know you have a bad transponder signal. Other aircraft in the area will not be able to see you either.

Why might your transponder be off frequency?

Over time, a transponder will drift off frequency because radio filter grounds become oxidized. We see this on about 90% of the TDR-94/94D transponders in our shop. We are able to fix this by cleaning the grounds on the shield.

If you have any questions about this squawk or other questions about your TDR-94/94D, including the upgrade required for ADS-B compliance, contact a Duncan Aviation Avionics Instruments Tech Rep.