Having trouble with widely varying altitude errors reported by the Garmin 327 transponder, installed in my Model IV-1200.

Last time up, ATC asked us if we were REALLY at the 2600' they saw on their radar - when we were actually at 1100 in the TPA. So the avionics shop took out the existing ACK Model 2 blind encoder, put it on the bench, and it wouldn't hold pressure values. Okay, easy enough, so a new Model 5 ACK encoder was bench-calibrated and installed in the KF.

Parked on the ramp, the panel altimeter and the 327 agreed with each other. (We made the correction for 29.92, etc.) Took her up for a spin and first verified that the steam gauge and the GPS altitude were in sync within 100'. They were, and they remained in sync for the remainder of the flight.

But the 327 was all over the map. For the first 10 minutes it was +1000 over the panel and GPS, then +300, then +500, etc. Then, for five minutes, the 327 was in perfect agreement with the panel and the GPS. Then it was all over the place again and we had to notify ATC (again!) of the discrepancy and head home.

I would have figured that if the plumbing had a problem, both the panel altimeter and the 327 would have been off, but the panel gauge was very steady and verified by the GPS.

Anyone had an experience like this?


Rich Cunningham
Lafayette, CA
Model IV-1200 80 hp UL