On our flight out to Knoxville a month ago, ATC called to verify my altitude. I thought that’s a little odd, but I responded with “we are level at five thousand, five hundred” They thanked me. 15 min later they asked me the same question, I responded with the same answer, but this time looked at the transponder (One of the nice pieces of information given on most newer transponders is the pressure altitude it is reporting to ATC) at it is reporting four thousand three hundred feet. Uhh oh. I call back and inform ATC that it appears that the Alt Reporting is off. I try resetting the transponder, but no good. So call ATC and tell them I would be off for a few minutes while I reset the the Avionics. I hit the Avionics master, and leave it off for 60 seconds, Flip it back on and …. looks like it’s working, But I continue to monitor it and after about 5 min it starts slipping again. Now the transponder is reporting my altitude +/- 500 ft, but is usually off by +/- 200 ft. for the rest of the trip. On the return trip it never takes a thousand foot excursion, but is still all over the place. Fortunately I am VFR and I am not in class B airspace so no big problem, but still not right.
After our return I do some more flights around the Bloomington area, and it’s not getting better. I think it can’t be a static leak, or I would see the same fluctuations on the altimeter. A little research and discovered that my encoder is a very old Narco AR-850 and this is a common failure mode for these devices.
I start looking at for a new encoder and find a pin for pin replacement. Perfect! I call my avionics shop tell them symptoms and the model I have and the encoder I would to replace it with. He agrees that it sounds like an encoder, but there are two versions of my old encoder. He asks do you have the 25 or 15 pin connector? I don’t know, but I go out to the plane and quickly count more than 15, so it must be the 25pin. I call my shop and inform him of my findings. ‘Umm, that’s too bad he said’. What? ‘we will have to redo the harness’. well poop!
After my conversation with the shop, I start thinking… We already have to redo the harness and pull apart the instrument panel, maybe now is the time to get the ADS-B compliance out of the way. I setup a meeting tomorrow with the shop to talk solutions and pricing. I will update with the conclusion of that discussion soon.