Transponder transmit incorrect altitude

2Airtime2

2Airtime2

Pre-takeoff checklist
Joined
Nov 5, 2015
Messages
197
Location
Raleigh, NC
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Cherokee 180c
I have a Narco transponder which tells atc that I'm about 1800' higher than I actually am. On 4 occasions I have been verified higher by 2 different atc's. Two times I was at 4500' and they were showing 5800'. I pulled the transponder and the encoder and sent them to Ellison in Texas (one of the few shops that still work on Narco). They can't find anything wrong and I decided to have them do an alignment. I still have the exact same issue.

The plane is a '66 Cherokee 180c and the encoder connects to the common static pressure line which goes to the altimeter, air speed, and vertical speed indicator. All of those other instruments read correctly as verified by the electronic panel on my Garmin GPSMAP696 so I assume the encoder is getting the correct pressure.

There is one thing I forgot to mention to Ellison. My altimeter sticks a little. I can tap the face of it and the indicator will jump to where it is supposed to be. Could that somehow effect the pressure the encoder is seeing? It's not bad, the dial might lag behind a hundred feet or so but will correct itself with the normal plane vibrations even if I don't tap the face.

The transponder antenna had grime on it but I have kept it clean since discovering this problem. If the encoder is getting the correct pressure yet the transponder transmits the wrong altitude, what else is left? The antenna? Has anyone ever heard of a transponder antenna going bad? Ellison never mentioned antenna to me and I didn't ask. I have verified continuity on all the wires going from the encoder plug to the transponder tray/plug.

Any guesses?
 
Often the antenna pigtail can go bad, resistance changes over time and with heat, but I don't think that would change only alt tx. Still, if everything else has been looked at, making up and installing a new pigtail is cheap, easy and won't hurt nuthin'.
 
the encoder see's static pressure....so either something is wrong with your static pressure plumbing or your encoder is going bad. They do go bad.
 
I had the same problem a couple years ago. Tests at the airplane showed the static line was good. Simple and cheapest solution was a new encoder.
 
I chased a transponder problem for months except mine was a little more serious. It was reporting me in the flight levels when I was at 3k msl. Went to the radio shop 4 times and each time they came back and said they didn't see any issues. Finally took it upon my self to resolve and found a corroded connection on one of the pins that activates at the 14k mark. I don't plan to be that high any time soon so I just cut the wire for now.
 
Sounds like your altimeter is off. Get that tested.
 
It needs to be tested in the aircraft. The issue could be with one of the pins that sends the grey code from the encoder to the transponder - it's a parallel connection and if the transponder is not getting all the bits from the encoder, then it transmits the wrong altitude.

Altimeter sticking will have nothing to do with the pressure to the encoder. But an altimeter that is not reading correctly could put you at an altitude that does note match the actual altitude reported by the encoder.

Taking the transponder out and getting it "checked" is not likely going to fix the problem - the system needs to be tested in the aircraft - and any avionics guy can do that whether they "work on Narco's" or not since the transponder is the least likely component to be at fault.
 
sounds like a plumbing issue if the encoder bench tests good.

I had the static line to the encoder melt once because it was too close to the cabin heat outlet...every time I used cabin heat, ATC saw me suddenly several thousand feet higher.
 
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