Altitude encoding errors... where do I start looking?

As there is only one very busy avionics guy at my airport, it helps a lot if I can debug things on my own... hoping someone can point me in the right direction.

On my last flight ATC kept telling me they were seeing me on radar at around 6000 but I was at 2000-2500 at the highest. I cycled the transponder mode but to no avail. I was "that guy".

Oddly, this is the first time this has happened right after the avionics guy took a look at an unrelated issue (my air data computer not reporting the temperature correctly). I don't think he actually did any work or moved any cables but he may have. He says he can't be responsible for this altimeter/transponder issue but for reasons I won't go into, I don't entirely trust him.

Maybe he knocked some cables or reset something? Where would I look first to get this sorted out?

He's proposing installing a new pressure sensor and tying it to the transponder. Not a very complicated or expensive fix but I wonder if there's something else to check before jumping right into this and spending time and $. Avionics man is super busy and isn't going to do much debugging.

I have a GTX-33 remote transponder and a GNS-480 GPS that controls it. The altimeter is a KEA 130A encoding altimeter, I believe. Aircraft is a 1986 Mooney 252/Encore conversion.

 

I assume your altimeter outputs a grey code to the transponder? If so:
This is a parallel data transmission system where each wire represents one bit of data. So, if one of the wires in the bundle is reading high your altimeter will read high. Now, I don't know what the digital inputs to th GTX-33 have on them - pull up, pull down, or nothing, so I can't predict what the result of an open input would be. But in any case, the place to start is the harness then the altimeter.
http://www.mglavionics.com/kb/article/AA-00208/0/Transponder-Gray-Code-Gillham-Code-Reference.html

Difference between 2000 and 6000
6000 000001001010
2000 000000101010
One would want to look at the

The problem, from a paperwork perspective, is that, unlike radios, you can't just replace the transponder in the tray if you pull it out - it has to be checked by your avionics guy to confirm that it is properly transmitting the altitude.

Consider looking for a new avionics guy. You can fly with the transponder off (if you are under class B, just call ATC first) to some other airport if there are no trunkers that will come to you (Your A&P might know one, or ask around for where other owners get transponder checks - I know of two guys that will come by my home drome and work in my hangar...)

 

Thanks Captain Thorpe! I'm an electrical engineer so that background was very informative.

Alas there is only one avionics guy nearby, and he was recommended by my A&P. I moved recently to Northern Spain so getting permission to fly to Southern France (nearest airport with decent maintenance facilities) is not a prospect I relish given the bureaucracy here. Hence my post. But it may just come to that.

 

Digital encoders are cheap. Probably save money and get increased reliability by just replacing the gray code encoder input from the altimeter with a TransCal ssd 120, run an rs232 Cable to the xponder and be done with it. That's probably hat your guy is recommending.

 

Ok - not quite the same situation as here in the motor city...

And, I should make it clear that I ain't no A&P...

 

You know, that actually makes me feel much more comfortable about what he proposed, thanks. Think there's anything fishy about the timing, though? Strange to me that it would fail now after he was meddling.

 

Stuff happens. Encoding altimeter's can have all sorts of calibration issues. Sooner or later it'll need work and cost you a bunch more than just installing a solid-state blind encoder now. Heck, troubleshooting the current issue will probably cost you more than a modern encoder, and it'll still be a problem at some point.

If it makes you feel better, unplug the connectors, clean them up with contact cleaner, plug them back in. If that doesn't fix it, go with the modern encoder.

 

Ok thanks for everyone's comments. I'm going ahead with putting my faith in the avionics guy and installing the new altitude encoder. Fingers crossed.

 

With the altitude reporting higher, especially this time of year, it's possible that there is a break in the static line to your encoder that's near a cabin heat output...the hot air is less dense and so will indicate higher. Try shutting the heat down and see what happens.

In my case, it was a 182 reporting in the high teens/low 20s.