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Altimeter Blind Encoder (possible) issue

Dear all,

Recently I have been dealing with some issues on my transponder. It shows my altitude with a considerable differance between the altitude that my altimeter reads and the altitude that the transponder shows. Quite often, the ATC reads back my altitude on their radar wrongly and I already had some occasions were they asked me to switch off my transponder’s mode C.

I have been searching a bit and found out that it might be related with some kind of issue on the blind enconder. Before starting to buy a new one or spending some money, I consider doing some troubleshoot:

- Do this kind of equipment need a proper calibration?
- In Europe, does anyone know a shop where I can buy a new one?
- If I buy a new one, do the instalation shop need to do a calibration after the installation?

Just FYI, my blind encoder is this one

My transponder is a trig TT31.

Many thanks for your help!

Portugal

Both the local (and they are actually not that local to me) maintenance outfits are able to calibrate altitude encoders.

Although I would add looking at the link you sent from eBay your unit looks a right museum piece. Perhaps a new solid state one is in order?

Of course if you are UK based you now have a perfect excuse to fly around Mode Alpha only. Which I am led to believe is very poplular these days.

Last Edited by Bathman at 28 Jul 07:52

miguel22 wrote:

- Do this kind of equipment need a proper calibration?

They need periodic checks, just like altimeters. (The encoder is an altimeter, after all.) It it fails the check, it has to be repaired or replaced.

- In Europe, does anyone know a shop where I can buy a new one?

Basically any shop that carries avionics, e.g. Aircraft Spruce. And as avionics parts go, they’re not expensive. There are two standardised interfaces (serial and parallel — the TT31 can use either) so essentially any encoder would work.

- If I buy a new one, do the instalation shop need to do a calibration after the installation?

I would assume they have to make a check.

But the problem may not be the encoder itself but the wiring between the encoder and the transponder. If the difference is consistent, then it’s likely not the wiring, but if it is correct for some altitudes and wrong for others it is likely the wiring.

Last Edited by Airborne_Again at 28 Jul 08:21
ESKC (Uppsala/Sundbro), Sweden
I have an ACK for encoder ,which also started showing stupid data.. It was repaired and recalibrated by my mechanic, no problems so far. If I remeber correctly, it contains some kind of heating element which had failed. We also tried to order a new one from Spruce,but it did not arrive in a reasonable amount of time (read 2-3 months).
EETU, Estonia

Airborne_Again wrote:

But the problem may not be the encoder itself but the wiring between the encoder and the transponder. If the difference is consistent, then it’s likely not the wiring, but if it is correct for some altitudes and wrong for others it is likely the wiring.

That’s exactly my problem! In some altitudes it shows the correct altitude but in others it has around 2000’ ft of difference. I believe when the TT31 was installed they kept the existing wiring and just plugged directly to the old encoder that was on the airplane.

If I order a brand new Encoder (let’s say ACK) will it come with the necessary wiring to make the instalation from the Altimeter to the Encoder and then from the encoder to the transponder? I believe it should be a generic wiring right?

Thank you all for the help!

Portugal

If it’s suddenly jumping by a couple of thousand feet, it can be a problem with the Gray code encoding (either due to a wiring fault, or a fault in the encoder) rather than any calibration issue.

For example, the Mode-C binary code for 2300 feet is:

0 0 0 1 0 0 1 0 0 0 0 0

The Mode-C binary code for 3200 feet is:

0 0 1 1 0 0 1 0 0 0 0 0

So all it takes is the 3rd bit from the left to be flipped incorrectly to a logic 1 state for your altitude to show off by 900 feet. If you know which altitudes it’s misreporting on, and what altitude you were actually at, you can probably figure out which bit is getting set/reset incorrectly and when. Additionally mode-C transmissions have no error correction, so all it takes is a bit being flipped by interference to show you potentially off by a lot (I’m guessing ATC’s systems have some form of sanity checking, e.g. rejecting aircraft that appear to suddenly climb 10,000+ feet between mode-C replies).

Andreas IOM

miguel22 wrote:

I believe it should be a generic wiring right?

The wires themselves are generic, but the connectors on the encoder and transponder are not, so you must expect to make new cabling.

ESKC (Uppsala/Sundbro), Sweden

There should be a plug and play solid state type encoder. This happened to me and I swapped it out with no wiring changes

United Kingdom

Archer-181 wrote:

There should be a plug and play solid state type encoder. This happened to me and I swapped it out with no wiring changes

And which encoder did you buy? I was thinking buying this one

I heard from a friend of mine that works very well and he installed also with a TT31.

Any other recommendations?

Thanks a lot everyone for all the help!!

Portugal
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