Transponder Gremlins
 

Yesterday I was alerted by a helpful ATCO that my mode C was returning erratic altitudes ranging between FL11 and 4500ft when actually I was level at 1300ft.

Before I commit to having it investigated and repaired has anyone here experienced similar goings on and if so what was the fault? I should say that it's a King KT76A not sure what the ALT encoder is though.

We have one for sale at £250 complete with tray and connectors if it's the box rather than the encoder send PM if interested

Thanks will bear that in mind!

My understanding is that the transponder will just output whatever the encoder says, so its more likely to be the encoder rather than the transponder.
The encoder outputs an ASCII code across its pins (see http://www.airsport-corp.com/modecascii.txt ).
So firstly I'd check for a loose cable!
However if its varying constantly, rather than jumping between two levels (which could suggest the dodgy cable), I suggest there is a problem with the encoder itself. That sort of variance is probably more than a dodgy static connection would bring. I suspect a new encoder is in order.

Please note though - I've not yet even got a PPL so I'm not a reliable source of info!

Probably the encoder.

Rod1

Most mode C/S transponder show the FL on the instrument itself. Was that number jumping up and down too? 'cause if it wasn't, then the error is probably in the transponder itself.

But other than that, I agree. It's most likely the encoder or a loose/dodgy wire between the encoder and the transponder.

The transponder in question doesn't display levels, but the encoder is easily accessible so I will take a look for anything obvious.

Thanks for the suggestions.

The encoders use a multiwire system and as you increase altitude the code changes on one wire each time. This means that if you have a bad connector on one wire then the altitude will jump up and down a lot. I would suggest unplugging and then plugging it in at both ends (transponder and altitude encoder) several times to clean them. Also check for damaged or bent pins.

Altitude encoders are much cheaper (£150-200) than transponders so I suggest changing that first if the above doesn't help.

ps you can also just operate mode A. Now that all the TMZs require mode S there isn't a lot of advantage in using Mode C over Mode A in terms of where you can fly.

Sounds identical to an encoder problem we had with the 182R. I got a serviceable encoder for 70 quid off an avionics shop and had it replaced at annual.

Think you'll live

The difference in encoder grey code between FL11 and FL44 is just one bit of the 12 bit code, specifically the A4 bit.

That suggests that somewhere in the chain that bit isn't transmitting correctly.

It may be worth cleaning and re-seating all the connectors between the encoder and transponder. An encoder is a relatively cheap bit of kit so maybe worth swapping out if you have no joy with the connectors.

Almost certainly the connection between the encoder and transponder.

Some time ago, I had a similar problem and found a poor connection in the plug at the transponder end and just a couple of weeks ago a friend had the same problem in his aircraft and found a wire in the plug no longer connecting properly.

Taken all on board, thanks, I've had a problem with the radio previously which was down to oxidization on the plug terminals, so hopefully thats all it will be.

Just one thing, there is a small pipe coming out of the encoder which looks as though it's tee'd into the static air system, what is it's function?

Switch cleaner
 

Servisol Switch cleaner is Good Stuff to have around especially for low voltage/low current applications. A few operations - plugging/un-plugging might clear the problem, but a squirt of switch cleaner too should fix it for a good while.
RF

Um - an encoder's job is to convert flight level (ie - altitude at standard pressure) into a code. It does this by measuring the air pressure as supplied through the static pressure line.

Of course - it could just use the air pressure from around the encoder. However this would be subject to various errors (the air in the cockpit will be a slightly different pressure than outside) - hence why you have a static air pressure system.

Riverrock, thanks, did wonder if there might be an obstruction in the pipe, but from your explanation thats unlikely.. Intending to have a poke around this weekend, will report back.

It might be worth mentioning that a thin film of oil on the antenna can cause transponder problems. We now include a quick wipe as part of the pre flight inspection.

Pulse1, that is interesting, mine is located behind a Gipsy Major, so sometimes has more than a thin film of oil on it. Hasn't been an issue before though.

Since ATC did not report any problems with your sqawk code, the RF part of the transponder must be in order. So the alt-encoder looks like the culprit.
As stated before, a single bit error on the alt can yield some large altitude jumps.

Hi, grey code is unlike binary, in that only one digit changes at once.
With binary the next step from 00111 is 01000 i.e. 4 digits change over.

The OP said..
Yesterday I was alerted by a helpful ATCO that my mode C was returning erratic altitudes ranging between FL11 and 4500ft when actually I was level at 1300ft.

So there are three altitudes mentioned, therefore there is more than one loose connection in the grey-code cables. Try lots of switch cleaner...!

Actually 1300 feet on the QNH might have been the equivalent of FL11 on that day. Transponders and blind encoders don't care about RPS, transition levels and such.

Did the QNH happen to be around 1006 by any chance?