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Stec 55x and Altitude Preselector ST-360, and periodic altitude fluctuations


I am looking for an Installation Manual of those.
I have ST55x and ST-360 in my plane but there is a problem with communication between those units. AP doesn’t “see” the preselector, so ALT Capture doesn’t work.
I would like to recheck the wiring but don’t have the documentation.



Does this help:

Avionics geek.
Fairoaks. EGTF

Thank you very much!!


Another problem with this AP I have are some altitude fluctuations in ALT mode +- 20-30ft which is quite annoying. But sometimes everything is fine.
I checked the pitch trim and elevator cable tension and they are ok.
I suspect something in the electronics or the Absolute Press Transducer.
Normally when I fly in stabilised ALT mode – when I switch to VS – it should display “0” as I’m already in level flight.
However when the fluctuation starts, for example the aircraft starts to pitch up – when I press VS button – I see about “-5” hundred feet/min.
So the AP thinks – we are descending so it has to pitch up. Then after few seconds it realizes that it has already went too high and begins to pitch down.
I have an auto trim, which works fine. Tried to switch it off but doesn’t help.

Anybody experienced similar problems?


Are these fluctuations periodic?

I had this and a part of the solution was the replacement of most of the static pipework. It is possible something got pinched, or there was a crack.

The issue existed only in ALT mode.

I don’t know about STEC but I think King autopilots pick up the current VS value when VS is pressed.

Note that VS=0 will not hold altitude.

Shoreham EGKA, United Kingdom

Thanks Peter,

Yes they are periodic. Let say about 50% of flight time.
When I press VS button I think I read the current vs rate – as you described. Normally it is “0” but when the fluctuations come – I read different values – from minus 6 to plus 6 hundred feet/min when pressing VS.
VS=0 of course will not hold altitude. The plane very slowly drifts up or down but it generally helps to reduce variations.


Roughly what frequency?

Shoreham EGKA, United Kingdom

More or less the same or a bit slower than on your video.


How easily can you arrange a static system leak test? I would do that first.

Then change any tubing where creepy crawlies might have crawled into and blocked it.

Shoreham EGKA, United Kingdom

Peter wrote:

How easily can you arrange a static system leak test?

I will find out.

Do you think it could be also bad electrical connection between press transducer and the autopilot?
That I could easily check.

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