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I should like to ask for any suggestions on the following fault. I have an STEC 55 which works absolutely fine until you climb into thinner air. There is no particular level where the problem occurs except to say anything above 3000 to 5000 feet then it "hunts" around the course set on the bug. It will go to about 4 degrees either side of set course before trying to correct. Anything obvious to look for? Perhaps the explanation is poor but I think you wil get the gist. All pre-flight checks are 100% OK

UK, United Kingdom

The standard tips for this kind of thing are

  • A grossly insufficiently tensioned bridle cable. [Here]($(KGrHqFHJEgE-muhtjmCBPz3qqjptQ~~60_12.JPG) is a pic which gives a general idea of what the cable looks like

  • A defect in the roll servo. Here is a pic of the internals of a KS270C pitch servo where a wire came off the tachometer, causing the servo to run about 3x too fast, making the plane unstable in pitch.

  • An incorrectly configured or faulty autopilot. When an autopilot is installed, it needs to be configured for the characteristics of the aircraft. Maybe this config has been modified or corrupted? It may be done digitally (with a laptop) or with trimmers.

  • In old (analog) autopilots, the control loop behaviour was determined by a mixture of resistors and electrolytic capacitors. The capacitors would dry up over time (10+ years) causing the loop time constants to shorten, which (if gone bad enough) makes the aircraft unstable on that axis. This (stupid design) is a really common issue with the older autopilots but I thought the 55 was digital internally...

  • In autopilots which use a TC as their roll reference, it's obviously important that the TC outputs the correct voltage per degree roll (or whatever). If it was changed for a different type which outputs say 2x as much, that is going to reduce (or remove) the stability margins of the control loop.

Does it do it in just the HDG mode, or the ROL (wings level mode) as well? If HDG only then maybe the HDG output from the HSI is too big.

I often hear of STEC autopilots going slightly unstable in pitch or roll, in certain loading (W&B) conditions, which I would put down to them not having been sufficiently flight tested in every airframe for which they got an STC. Just my opinion of course

Shoreham EGKA, United Kingdom

There are several common areas that induce the Stec autopilots to S turn. Among them are a roll servo with a high startup voltage, incorrect tension on the aileron cables, roll center adjustment not in proper adjustment. You can download a copy of the field service manual which should cover most of the normal issues at

KUZA, United States

Thanks for the replies. The problem does seem to be at its worst in HDG mode but I cannot understand why it gets worse as one climbs. BTW I just love this forum Peter, no idiot responses (so far) from anyone in any post I have read. Keep it up.

UK, United Kingdom

Does it get worse during the climb, or does it get worse at altitude?

If the former, it could be because you have a marginal stability situation and the roll stability of an aircraft is worse during a climb than during a descent (so I gather - can't explain it).

Shoreham EGKA, United Kingdom

It is worse at altitude and as far as I can tell climb and descend are bothe quite stable but start to lose heading stability as we climb higher. I am no engineer but this just seems so implausible.

UK, United Kingdom

NCYankee that was most helpful, thankyou. Now can any one suggest a good A/P engineer please. I am aware of International Aerospace at Cranfield but I think their charge may be somewhat excessive based on previous experiences.

UK, United Kingdom

Fellows, A freind of mine, installed the S-Tec A/P 50 in his Lancair 360 A/C. He asked me to check his A/P and i found that the OPR switch (ON - OFF - TEST) operates the A/P only in TEST mode but no operate the A/P in ON. I checked the wires from its connector and found thaqt all wires are connected as per the installation diagram. Does any one of you have any idea what shel I do to solve the problem? Thanks Ariel

I don't have the answer but I am emailing you some manuals for these autopilots.

Shoreham EGKA, United Kingdom
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