autopilot s-turning down the ILS

anybody else have this problem? coupled approaches with the ILS finds the S-tec 55X (on my SR22) hunting and overcorrecting all the way to dh…glideslope stays nailed but localizer a mess. I’m working on it with my avionics guy but we’re stumped so far (all Sandel settings to factory specs).


I had a problem like this (mine hunted in all modes). I am willing to bet that it is you autopilot. You will probably need a new one. That is what fixed my problem.


SR22 #307 N258ST

Is your autopilot only having this problem on ILS approaches?

Mine has this type of problem, but in all modes (HDG, NAV, NAV GPSS…).

I’ve been working on it for awhile, many items replaced (including the STEC-55X),
but I still have the problem.


In my '22, I had a very similar problem in the ILS/Approach, VOR modes, but not the GPSS or HDG. The A/P oscillated in ever increasing deviations, typically holding to the left of the course.

It turned out that it was the Sandel which, I was told was set up to emulate an incorrect model HSI (Century maybe?). Since the problem seemed to get worse as opposed to be a steady state, I had my doubts. This started happening some time after the engine monitoring was added to the ARNAV, but not immediately after. As I said the problem seemed to degenerate.

However, the Avionics shop did whatever they did and the problem was fixed and has not returned. I don’t argue with success.[:)] Good luck with yours.


Not necessarily the a/p. N747TG had a bad spherical bearing in one aileron, resulting in heavier control forces to the right than to the left, which confused the a/p. A new bearing solved the problem immediately.

Timm Preusser


What is the variability in your oscillations (how may degrees)? I am very surprised that exchanging the autopilot did not help resolve your problem.

SR22 #307 N258ST

One example: If I set the HDG bug 60 degrees left of my present heading, the
AP turns me 20-30 degrees past the HDG bug, before leveling and then turns
back to the HDG. It eventually gets there.

Turning to the right, however, it levels approaching the HDG, and does a good job
of intercepting the HDG without overshoot.



I have had past Sandel issues that were resolved with software updates…and all Sandel settings (inc emulation) are set to fac specs…Everything works perfectly except the localizer function…so, it almost assures me that it’s the autopilot.


My airplane’s autopilot tracks the ILS perfectly - localizer and GS, no problem.
Its real pilot, on the other hand, S-Turns simultaneously in both the horizontal and vertical planes. Cirrus denies all responsibility. Whazzup with that? [;)]


During instrument training, my instructor said my localizer and GS needles looked like two people swordfighting.


Smack it across the top of the unit, maybe it’ll reset and work better :slight_smile:

I get embarrased when someone asks about a nice ILS and ask if I was
hand flying it, and I have to say no, it was the autopilot. I never seem to
get that comment when hand flying.


Could you feel the difference (one direction vs the other), when hand flying
the airplane?

Did the electric trim work full range in both directions?

I’m looking for a way to determine if this is a problem on my airplane, which
still has an AP related issue


What you could feel was that the neutral position of the yoke was sluggish somehow because the cartridge springs were in force in-balance; and yes, a very small effect on aileron forces when hand flying, but barely noticable. (Logical, because the slipstream over the wings will neutralize the ailerons, or at least help to do so). What you could see when on the ground was that when you manually lifted up an aileron (I mean the control surface, not the yoke), it would not return into neutral position with the springs only.

Hope that helps.


Timm Preusser

no, it’s not an isse hand flying. In fact, I rarely couple approaches (prefer the to hand fly for the experience) so this problem could have been around a long time (290 hrs on the airplane and maybe one couple approach 9 months ago.

Also…found out today the the VOR/autopilot mode does the same thing…will begin to track (turn to) the VOR and then just blow right through and “hunt” (small movements both directions on the yoke) for the needle. Again…never fly VOR’s anymore so I’m not aware how long this has been going on.

Thanks, yes that helps.

I’ve tried that and I don’t have what I’d call friction in the system that would
prevent the spring cartridge from returning to the neutral position.

My issue must be something else. I’ve also lubed those bearings, and the
others in the aileron sector, with no improvement.