Autopilot pitch stability problems
 

I am flying the Metro 23 with the S-tec 65 autopilot.

http://www.s-tec.com/pdf/System_Sixty_Five_(3rd_Ed).pdf

When the metro 23 is heavy or CG is aft, the autopilot are having pitch stability problems both in climb and cruise. It gets into a series of incresing ossilations witch requires disconnect of the autopilot, retrimming and reconnection of the autopilot.
Somtimes it is impassible to make it work and we then fly it in heading mode and manually maintains altitude, or disconnects the auto-trim funktion and manually trim the aeroplane following autopilot trim commands.

I have come to the conclusion that the problem is the auto-trim funktion, when conducting preflight test of the autopilot the auto-trim funktion is testet by applying presure to the stearing wheel in the pitch direction, after 3 seconds the auto-trim funktion should then trim the forces away from the stering wheel, but it starts doing that after 1 second.

What are the oppinion of this forum, will to fast applikation of trim cause this tendency of incresing ossilations ???

Our mechanics tell me thet the timing funktion for pitch force is in the pitch flight guidance computer and that they cant change it.

Should there be a S-tec autopilot specialist out there who knows how to solve the problem it would be nice to here from you.
regards

S-tec has just dismissed their tech rep that I knew (poor business sales, unfortunately) however...S-tec has had a known problem with pitch occilations in many aircraft with their series 55 and 65 autopilots.
However, many of these problems can be eliminated, he reported, if the aircraft and autopilot control cable tensions are checked carefully, and adjusted to specification.
IE: tighter (more tension) is better.