Stability problems

[dirkdej]

I can now manage to stay on the runway when taking off and landing.

In cruise flight, in good weather, no turbulence,  at medium power setting, say about 180 KIAS, I can trim the aircraft to fly hands-off for some time, with only minor touches needed to keep straight and level. When I switch on the autopilot, the aircraft starts to pitch up and down at about 2Hz; if I switch off the autopilot, the aircraft that was previously trimmed to fly almost hands-off will start a steep climb of 60° up and then fall off on the left wing. In my opinion, a functioning autopilot should leave the aircraft in trim after disconnection.

Any suggestions?

[dirkdej]

I did some more flying in the TBM900;

I fly with mouse and keyboard, mostly to practice IFR procedures. After top of climb, I set up the aircraft in cruise, trimming manually to maintain S&L in good conditions. After a couple minutes of manual flight, in trim, I switch on the AP. The aircraft remains stable except for some up and down pitching, maybe half a degree, twice per second. When I switch off the AP and want to resume manual flight, the aircraft pitches strongly up or down, even full up or down elevator is not enough to keep control.

To me it seems that either the automatic pitch trim is running away or not working. If I switch off the autopilot in any of the aircraft I fly, I get an in-trim aircraft back.