Flying back from Louisville KY to my home base in Northern, VA in my 340A, I lost the altitude hold function after about 15 minutes of flight time on the 400B Auto Pilot. The Auto Pilot started to trim up and down drastically. The aircraft had been sitting outside in the rain for about 36 hours and the initial hour of flight was through heavy precipitation. I would appreciate any thoughts.
If the rain was a driving rain aimed perpendicular to the plane it is possible the static port on the rain side will suck in the water, collecting more than you would think. This is because if it is windy enough the static port on the other side is at a slight negative pressure. Doesn’t take much of a differential to start pulling in some water – and the altitude hold systems are very sensitive. This is one reason why the AP should not be turned on just after departure.
If your 340 is similar to a 421, your altitude hold box is in the tail and Tee’d into the static system with a short line.
If the problem goes away on the next flight it was probably water as previously described.
If not, it could be an elevator servo. I had one go out with similar results – altitude hold would capture, then oscillations in pitch would start and continue to get worse until the AP was disengaged.