My Aztec has the above autopilot. The manual says to use it, 1) turn on autopilot master switch, then 2) use the pitch thumbwheel to center the trim needle on the AP control panel, and then 3) press "AP Engage" and then 4) rotate the pitch thumbwheel to the center detent. This unit was overhauled just before I bought the airplane, but if I follow the above, upon engaging the A/P the airplane always immediately descends about 200' before settling on an altitude, which it then holds reliably. At the suggestion of another Aztec owner I tried it without centering the needle, but it still does the same thing. I plan to experiment some more with it today, but any ideas would be gratefully accepted.
