Peter R. wrote:

Speaking of the S-Tec 60-2, I am transitioning to a Bonanza V35B that is
equipped with this AP. The previous owner demonstrated an idiosyncrasy
with the AP that I would like to correct, if possible.

Using the AP to hold VSI, the aircraft is in a climb. Upon reaching the
assigned altitude, I push the ALT button on the AP. The aircraft, since it
overshot the altitude by 100 feet or so, descends and levels off at the
altitude where I engaged the ALT hold, then begins a speed increase to
cruise speed.

Now for the idiosyncrasy: The AP is engaged and the aircraft is holding
perfectly at the desired altitude. However, if I disengage the AP, the
aircraft immediately begins a climb, suggesting that there is a slight
nose-up trim situation that was being held back by the AP.

To overcome this, the previous owner suggested that after level-off,
disengage the AP, manually re-trim, then re-engage the AP.

How can the S-TEC AP be adjusted to hold the altitude after a climb without
the nose-up trim problem? It seems to me that, over time, this problem
could result in premature wear and tear of the AP control servos, no?


You don't mention whether your a/p installation includes the *optional*
"automatic electric pitch trim" as specified on the STEC web site
http://www.s-tec.com/products/ap2rpff.html

The behavior you describe is exactly what I see routinely with the
System 55 a/p in my plane, and is normal without the automatic electric
option, which I plan to have installed soon.

Trim setting for climb power and airspeed is not usually where trim
will be once you level and set cruise power, and that variance is what's
causing the phenomenon you're seeing.

The a/p is capable of holding altitude even with some relatively high
pitch forces, but I agree with you - what's the sense in asking the
pitch servo to work against those pressures? I always disconnect and
retrim when I level out for cruise.