First, what is the make and model of the autopilot?
For my respsonse, I'll assume you have a Piper autopilot,
which is based on the Edo-Aire / Mitchelle designs.

In general, a hard bank to one side indicates that that
roll servo is being driven that direction by the amplifier. (Obvious,
uh?). The question is why.

In my Piper Altimatic II, the roll servo is a motor that is driven by
an H-Bridge type of amplifier. This input to the final motor
driver amplifier is a left / right signal which is derived from the
difference in the heading bug and indicated heading, and if in
NAV, OMNI, LOC or REV LOC mode also the left / right
signal from the nav output. Therefore, your trouble could be in
the console amplifier itself or the input(s) to the amplifier.

If the alierons are driven to their mechanical limits, I'd suspect
a bad transistor in the console amplifier output stage (H-Bridge).

If the alierons are driven to their maximum bank position but not
the their mechanical limits, (usually about 30 degrees of bank), then
I'd suspect a stage before the final amplifier.

Since this happens in heading mode, the problem could the the
resolver in the DG, or any of the electronics in the autopilot console
amplifier or the radio coupler.

Ronnie


"William Snow" wrote in message
...
I have a piper Arrow and out of the blue...(no pun intended)... the
Autopilot banked hard left (30 deg.) for no apparent reason. It was on Hdg
Mode and the bug was centered.

Each time it was shut off and re-engaged it continued to bank hard left.

I have a long trip to FL planned over the school break and sure would like
to have the Autopilot working.

Does anyone know why this would happen?

Thank You All.
Bill Snow