Autopilots... failure modes

john smith wrote:
I am interested in learning from everyone...
a. What autopilot your aircraft is equipped with?
b. What aircraft your autopilot is installed in?
c. What does your autopilot use for heading/altitude/attitude reference?
d. What does your autopilot use for loc/gs reference?
e. If your heading/altitude/attitude reference fails, what functions
does you autopilot still provide?
f. With failed heading/altitude/attitude reference, does the loc/gs
still function?


Good questions all - for me, this is an item of emphasis when providing
Cirrus (SR-20 or 22) transition training. The failure modes are
relatively varied and sophisticated, and really require an understanding
of how the S-TEC 55X interfaces with the PFD, air data computer, and the
Garmins 430s.

Assuming the discussion is GA-centric, you can boil this down into two
"basic" categories of autopilots - rate-based, and attitude-based.
S-TEC is rate-based, which essentially means it's measuring your rate of
turn for roll and heading control. Century's autopilots (which I'd
consider to be old-tech equipment) look at the attitude indicator. If
the AI is getting tired and leans a little to the left when the wings
are level, the AP will fly the right wing low when trying to maintain
heading.

I fly several autopilots regularly. I fly the old (Altimatic, i.e.
Century), the recent (KAP 140, the ubiquitous King AP installed in most
current vintage single-engine Cessna airplanes), and the modern (S-TEC
55X, the best of the bunch in my opinion.) The answers to your
questions would be largely different for all of them. Also, there are
more questions you'd need to ask to completely understand the failure
modes. For example, the S-TEC 55X will continue to fly the airplane in
GPSS mode if the PFD or ADAHARS fails, but heading mode would be
unavailable due to the lack of heading input. If the AP lost heading
data during HDG mode operation, it would begin a large circling
maneuver. If it was in GPSS mode at the time of the failure, it would
continue operating seamlessly.

Anyway, I don't have time to answer all of those questions for each of
the autopilots I fly, but there you have some thoughts on the matter.

-Ryan
ATP/CFII (airplanes and helicopters)