Autopilots... failure modes

Actually, the Century I is rate based (turn coodinator based). In fact
the whole autopilot (except the servos) are contained in the turn
coordinator. It does not, in my case, couple to the DG. I like this,
it isn't dependent on the DG, and thus not dependent on the vacuum
pump. As it is now, it is totally electric. So I have it if I lose the
vacuum gyros (AI and DG). It couples to either the VOR (gets its left
and right from the VOR head), and the IFR GPS (gets its left and right
from the OBS indicator). If you uncouple it, it uses the TC only to
fly without turning (it will drift, and I don't mean wind drift). All
aircraft are a little out of rig. There is a "trim" knob for the
autopilot that will bias it left or right. There is also a knob that
you can turn and the autopilot will turn the airplane, up to standard
rate. It uses the VOR head for right/left, and works with this if the
VOR is on the localizer. It's a simple setup. No altitude hold or
altitude anything. The servos turn using the ailerons.

If the autopilot gets a left signal from the OBS indicator (or VOR
head), it starts a turn. It then looks to the TC to see if it is
making progress. It turns constantly, first one way, then the other
(very small turns). In calm air, it can outfly any pilot, especially
if coupled to the GPS.
If the TC stops working, the autopilot keeps turning the plane, all
the way to a a very steep bank. Bad failure.

If the GPS stops working, the autopilot would get no left or right
from the OBS, and just keep flying straight ahead (with small
corrections), pretty much like it does when it is uncoupled.

I do not know what it would do if the servos stopped working.
Just make sure you watch the AI and DG while the autopilot is on, as
these are the gyros it does not use. You can ignore the TC.



Ryan Ferguson wrote in message ...
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)