Autopilots... 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.


I realize you probably left this out on purpose to be brief, but technically
this is only true if GPS1 was already driving the autopilot or if GPS1 and
GPS2 have identical flight plans loaded and activated. It's not true if
GPS2 is flying the plane and GPS1 is inop or has a different flight plan
active. The Stec 55X will also still work in altitude hold or vertical
speed modes because its uses its own static pressure sensor separate from
the EFIS. These are the kinds of details autopilot users ought to know
cold.

Cocomo wrote:

I realize you probably left this out on purpose to be brief, but technically
this is only true if GPS1 was already driving the autopilot or if GPS1 and
GPS2 have identical flight plans loaded and activated. It's not true if
GPS2 is flying the plane and GPS1 is inop or has a different flight plan
active. The Stec 55X will also still work in altitude hold or vertical
speed modes because its uses its own static pressure sensor separate from
the EFIS. These are the kinds of details autopilot users ought to know
cold.

Thanks for the reply.

Yes, that gets a bit too Cirrus-specific when answering to the general
autopilot question. But since you bring it up, I'm not sure what your
point is - the example referred to a PFD or ADAHARS failure resulting in
a loss of heading data sent to the 55X, which, if the 55X is currently
in HDG mode, will always result in the behavior I described. If the
autopilot is in GPSS mode, regardless of which GPS is providing the
steering input, loss of heading data will not affect that mode of operation.

-Ryan

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)

Basically, autopilot failure modes can be summarized as these:
Turns right instead of turning left
Turns left instead of turning right
Goes up instead of going down
Goes down instead of going up
An airplane is ALWAYS turning and going up or down, it's never really
in steady state. These pertubations can be small or large. (like a
steep turn descending spiral or a gentle turn).

john smith wrote in message ...
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?

"Matt Barrow" wrote in message news:...

You might want to take a gander at this article. I'm not sure what you
mean
by slow corrections, but evidently S-TEC (taken with a grain of salt) has
optimized the corrections, as well as smoothed them to avoid overstressing
the aircraft in turbulence.

See Pages 6 & 7.

http://www.s-tec.com/pdf/autopilotbook.pdf


Addendum to lost (by my ISP) post.

Meggitt 1500 & 2100 AP's use a hybrid of attitude and rate information
through the ADAHRS system. Sorta of "best of both worlds"?