Autopilots... failure modes

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?

correction...

f. With failed heading/altitude/attitude reference, does the loc/gs
tracking still function properly?


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?

Here is a link to a very good article on autopilots on the Avionics West web
page. I think it answers a lot of your questions.

http://avionicswest.com/articles/kno..._autopilot.htm

-Brenor

"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?

Autopilot failure modes are a bit of a mystery, but I can tell you
from first hand experience, some failure modes would be very bad
indeed. One absolutely needs to keep an eye on the gyros the autopilot
DOES NOT use and turn the autopilot off if there is disagreement.

My answers below:
john smith wrote in message ...
I am interested in learning from everyone...
a. What autopilot your aircraft is equipped with?
Century I
b. What aircraft your autopilot is installed in?
Aviat Husky
c. What does your autopilot use for heading/altitude/attitude reference?
Uses both the electric TC, and if coupled then either the GPS obs
indicator or the VOR for right and left
d. What does your autopilot use for loc/gs reference?
It uses the VOR indicator for right and left, no altitude
e. If your heading/altitude/attitude reference fails, what functions
I am not sure of this failure mode. The autopilot uses both the TC and
the GPS for it's right and left.
does you autopilot still provide?
f. With failed heading/altitude/attitude reference, does the loc/gs
still function?
Yes, it still functions

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)

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.

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)

"Brenor Brophy" wrote
Here is a link to a very good article on autopilots on the Avionics West web
page. I think it answers a lot of your questions.

http://avionicswest.com/articles/kno..._autopilot.htm

That's not a bad article. I do have some issues with it.

First off, it ignores the most important difference between rate-based
and attitude-based autopilot - performance in turbulence in a slick
airplane. Attitude-based autopilots (those that use pickoffs on the
horizon gyro) work all the time. Rate-based autopilots (those that
use pickoffs on the turn coordinator) work in smooth air or with
light, draggy airplanes. In a heavier slicker airplane, especially a
twin, turbulence makes for a very uncomfortable ride because all the
corrections are too much too late. There are NO rate-based autopilots
in the transport category - they just wouldn't work.

The whole system failure issue is more complex that it seems. The
reality is that ALL autopilots built for GA use can roll you over in a
heartbeat - every one of them without exception. Vacuum failure is
only one mechanism for this - it causes the gyro to provide incorrect
bank information. Here are some others:

Bad connection. Really. A bad conenction to the attitude indicator
(AI) or turn coordinator (TC) means the system has no idea what the
bank angle or rate of turn is - but it thinks it does. Over you go.

Bad brushes/dirt. A TC with bad brushes can come to a halt - and not
flag out. An AI with dirt/water sucked in can stop spinning or go
nuts because of the pendulous vanes - and not flag out. Warning flags
on GA gyros are near-useless - they monitor only the power source, not
actual rotation.

Electronic failures - there is lots of circuitry required to read that
AI or TC. Any of it can fail. Over you go.

Bad computer. The modern autopilots are computer based. The sytem
crashes, and over you go.

The bottom line is that with any GA autopilot system, you as the pilot
must continuously and carefully monitor whichever gyro(s) the
autopilot is NOT using. The autopilot is only a way for you to take
your hands off the controls, not a way to stop the scan.

Michael

In article ,
(Michael) writes:

The bottom line is that with any GA autopilot system, you as the pilot
must continuously and carefully monitor whichever gyro(s) the
autopilot is NOT using. The autopilot is only a way for you to take
your hands off the controls, not a way to stop the scan.


Hip, Hip, Hurray!!

Chuck

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