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?

"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

"Michael" wrote in message
om...

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.

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


Matt
---------------------
Matthew W. Barrow
Site-Fill Homes, LLC.
Montrose, CO

Matt,

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

....but the S-Tec related Meggit line of products just offerd an autopilot
aimed at the multi/turboprop market, and it's attitude(position) based.

Seems the realized that a rate-based auto is not on par with the airplane
when installed in, e.g, a fast single-engine turboprop, just as Michael
said.

Regards

Kai

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

(Doug) wrote in message . com...
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.

Mine once turned the yoke full right to the stops and rolled us into a
steep turn until I was able to grab it and right the craft. I was
following a VOR and the VOR went out of range. However, that isn't
what the autopilot is suppose to do in that situation.

-Robert

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)