Sure Jay, he attitude gyro tells the autopilot which way is "up." It is used
for pitch AND roll control in a position-based system, like Century (nee
Mitchell or Edo-Aire) and King.

For instance, in roll, the autopilot will use the gyro angle to cancel the
heading error. For instance, if you are 10 degrees off the selected heading,
the autopilot will roll in 10 degrees of bank until the heading select error
starts to decrease. On heading, the bank angle becomes zero.

As you said, the vacuum is one of the least reliable systems, but electric
gyros are very expensive, while vacuum powered ones are simpler, and more
robust. And attitude information was required in these systems.

S-tec avoided the attitude gyro necessity, by using RATE based systems and
pressure transducers.
--Gary

"Jay Honeck" wrote in message
news:4DG6b.381210$Ho3.57121@sccrnsc03...
So my AI has been going whacky for several months now -- falling over on
its
side, then erecting, and so on.

We finally realized that, with night-flying season nearly upon us, we'd
better replace that boat anchor with an instrument we can trust.
Although
we don't fly IFR, we DO occasionally fly at night, and in rural Iowa, that
can be essentially IFR. So, I asked my A&P to order the correct one for
me,
cuz I couldn't see a model number or any identifying marks on the face.

Well, it turns out my autopilot is connected to the back of this
instrument
with a multi-prong plug, necessitating (of course!) the more expensive AI.
This also explains (I hope) why my autopilot has been acting erratic, so
I'm
looking forward to fixing two problems with one purchase. (Ha! Wake up,
Jay!)

We ended up ordering a rebuilt unit from the Gyro House, in northern
California. You pay $1100, and they credit you $600 when they receive
your
old one back in the mail. Even though I told them there was no rush, the
instrument arrived within two days -- before my shop was even ready to
install it. It was shipped in a gigantic box, with two huge pieces of
foam
holding the instrument in the middle of a marshmallow-like suspension. So
far, I'm very impressed with their helpful attitude on the phone, and
their
quick service. I hope it will be installed Monday or Tuesday.

Question: Does anyone know why the old autopilots were hooked up to a
vacuum
instrument? This seems like the height of folly, since a vacuum pump
failure takes down EVERYTHING, as opposed to the newer units that work off
the electric turn & bank.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"