Following another thread here, this article is an interesting summary of the sort of thing a pilot ought to know - even on a "simple" aircraft.
On a modern aircraft, G1000 etc, one needs to know a lot more.
One specific thing which every pilot must know is that every autopilot is supposed to have slipping clutches on the servos.
There is a preflight (ground) check whereby you engage the autopilot and move the yoke in pitch and roll and make sure you can overpower it. This tests the slipping clutches. I know for a fact that a lot of pilots don't do this test. Often they are not aware of it, and sometimes they have been told by some "expert" that it should not be done. This is a life critical preflight test.
One aircraft I saw had one of the clutches totally seized and the pilot didn't know anything about it; he never tested it. Had the autopilot failed badly, he would have got killed, because there is no way you can overpower the servo if the clutch is seized up. The gearing between the servo motor and the servo output gear is something like 10000:1 to 20000:1 and it probably has enough power at the servo output gear to bend the control linkages.
There is an additional electromagnetic clutch on each servo (sometimes inside it, sometimes not) which is supposed to disengage when the autopilot is OFF. So if the slipping slutch has seized, you should be able to survive by very rapidly disconnecting power to the autopilot. So knowing where the master autopilot switch is, and where the circuit breakers are, is important.
I know for a fact that a lot of pilots don't do this test.
Me included. We operate our aircraft as per AFM, checklists and type rating course. There is absolutely no reference to this kind of test anywhere. I know that they do it during maintenance sometimes, and after the servos have been overpowered by the operator, the autopilot needs to be completely reset (which is not something you want to do during normal operation).
With the Cessna 300A I used to have, I would regularly forget to disable the autopilot on approach and then after landing wonder why the yoke always wanted to go to one side. At one point I even suspected some mechanical imbalance. It was really not very strong on the servo.
The current S-TEC servos are much stronger but they have an electromechanical clutch which is clearly audible. I think in addition to the slip clutch and the electromechanical clutch, they also have a joint which is supposed to shear when you overpower it.
The AP is a very critical system for IFR pilots. The first 6 months after getting my IR, I used to have those "what the f*** is the AP doing now?" moments on pretty much every flight.
We operate our aircraft as per AFM, checklists and type rating course. There is absolutely no reference to this kind of test anywhere.
The TB20 PIM mentions this test. I suspect I am the only member of my syndicate who performs it every time they fly.
We operate our aircraft as per AFM, checklists and type rating course
Yes, but that is a jet. I am sure one doesn't check the servo clutches in a 747, either 
All the common piston GA autopilots have the servo clutches.
Oddly enough on the TBM, Socata require the servos to be removed at every Annual and the clutch slipping torque to be checked, on a very overpriced bench rig. However I don't know if the TBM POH calls for the preflight clutch test. If it doesn't, that might explain the annual servo check requirement - which is quite time consuming. It takes about 2 days to do that job. The TBM (KFC325 version) uses almost the same servos as the KFC225 etc.
I suspect I am the only member of my syndicate who performs it every time they fly.
Most TB pilots don't do it; that's for sure.
@ Rich
The TB20 PIM mentions this test ( Peter: move the yoke in pitch and roll and make sure you can overpower it. This tests the slipping clutches.)
reg. TB20 I only find in supplement 8 / KFC150
a) actuate left side of split switch unit to the fore and aft positions. The trim wheel should not move on its own. Rotate the trim wheel manually against the engaged clutch to check the pilot's trim overpower capability.
I do not find a procedure saying to move the yoke against the clutches while a/p is engaged. May be its a KFC225 specialty.
Nobbi,
I have a 1992 TB20 with a KAP 150, the manual on the AP in section 8 is dated 31. January 1988 and is for both KFC 150 and KAP 150.
In this version it says, in addition to the checks you described with the trim wheel: Engage AP, move yoke left to right, disengage AP.
There is no reference to moving the yoke in pitch, though.
Thanks RXH, somehow I had overlooked it -
And it even says " FLIGHT CONTROLS - MOVE fore, aft, left and right ..." So pitch is included.
Any autopilot with a pitch servo should have the pitch overpower test.
Another test, for systems that have a pitch trim servo: when it is engaged in the basic AP mode, press on the yoke and after a few secs the pitch trim runs backwards. If you pull on the yoke, it should run forwards.
Strange, I wonder why different versions of the manual for the AP exist. Mine definitely has no mention of the pitch movement and refers to roll only ("Move the yoke left and right - Disengage").
Will include the pitch movement now into the preflight checks.
Thanks, Peter and Nobbi.
