Autopilot fighting for control

In simulation, if I set the autopilot to maintain the altitude, and
then try to change pitch with the controls, the control surfaces
barely move, and I can hardly have any effect on pitch.

I think this is a limitation of the simulator. My guess is that, in
real life, the autopilot continuously adjusts trim, and so, if you try
to change pitch with the yoke, the A/P retrims to cancel out your
efforts, and the net effect is that you can change pitch, but you must
exert very high control pressures to do it (in order to overcome the
trim that the A/P is applying to neutralize your efforts).

Is this correct? I can see why this is difficult to simulate with
controls that don't provide feedback, but I want to make sure that I
understand the difference between the sim's way of doing it and the
real thing.

--
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Mxsmanic wrote:
In simulation, if I set the autopilot to maintain the altitude, and
then try to change pitch with the controls, the control surfaces
barely move, and I can hardly have any effect on pitch.

I think this is a limitation of the simulator. My guess is that, in
real life, the autopilot continuously adjusts trim, and so, if you try
to change pitch with the yoke, the A/P retrims to cancel out your
efforts, and the net effect is that you can change pitch, but you must
exert very high control pressures to do it (in order to overcome the
trim that the A/P is applying to neutralize your efforts).

Is this correct? I can see why this is difficult to simulate with
controls that don't provide feedback, but I want to make sure that I
understand the difference between the sim's way of doing it and the
real thing.

It takes pretty minimual control to over power the autopilot. However,
if you disrupt it a great deal (make a big pitch change for instance)
the autopilot may overcorrect a few times before settling back to the
correct pitch. The ability to return to the correct pitch with minimal
over correcting is a function of the quality of the autopilot.
Sometimes when you make changes such as lowering flaps you need to
"help" the autopilot but holding the yoke in the right place for a bit
until the autopilot figures out what is going on. This even with the
more expensive autopilots (like the new KAP140).

However, it is very easy to hand fly the plane while the autopilot is
on. The Cessna training instructors told us that they routinely hand
fly procedure holds on approach w/o changing the mode on the autopilot,
just over powering it.
In my plane I'll hand fly through turb but I'll leave the autopilot on
because it gives me a nudge back to course (i.e. I can feel it trying
to pull me back on course). If I take my hands off during turb though
it will make the turb worse.


-Robert

Robert M. Gary wrote:

Sometimes when you make changes such as lowering flaps you need to
"help" the autopilot but holding the yoke in the right place for a bit
until the autopilot figures out what is going on. This even with the
more expensive autopilots (like the new KAP140).

I don't have to do this with the 55X provided that I make a power
adjustment to go with the new configuration.

However, it is very easy to hand fly the plane while the autopilot is
on. The Cessna training instructors told us that they routinely hand
fly procedure holds on approach w/o changing the mode on the autopilot,
just over powering it.

Ick. What's the point? Which mode where they in to start with?

Ron Natalie wrote:
Robert M. Gary wrote:
However, it is very easy to hand fly the plane while the autopilot is
on. The Cessna training instructors told us that they routinely hand
fly procedure holds on approach w/o changing the mode on the autopilot,
just over powering it.

Ick. What's the point? Which mode where they in to start with?

The autopilot is in nav/approach mode. This is coupled with a G1000 so
it doesn't do holds on its own (its based on the 430 software, not the
480 software). So, rather than switch the autopilot into heading mode,
and then back into approach capture mode, they leave it in approach
mode. That way, as soon as you turn in bound you can just let go and it
flys the approach (KAP140 has pitch control and can fly a GS). It just
saves you from leaving nav mode and havnig to go back into capture
mode. I think part of their point is that it doesn't do any harm to out
mussle the autopilot.

-Robert

If the autopilot is flying, use the autopilot console to fly
the airplane. Over-powering the autopilot is always BAD
technique. Many autopilots will disconnect if they sense a
problem which can include input on the control wheel.

Autopilots have a supplement in the POH, it should be read
and understood.



"Robert M. Gary" wrote in message
ups.com...
|
| Ron Natalie wrote:
| Robert M. Gary wrote:
| However, it is very easy to hand fly the plane while
the autopilot is
| on. The Cessna training instructors told us that they
routinely hand
| fly procedure holds on approach w/o changing the mode
on the autopilot,
| just over powering it.
|
| Ick. What's the point? Which mode where they in to
start with?
|
| The autopilot is in nav/approach mode. This is coupled
with a G1000 so
| it doesn't do holds on its own (its based on the 430
software, not the
| 480 software). So, rather than switch the autopilot into
heading mode,
| and then back into approach capture mode, they leave it in
approach
| mode. That way, as soon as you turn in bound you can just
let go and it
| flys the approach (KAP140 has pitch control and can fly a
GS). It just
| saves you from leaving nav mode and havnig to go back into
capture
| mode. I think part of their point is that it doesn't do
any harm to out
| mussle the autopilot.
|
| -Robert
|

Jim Macklin wrote:
If the autopilot is flying, use the autopilot console to fly
the airplane. Over-powering the autopilot is always BAD
technique. Many autopilots will disconnect if they sense a
problem which can include input on the control wheel.

I'm simply quoted chapter and verse what the Cessna factory instructors
are saying. If you have a disagreement with the technique you are free
to discuss it with them.

Jim Macklin wrote:
If the autopilot is flying, use the autopilot console to fly
the airplane. Over-powering the autopilot is always BAD
technique. Many autopilots will disconnect if they sense a
problem which can include input on the control wheel.

Yep, caused an airliner to crash in Russia a few years back
(among other issues).

Robert M. Gary wrote:
Ron Natalie wrote:
Robert M. Gary wrote:
However, it is very easy to hand fly the plane while the autopilot is
on. The Cessna training instructors told us that they routinely hand
fly procedure holds on approach w/o changing the mode on the autopilot,
just over powering it.
Ick. What's the point? Which mode where they in to start with?

The autopilot is in nav/approach mode. This is coupled with a G1000 so
it doesn't do holds on its own (its based on the 430 software, not the
480 software). So, rather than switch the autopilot into heading mode,
and then back into approach capture mode, they leave it in approach
mode. That way, as soon as you turn in bound you can just let go and it
flys the approach (KAP140 has pitch control and can fly a GS). It just
saves you from leaving nav mode and havnig to go back into capture
mode. I think part of their point is that it doesn't do any harm to out
mussle the autopilot.

Still Ick. I don't know about the KAP150, but switching autopilot
modes is just a button push (well, two, but you can mash NAV and
APR at the same time, I even can do HDG+NAV+APR together without
strain). Switching the autopilot modes is a lot easier than
fighting the thing trying to drive you back to the final course.

Ron Natalie wrote:
Still Ick. I don't know about the KAP150, but switching autopilot
modes is just a button push (well, two, but you can mash NAV and
APR at the same time, I even can do HDG+NAV+APR together without
strain). Switching the autopilot modes is a lot easier than
fighting the thing trying to drive you back to the final course.

It just have to recapture.

Robert M. Gary wrote:


However, it is very easy to hand fly the plane while the autopilot is
on.
-Robert

Ouch.

My old Skylane has a KAP-150.

"Easily" over power the yoke/trim with the AP on? No.

I have a nice big red circuit breaker handle on the trim breaker just
for this reason.

Pre-start includes saying out-loud: "Run-away trim, pull red breaker."

Fly an approach while overpowering the AP?

Who are these Cessna instructors?

They might want to RTFM, that's what CWS is for.

Adam K.