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

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.


There's a big difference between airline control movement and it's
relationship to trim than light aircraft. I'll restrict our talk
to light aircraft.

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).

My autopilot has both pitch servos and trim. You push on the yoke
it has the same impact that it would if the autopilot wasn't there.
However, what the autopilot does endeavor to do is adjust the trim
so that it's always "trimmed" which means it may try to trim away
your force (if you are pitching it away from where it thinks it
should be). The autopilot does this so that if you kick off the
autopilot it's giving you a plane that is in trim.

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?

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.

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

Ron Natalie wrote:
My autopilot has both pitch servos and trim. You push on the yoke
it has the same impact that it would if the autopilot wasn't there.
However, what the autopilot does endeavor to do is adjust the trim
so that it's always "trimmed" which means it may try to trim away
your force (if you are pitching it away from where it thinks it
should be). The autopilot does this so that if you kick off the
autopilot it's giving you a plane that is in trim.

Some autopilots don't have trim servos and have "up" and "down"lights
on the autopilot display asking the pilot to retrim. The autopilot
normally flys a bit out of trim but anything outside the params results
in a request of the pilot to trim. The purpose is just as you
mentioned. You would be sad to find yourself in a nose down dive after
disconnecting the autopilot.
The autopilots that have "pitch control" (terminology used to denote
that it does more than hold altitude, it holds VS) usually have trim
control as well. The familiar "Trim, in motion" annunciator that seems
to go off everytime I click the mic to call tower in the G1000
aircraft.

-Robert

Mxsmanic wrote in
:

but I want to make sure that I
understand the difference between the sim's way of doing it and the
real thing.

Why do you need to understand?

What's the difference if you are never going to sit in a REAL PLANE.

Allen

Ron Natalie writes:

There's a big difference between airline control movement and it's
relationship to trim than light aircraft. I'll restrict our talk
to light aircraft.

The 737-800 sim has a very fancy autopilot (like the real thing), and
when you touch the controls, it switches to CWS pitch or roll mode.
Unfortunately, I don't understand how that works yet (especially when
it comes to getting the A/P to take over again), so I don't use it
much.

The A/P in the Baron seems straightforward enough. I note that if I
set it to hold altitude only, it resists any change in heading as
well; not sure if that is the sim or an accurate representation of
real life (the A/P is marked KFC225, whatever that is).

My autopilot has both pitch servos and trim. You push on the yoke
it has the same impact that it would if the autopilot wasn't there.
However, what the autopilot does endeavor to do is adjust the trim
so that it's always "trimmed" which means it may try to trim away
your force (if you are pitching it away from where it thinks it
should be). The autopilot does this so that if you kick off the
autopilot it's giving you a plane that is in trim.

Do you feel more pressure in the controls as the A/P tries to trim
away your pitch change?

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Robert M. Gary writes:

The autopilots that have "pitch control" (terminology used to denote
that it does more than hold altitude, it holds VS) usually have trim
control as well. The familiar "Trim, in motion" annunciator that seems
to go off everytime I click the mic to call tower in the G1000
aircraft.

Doesn't trim make its own noise? I've read notes in voice recorder
transcripts that mention "trim wheel moving at autopilot speed,"
although that is in large aircraft. I assume it must make quite a
noise if this gets recorded.

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