Autopilot fighting for control

The Cessna instructors are probably showing that the servos
can be over-powered with pilot muscle, no reason to lose
control. But I'm sure they also teach the location of CWS,
the trim switch, the red disconnect button and the A/P CB
and avionics master.


wrote in message
ups.com...
|
| 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.
|

On Tue, 10 Oct 2006 21:35:47 -0500, "Jim Macklin"
wrote:

The Cessna instructors are probably showing that the servos
can be over-powered with pilot muscle, no reason to lose
control. But I'm sure they also teach the location of CWS,
the trim switch, the red disconnect button and the A/P CB
and avionics master.

There's no CWS on the KAP140 install in the NAV3 equipped Cessna line.
Probably on the Mustang when it comes out, but not the SEPs.

It's been a few years so things might be different now, but IIRC, the
class doesn't teach leaving things in NAV mode to fly procedure turns,
but teaches doing a partial-panel approach by selecting wing-level and
flying it that way. Personally with a working aircraft I always fly a
procedure turn w/ autopilot by changing it to HDG mode, flying the
procedure turn with the bug, and then re-arm APR on the inbound
intercept.

Peter Clark wrote:
..

There's no CWS on the KAP140 install in the NAV3 equipped Cessna line.
Probably on the Mustang when it comes out, but not the SEPs.

I've got CWS on my 55X. Frankly, I never use it.




Personally with a working aircraft I always fly a
procedure turn w/ autopilot by changing it to HDG mode, flying the
procedure turn with the bug, and then re-arm APR on the inbound
intercept.

That's what works for me. On the 55X, you can HDG+NAV+APR
at the same time and it holds the heading until the needle comes
alive (if you just NAV+APR, it may make a more agressive turn
towards the course).

If the autopilot is over=powered the auto-trim will run to
try to get control forces back in the acceptable range.
This will place the airplane seriously out of trim. A pilot
can over-power the autopilot, but if this is done it should
be either as a demonstration during checkout or in response
to an autopilot actuator run-away. The pilot should
maintain attitude control, over-powering the autopilot while
they
1 push the disconnect button
2 pull the circuit breaker
3 manually retrim
4 land ASAP



--
James H. Macklin
ATP,CFI,A&P

"Robert M. Gary" wrote in message
oups.com...
|
| 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.

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

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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Mxsmanic wrote:
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.

Its an annunciator just like any other. There is one annunciator that
has multiple preprogrammed channels. I believe mine has 40 different
things it can say, although few are hooked up. A signal goes down the
wire to the annunciator and the voice speaks. Its not part of the
autopilot or trim system itself.
Mine often says "Check landing gear" just as I call up tower when on
approach. Its a bit annoying.

-Robert

Robert M. Gary writes:

Mine often says "Check landing gear" just as I call up tower when on
approach. Its a bit annoying.

The simulated Baron I have says "Check gear!" constantly below certain
throttle settings (I'm not exactly sure what sets it off, though).
It's very irritating, but if it's in the sim, it must be doing that in
real life as well.

--
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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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