Autopilot fighting for control

Robert M. Gary wrote:
Ron Natalie wrote:
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).

Ok, if you're going to bring up an incident tell the entire story. The
captains 15 year old son was at the controls at the time.

Yes, that was the cause of the AP disconnect. The 15 yo overpowered
the autopilot causing it to silently disconnect the aileron servos.
Neither the right seat pilot nor the captain standing behind noticed
this. When the airplane started roll subsequently they assumed they
had somehow commanded an autopilot-controlled hold entry. They
then allowed the bank to progress to 50 degrees.

This leads to an incipient problem you can have in a private aircraft
as well. Without sufficient power, the autopilot trying to maintain
altitude can drive the aircraft into a stall. It was finally at the
onset of the prestall buffeting that the copilot started to try to
recover, unfortunately while you can overpower an autopilot easily,
overpowering a 15yo holding the other yoke is not as easy.

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:
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.

eh?

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.

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).

On Thu, 12 Oct 2006 15:35:28 -0400, Ron Natalie
wrote:

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).

Yea, same on the KAP140. I use HDG+APR and try for a 30-40deg
intercept angle. Seems to work.

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
|