Coupled approach?

I don't fly too many coupled approaches, so forgive me if this is a
simplistic question.

We were flying a coupled VFR/practice ILS in a Debonair with a
Century-2000 A/P. The heading bug was set about 10 degrees to the left
of where it should have been, resulting us tracking to the left of the
localizer centerline. The CDI was about 1/2 scale "fly right"
deflection but stable.

Had we continued that way, it seemed that we would indeed eventually get
to the threshold, but following an offset course. This would have
required about a 5 degree heading change at DH to line up with the
runway. I suggested resetting the heading bug, which the other guy did,
when we were about 2 miles out. This resulted in the A/P over
controlling, flying us back through the localizer centerline, and then
starting what looked like it would end up being a series of S-turns
through the localizer. We disengaged the A/P at that point and flew the
last bit by hand.

So, my question is, what would have been the right thing to do?

One possibility would have been to do nothing, and just be content to
keep 1/2 scale deflection all the way to the DH. As long as it was
stable, we would have certainly found the runway.

Another possibility would have been to correct the heading bug, but to
do it in smaller increments, moving it a couple of degrees at a time to
let the A/P catch up.

I suppose a third possibility would have been to just be patient and let
the A/P damp out the oscillations on its own, but I think that would
have been a poor idea.

Roy Smith wrote in news:roy-E354B6.12140925092004
@reader1.panix.com:

I don't fly too many coupled approaches, so forgive me if this is a
simplistic question.

We were flying a coupled VFR/practice ILS in a Debonair with a
Century-2000 A/P. The heading bug was set about 10 degrees to the left
of where it should have been, resulting us tracking to the left of the
localizer centerline. The CDI was about 1/2 scale "fly right"
deflection but stable.

I've never used that autopilot, but the behavior is different from what I'm
used to. The flight directors I've used completely disregard the heading bug
when they couple to the localizer. I generally set the heading bug to
whatever course ATC give me for the intercept, and then when the a/p captures
the localizer, it turns to whatever direction it needs to hold the needle
centered, and the heading bug can be turned to any heading you want, with no
effect at all. The heading bug is only active when the a/p is set to hold
heading, not when tracking a nav signal.

What does the manual for the Century-2000 say?

--
Regards,

Stan

Don't know anything about that A/P, but I was under the general impression
that if an A/P is in APR or NAV mode, it is following the CDI needle (or
possibly the GPS?). The heading bug is used for HDG mode.

In article ,
"Jeremy Lew" wrote:

Don't know anything about that A/P, but I was under the general impression
that if an A/P is in APR or NAV mode, it is following the CDI needle (or
possibly the GPS?). The heading bug is used for HDG mode.

Well, the manual says:

"In systems equipped with a DG, during an instrument approach, the
heading bug must be set to match course for the segment of the approach
being flown when using the NAV, APR, or REV modes. course pointer."

Unfortunately, the manual is big on rote descriptions of which buttons
to push when, and says damn near nothing about how the thing actually
works inside. I'm left with guessing at its operating logic based on
observed behavior and some theoretical knowledge of control systems.

Clearly, setting the heading bug 10 degrees off the desired course was a
mistake, but the manual doesn't even begin to talk about the best way to
correct the mistake. Just resetting the bug to the right setting
resulted in course oscillations. What I'm trying to figure out is what
might have been a better course of action.

"Roy Smith" wrote in message
...



Roy, my Saratoga has an Altiimatic IIIC autopilot. It's heading bug is
active in NAV mode also. I believe the a/p sums the bug vs DG error and the
nav error input, and attempts to null the sum. Mine also oscillates if a
sudden change is made, but not too bad for a 10 degree bug change. I have
flown only a few coupled approaches so I don't really know how it behaves on
a localizer. I remember turning it off because I could hand fly it better.

In cruise using the GPS nav input, before I engage the NAV function, I first
get established on the course in heading mode and then engage the a/p. If
there is a strong crosswind, it will still oscillate some as it hunts for
the correct heading and track offset. If I have to make a substantial turn,
I sometimes will disengage the a/p or go back to heading mode, then reengage
NAV when established.

For your specific question about correcting the 10 degree bug error, I would
probably disengage the a/p and hand fly it. If I wanted to continue the
coupled approach, I would either ignore it or just slowly tweak it back.
The way you said would probably work fine, a couple of degrees at a time.
The thing is, in a crosswind, it will fly an offset from the track (at least
mine does) to null out the bug vs heading error. So the tweaking of the bug
setting has to be slow enough so that the nav offset gets established in
addition to the heading offset. It may take too much attention to be worth
it, just make the correction when you break out and disengage the a/p.
Maybe not too good at ILS minimums, but OK for higher.

Stan

Unfortunately, the manual is big on rote descriptions of which buttons
to push when, and says damn near nothing about how the thing actually
works inside.


This is true =everywhere=, and is =damn= frustrating. I don't want to know
what button to push, I want to know what happens when I push it.

Jose

--
(for Email, make the obvious changes in my address)

Roy Smith wrote in message ...
I don't fly too many coupled approaches, so forgive me if this is a
simplistic question.

I don't either, but I hope this info helps.

So, my question is, what would have been the right thing to do?

I have a KAP140. I think, in the situation you describe, my autopilot
would turn to intercept the localizer at a 45 degree angle (which it
does by default) and then try to turn on course per the heading bug.
At 2 miles out, I don't think it would make it.

What I would do on my machine would switch to HDG mode, turn to a
heading that would intercept the localizer at a small angle, switch to
ROL, hit APR, and move the heading bug to the localizer course. The
machine would intercept the localizer on the wings-level heading, then
turn on course when the localizer is intercepted.

Actually, what I would do is disengage and fly by hand, as you did,
but I'm pretty sure this would work. Now I'm gonna hafta try it next
time I'm out.

My two cents worth: any autopilot that acts in that manner is probably
best not used for "coupled" approaches. I put "coupled" in quotes
because a truely coupled approach would be independent of the heading
bug once the localizer has been captured.

The kludged-up situation you describe could be hazardous during
demanding circumstances.

Roy Smith wrote:

In article ,
"Jeremy Lew" wrote:


Don't know anything about that A/P, but I was under the general impression
that if an A/P is in APR or NAV mode, it is following the CDI needle (or
possibly the GPS?). The heading bug is used for HDG mode.


Well, the manual says:

"In systems equipped with a DG, during an instrument approach, the
heading bug must be set to match course for the segment of the approach
being flown when using the NAV, APR, or REV modes. course pointer."

Unfortunately, the manual is big on rote descriptions of which buttons
to push when, and says damn near nothing about how the thing actually
works inside. I'm left with guessing at its operating logic based on
observed behavior and some theoretical knowledge of control systems.

Clearly, setting the heading bug 10 degrees off the desired course was a
mistake, but the manual doesn't even begin to talk about the best way to
correct the mistake. Just resetting the bug to the right setting
resulted in course oscillations. What I'm trying to figure out is what
might have been a better course of action.

The simple answer is that the autopilot compares the heading bug to CDI
deflection to determine the amount and rate of aileron input. To
prevent a reoccurence of that problem, set the heading bug to the final
approach course (or whichever segment you wish to track) and you'll be set.

I have an Altimatic III (a Century product) in my airplane and it works
essentially the same way.

"Ryan Ferguson" wrote in message
...
The simple answer is that the autopilot compares the heading bug to CDI
deflection to determine the amount and rate of aileron input. To prevent
a reoccurence of that problem, set the heading bug to the final approach
course (or whichever segment you wish to track) and you'll be set.

I have an Altimatic III (a Century product) in my airplane and it works
essentially the same way.

Is this true of the newer S-TEC autopilots as well? My club plane is
getting a 55X installed soon.