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.

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)

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.

wrote in
link.net:

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.

I have to agree. If the heading bug is making a difference in the track,
then it isn't a fully coupled approach.

This all does bring up one important point, though. It is very, very
important to know exactly how the autopilot you're using works, and how it
does everything it does. If you don't know, then you're getting into
dangerous territory. Using an autopilot when you don't know how can kill you
very, very quickly.

--
Regards,

Stan

wrote
My two cents worth: any autopilot that acts in that manner is probably
best not used for "coupled" approaches.

I concur. But then I'm not a fan of doing coupled approaches with
old-technology autopilots anyway.

I put "coupled" in quotes
because a truely coupled approach would be independent of the heading
bug once the localizer has been captured.

Do you fly an approach without reference to heading once the localizer
is captured? Of course not - you fly a heading, and use the CDI as a
correction on the heading. So do most Century autopilots. There is
nothing wrong with using heading information to stabilize the
approach. The problem is with the way the unit does it.

Century 2000 sounds all nice and modern, but really it's the same old
analog control loop design going all the way back to the Century II
with a digital false face hung on it. Here's how it really works:

At its core, the device is a wing leveler, and a crude one at that.
It runs a roll servo to attempt to keep the bank angle at some
setpoint. That set point can be zero bank, it can be some bank angle
dialed in by a roll knob, or it can come from the heading gyro. In
that case, the set point is a bank angle proportional to the deviation
between actual and bugged heading, with a limit (usually 25 degrees of
bank). When a nav coupler is used, the nav deviation is used to add
an offest to the bug. What I mean by this is, let's say in LOC mode,
one dot is worth five degrees. If the loc needle is a dot right, then
the nav converter will make the autopilot think that the bug is five
degrees to the right of where you set it. In reality, it's usually
not linear - past some point (say 3/4 scale) it will start making each
additional increment worth a lot more degrees, so that it can track a
course somewhat even if the user set the bug totally wrong. So really
we have another control loop, and a non-linear one at that.

Now, obviously with this kind of scheme, you never actually eliminate
offset unless you set the heading bug to the correct heading (not
course) to fly. That's why the approach was being flown two dots out.
This is normal behavior for the Century system.

The problem is that when the heading bug was reset, the system went
into oscillatory behavior.

Realize, though, that this is a system with three nested control loops
- a tight inner loop doing wing leveling, a more damped loop modifying
the bank angle setpoint on the inner loop based on heading deviation
to accomplish heading hold, and a third loop modifying the heading
setpoint on the middle loop based on course deviation to accomplish
course tracking.

Now realize that ultimately, that inner loop is deflecting ailerons -
and air loads on ailerons at any given bank angle depend quite a lot
on airspeed, not to mention control system friction (which can vary
with rigging and ambient temperature). The system is expected to work
over a range of airspeeds and temperatures for a given make and model,
and with only minor tweaks it is the same system for many makes and
models.

Note that turbulence will affect bank angle - adding noise into the
system.

The rate of heading change as a function of bank angle changes with
airspeed. Once again the system is expected to work over a range of
airspeeds.

The rate of angular course deviation as a function of heading change
depends on distance from the navaid and groundspeed. The system is
expected to function over a range of distances and groundspeeds.

It's pretty damn tough to do a triple nested control loop with
reasonably fast response across a variety of conditions and not have
oscillatory behavior somewhere. There are lots of adjustments to be
made. Then rigging changes over the years, things maybe don't get
lubed quite so well as they sould, electronic components drift out of
tolerance due to temperature extremes as the plane sits out in the sun
and heats up to 130 or chills in the winter to -20.

So the bottom line is what you saw is not so unusual, and the correct
solution (assuming there is one that does not involve hand flying)
will depend on what has drifted out of tolerance. Changing heading in
steps may be the way - usually the smaller the setpoint change the
lower the chance of oscillatory behavior. But the reality is that you
have an autopilot that, while still usable for lots of things, is
probably not quite exactly right, and probably should not be used for
actual coupled approaches. Which is pretty much the norm for older GA
autopilots.

Michael

Michael wrote:
wrote

My two cents worth: any autopilot that acts in that manner is probably
best not used for "coupled" approaches.


I concur. But then I'm not a fan of doing coupled approaches with
old-technology autopilots anyway.


I put "coupled" in quotes
because a truely coupled approach would be independent of the heading
bug once the localizer has been captured.


Do you fly an approach without reference to heading once the localizer
is captured? Of course not - you fly a heading, and use the CDI as a
correction on the heading. So do most Century autopilots. There is
nothing wrong with using heading information to stabilize the
approach. The problem is with the way the unit does it.

I make reference to the heading, but once coupled I have no autopilot
input to modify or adjust heading in any manner, so I certainly do not
fly heading while in NAV, APPROACH, or AUTOLAND modes. If I am hand
flying using the flight director while in NAV or APPROACH modes then,
indeed, I am using heading to track the course (course and vertical path
in case of ILS). But, the autopilot completely takes over steering of
heading when its doing the task, as its alway been with any autopilot
I've used since 1960 or so.

Roy Smith wrote:


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.

That doesn't sound normal. Some hunting is to expected, but based on
your description the autopilot might need some attention from a Century
shop.