Coupled approach?

On Sun, 26 Sep 2004 11:03:49 -0400, "Jeremy Lew" wrote:

"Ryan Ferguson" wrote in message
m...
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.


I have a -50 and have not noted any effect of the heading bug in other than
heading mode.


--ron

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

Jeremy Lew wrote:

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

No, the 55X disregards the heading bug when the HDG mode is disengaged.
Overall, a much better, more modern autopilot compared to Century.

-Ryan

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.

In article . net,
Ryan Ferguson wrote:

Jeremy Lew wrote:

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

No, the 55X disregards the heading bug when the HDG mode is disengaged.
Overall, a much better, more modern autopilot compared to Century.

-Ryan

For sure, the Century is not a modern unit. Near as I can tell, the
design is about 20 years old. I gotta assume newer units are more
sophisticated.

Roy Smith wrote:


For sure, the Century is not a modern unit. Near as I can tell, the
design is about 20 years old. I gotta assume newer units are more
sophisticated.

Great thread, Roy. I fly coupled approaches with my Century 2K from time to
time, now I'll have to go experiment with setting the heading bug slightly off
course to see what happens.

I think the poster who explained that the heading deviation is summed with the
course deviation and that the A/P attempts to null the sum has it about right.
That's consistent with my observations.

The C2K is installed in lots of different airframes, so there must be some
provision to adjust the gain, which surely must be different from airframe to
airframe. I wonder whether the gain could be adjusted to optimize the damping,
giving the fastest possible convergence with the desired course without
overshooting and consequent oscillation. Unfortunately my intuition tells me
that the optimum gain setting might be dependent on how far out you are on the
localizer course.

Dave

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.

wrote in
link.net:

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.

But not so with most of the Century autopilots out there, including ones
installed as late as 1980's). While the newer ones do as you describe,
all the older ones utilize a combination of data from the AI, DG, and
NAV input.

The AI provides the "wing leveler" primary input, while the DG and NAV
signals are summed to provide a heading. Amazingly effective, for such
a trivial analog approach, it will (if carefully adjusted) intercept and
track a NAV signal. However, in a strong crosswind, it will do so "off"
the actual courseline (parallel to it). The Century manual states that
"if this bothers you" you should simply adjust the heading bug on the DG
to an amount "offset" the other way.

The two annoying features of this system a
1. Every time the desired course changes, you have to adjust the heading
bug on the DG accordingly.

2. Like all such systems, there is no turn anticipation. So it is
always a compromise on each new acquisition between undershoot and
overshoot. These systems have a sensitivity adjustment that sets this.
Problem is: a) too little, and it will fly S-turns about the desired
course line, and b) too much, and it will wing rock itself until you
toss your cookies. Getting it set "just right" is an inflight specific
adjustment unique to each individual autopilot unit and N# combo.

jmk

In article . net, Ryan
Ferguson writes:

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,
The Century IIIB in my 79 Archer gets its "main" info from the heading bug,
even when in the NAV position. The Nav instrument then adds a correction of up
to about 30 degrees. I try to get the heading bug ( and the DG) set to the
correct values before the approach. However, if I correct the bug and get a
"course turn" I find that the NAV instrument rather quickly removes its
"correction" and you are back on course within 10 or 15 seconds. Flying 10
degrees off course for 10 seconds doesn't really give you much error of
position tho if on an approach in the clag, it might seem like a lot. Moral,
don't make changes on final.

Chuck