KAP140 Autopilot Details

Peter R. wrote:
Dave Butler ) wrote:


I assume you meant "...to match your current heading".


Yes.


directs the AP to turn in the direction you turned the heading bug, away
form the desired course. Left alone, the AP will soon turn back and re-
intercept the desired course.

If you turned the heading bug in the direction to match your current *heading*,
and and the AP turns in the direction you turned the heading bug, That turn will
be *toward*, not away from the desired course.


No, that is not what I am saying. The AP is already tracking the current
course, which results in maintained heading. However, the heading bug
happens to be five degrees to either side of the current heading. This
could either be because of a crosswind or perhaps DG precession.

If the pilot moves the heading bug to match the current heading, which is
also the course being tracked by the AP, the AP momentarily turns *away*
from the desired course.

OK, I don't want to beat this greasy spot where there used to be a dead horse,
but I think it's important that we not misinform someone who might be reading
this and trying to learn from it.

What you are saying may be correct, if it's behavior peculiar to the KAP140. It
is definitely counter-intuitive to me, and not the way APs that I have used behave.

Maybe we're just tripping over imprecise use of terminology, so let's be very
precise in the words we use. Sometimes an example is easier to understand than
just discussing the principle in the abstract, so permit me to concoct an example.

Example: We want to follow an airway that goes TO a navaid along that navaid's
180 radial. That is, the desired course is 360, along the 180 radial from the
navaid. There is a wind from the west. The DG and the magnetic compass have been
adjusted so that they are in agreement.

We manually (no autopilot) maneuver the aircraft so that it is centered on the
180 radial and heading 360. The OBS is set to 360, and the heading bug is set to
360. The CDI is centered. Now turn on the autopilot.

The heading is 360, but the wind from the west results in a track of (say) 010.
The CDI starts moving to the left as we drift off the desired track to the
right. The autopilot responds to the left-moving CDI by changing the heading to
the left, say 350.

There may be some overshoot or some oscillation, but eventually the whole system
comes to some equilibrium where the nose is pointed (heading) to the left of
360, and the track is more or less aligned with the desired track, but offset to
the right. For argument, lets say the track is now 360 (but offset slightly to
the right of the radial) and the heading is 350. [Nitpickers will argue that the
track won't be exactly 360, and they're right, but it doesn't affect the argument.]

OK so far?

Now, I assert that if the pilot turns the heading bug to the left to 350 to
match the heading, the aircraft will turn to the left and will take up a new
equilibrium heading. The new equilibrium track is closer to the desired radial
than the track was when the heading bug was set to 360.

It seems to me you are saying something different. Please confirm.

Thanks.

Dave

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