First of all, thank you for your responses.

I want to make sure I understand what you are saying. The AP only gets one
piece of information from the CDI... the equivalent of the left/right
deflection of the needle. It is not getting the OBS setting. This piece of
information comes from the heading bug. When tracking a radial, though, it
would seem the OBS setting will tell you the desired heading. Also, the
manual instructs you to always set the heading bug to the same value as the
OBS. Intercepting the course works because when you first select NAV and
the HDG message flashes you set the HDG bug to the OBS setting, then it
basically ignores the HDG for a while and either uses the current heading
(all angles intercept) or computes a 45 deg intercept. Again, if the AP had
the OBS setting and the current heading, it could figure this out. It seems
to me this may be a cost issue. The way it is now the AP only needs the
following inputs:

Current Heading
Heading Bug
CDI deflection

If it was able to follow a course from the CDI/OBS alone it would need an
additional input.

Thank you for all the responses.

martin


"Mike Rapoport" wrote in message
k.net...
By setting the heading bug and the OBS to the same course you are
mimicking
a HSI. The data out of the OBS is only course deviation, it doesn't
output
the course that is set on the dial. If the numbers on the OBS were
covered
up and I turned the knob to a randome course, you would have a difficult
time flying that course. You might eventually figure it out, but you also
might make a bunch of 90 degree intercepts first. By setting both, you
are
telling it what the desired course is (HDG) and the current deviation from
that course (CDI).

Life is simpler with an HSI which combines the DG and OBS onto one
instrument...highly recommended.


Mike
MU-2


"News" wrote in message
.. .
I have a question about some nuances of the Bendix/King KAP 140
autopilot.
Since the ink is still wet on my IFR rating (actually I have not even
gotten
the new one in the mail yet!) I am working on a personal "post
checkride"
syllabus. Part of this self training is to fly different routes on an
IFR
clearance in VMC using the autopilot. In all my VFR days I never
touched
the thing and was not allowed to use it during my IFR training. While I
am
serious about keeping my hand flying skills sharp, I also want to know
how
to use the autopilot. After reading the manufacturer's "Pilot's Guide"
cover to cover I noticed a rather odd procedure when using the AP in
conjunction with a DG. When you select NAV the AP temporarily flashes
HDG
to remind you to set the Heading Bug to the same course as set on the
OBS.
I do this and everything works fine but I want to understand WHY you do
this. What algorithm is the autopilot is using? How does it uses the
Heading bug info in conjunction with the OBS and CDI? I noticed that
once
I
am tracking a radial if I change the HDG bug the aircraft turns. My
mental
model has the autopilot just using the CDI deflection and OBS setting to
hold a course, but obviously I am missing something. If anyone can
point
me
to some more detailed information on how the AP uses the Heading Bug
while
in NAV or APR mode I would appreciate it.

thanks
Martin Van Ryswyk
PPSEL Instrument