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