KAP140 Autopilot Details

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

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

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

News wrote:
snip
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.

I don't have specific experience with the KAP140, but it sounds like it may work
just like a couple of other ones I have used.

I think you understand how it works in HDG mode, right? There is an error signal
from the heading bug, and the aircraft is turned so as to correct the error. In
NAV mode, the CDI error is (conceptually) added to the heading error, so that
the effective zero-error heading will be slightly off the heading bug setting
when the CDI is not centered.

In a crosswind, theoretically the aircraft will track a course offset from the
radial on the downwind side, with a heading offset into the wind. The CDI will
be offset upwind, and the heading flown will be offset into the wind from the
heading bug. In other words, it doesn't track the center of the radial, it
tracks just enough off to the side so that the error signal from the CDI plus
the error signal from the heading bug cancel each other.

Dave

Remove SHIRT to reply directly.

As a 300 hour user of a KAP140 I would caution you on several things
depending on the
plane its fitted to.

The short list of gotchas a

Easy to inadvertantly fly an ILS instead of a GPS if you mistune the NAV
receiver

Because its pitch information is based on rate of climb , mountain waves,
buump clouds,
thermals can cause significant airspeed and pitch chnage attitudes,
including stalling the plane!

Inadvertant engagement of the AP on teh ground can cause uncontrollable trim
excursions

If you tweak a nav system whilst its connected it may appear to be connecte
dto teh njav solution but
its actually disenegaged

apart from thats its fine!

PS make sure you can hand fly in real iFR when it lets go!

peter


"Dave Butler" wrote in message
...
News wrote:
snip
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.

I don't have specific experience with the KAP140, but it sounds like it
may work
just like a couple of other ones I have used.

I think you understand how it works in HDG mode, right? There is an error
signal
from the heading bug, and the aircraft is turned so as to correct the
error. In
NAV mode, the CDI error is (conceptually) added to the heading error, so
that
the effective zero-error heading will be slightly off the heading bug
setting
when the CDI is not centered.

In a crosswind, theoretically the aircraft will track a course offset from
the
radial on the downwind side, with a heading offset into the wind. The CDI
will
be offset upwind, and the heading flown will be offset into the wind from
the
heading bug. In other words, it doesn't track the center of the radial, it
tracks just enough off to the side so that the error signal from the CDI
plus
the error signal from the heading bug cancel each other.

Dave

Remove SHIRT to reply directly.

Peter Bondar ) wrote:

As a fellow few hundred hour KAP140 user, I have a couple of comments:

Easy to inadvertantly fly an ILS instead of a GPS if you mistune the NAV
receiver

Huh... I have heard of accidently flying the GPS course instead of the ILS,
since transitioning from GPS to ILS is more common than the reverse, but I
have never heard of the opposite. What scenario would allow this to
easily occur? I cannot think of one, other than the pilot was seriously
confused.

Furthermore, the AP will not fly the glideslope unless APR (a separate AP
mode button) was pushed. If someone *accidently* did this, then they would
be best served voluntarily grounding themselves until they had remedial
cockpit management training.

Because its pitch information is based on rate of climb , mountain waves,
buump clouds,
thermals can cause significant airspeed and pitch chnage attitudes,
including stalling the plane!

True enough. A pilot flying with the KAP140 needs to be ready to adjust
the throttle and disengage the AP at a moment's notice.

Inadvertant engagement of the AP on teh ground can cause uncontrollable trim
excursions

There was a C182 accident somewhere in England a couple of years ago where
this was the probable cause. The theory was that the pilot hit the AP
button with his knuckle when he pushed the throttle forward for the runup,
then never noticed the trim at full deflection.

In response to the investigation, B/K made a few modifications to the AP to
prevent this type of accident. I believe there was an A/D for the changes,
but my KAP140 was produced after the changes were implemented (fall 2002),
so I do not have the specifics on the A/D.

One of the changes B/K made was the method in which a pilot engages the AP.
No longer does the switch on the yoke engage the AP; it only disengages it.
Additionally, the AP button on the AP itself must be held for three seconds
to engage the unit.

Another change was the addition of an audio warning ("Trim in Motion" over
the intercom) if the trim is in motion for more than 4 seconds, presumably
to call attention to the AP if it were inadvertently engaged on the ground.
There is also an annunciator panel light that flashes when the trim is in
motion for greater than four seconds.

And finally, a non-AP change as a result of this accident was that Cessna
was asked to change the order of the pre-takeoff checklist to move the
"Trim set to TO" after the runup portion of the checklist. If this was
done, it was done after the 2002 model year, as my checklist still has this
item towards the beginning of pre-takeoff list.


--
Peter












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Dave,

You have confused me even more!
If I am flying a radial with a cross wind, should the HDG bug be on the
radial or should it be on the estimated course made good with the wind
correction?

Thanks,
Ross
"Dave Butler" wrote in message
...
News wrote:
snip
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.

I don't have specific experience with the KAP140, but it sounds like it
may work
just like a couple of other ones I have used.

I think you understand how it works in HDG mode, right? There is an error
signal
from the heading bug, and the aircraft is turned so as to correct the
error. In
NAV mode, the CDI error is (conceptually) added to the heading error, so
that
the effective zero-error heading will be slightly off the heading bug
setting
when the CDI is not centered.

In a crosswind, theoretically the aircraft will track a course offset from
the
radial on the downwind side, with a heading offset into the wind. The CDI
will
be offset upwind, and the heading flown will be offset into the wind from
the
heading bug. In other words, it doesn't track the center of the radial, it
tracks just enough off to the side so that the error signal from the CDI
plus
the error signal from the heading bug cancel each other.

Dave

Remove SHIRT to reply directly.

Ross Magnaldo wrote:

If I am flying a radial with a cross wind, should the HDG bug be on the
radial or should it be on the estimated course made good with the wind
correction?

When using NAV mode, the heading bug should precisely match the desired
radial which is selected by the OBS. The airplane will then seek a heading
which makes good the selected course, and that may not be heading selected
by the bug if there is a crosswind.

"Craig Prouse" wrote:
When using NAV mode, the heading bug should precisely match the
desired radial which is selected by the OBS. The airplane will then
seek a heading
which makes good the selected course, and that may not be heading
selected
by the bug if there is a crosswind.

The heading bug plays no part in NAV tracking on S-Tec 30 or 50
autopilots. Why is it necessary on tha KAP-140?
--
Dan
C172RG at BFM

"Dan Luke" wrote:

The heading bug plays no part in NAV tracking on S-Tec 30 or 50
autopilots. Why is it necessary on tha KAP-140?

If I recall correctly, those S-Tec autopilots require the pilot to establish
the aircraft on course before engaging the NAV course tracking mode. Once
on course, the error signal from the CDI is probably sufficient to remain
there.

The KAP 140 has the capability to compute an automatic intercept heading and
will take you to the desired radial or DTK then turn on course and track.
You can be anywhere, pointing in the wrong direction altogether, with a
full-scale deflection of the CDI, and the autopilot will turn you around and
get you on course. This requires a reference heading for orientation.