KAP140 Autopilot Details

[News]

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

[Mike Rapoport]

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
>
>
>
>

[Dave Butler]

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]

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 are:

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 R.]

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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[Ross Magnaldo]

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.
>

[Craig Prouse]

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.

[Dan Luke]

"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

[Craig Prouse]

"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.

[Ron Natalie]

"Craig Prouse" > wrote in message ...

> 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.

Depends which model you have. The 20/30 will not intercept a course, just hold it.
The more expensive ones in the line will intercept the course.

[Craig Prouse]

"Ron Natalie" wrote:

> Depends which model you have. The 20/30 will not intercept a course, just
> hold it.

Yes, when I said "those S-Tec autopilots" I was referring specifically to
"those," the 20/30.


> The more expensive ones in the line will intercept the course.

The KAP 140 is similar in many respects to the S-Tec 55. Actually the 55 is
a little nicer in several ways. I expect that the 55 would also require a
heading reference to support its intercept feature.

It seems like a heading reference might improve the ability of an autopilot
to maintain course precisely or at least reduce any tendency to weave or
hunt while bracketing the course. Would anyone with direct experience like
to offer a comparison?

[Dan Luke]

"Craig Prouse" wrote:
> It seems like a heading reference might improve the ability of an
> autopilot to maintain course precisely or at least reduce any
> tendency to weave or hunt while bracketing the course. Would
> anyone with direct experience like to offer a comparison?

I can only testify about my S-Tec 50, which is the same in this
respect as the 20 or 30. It works strictly off CDI deflection and rate
of turn. I don't think there is room for improvement. Using the GPS
CDI, once established on a track, it will maintain a xtrack error of
just a few feet without hunting. That is with the autopilot set to
"approach" sensitivity and the GPS CDI scale set to one mile.
--
Dan
C172RG at BFM

[News]

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
> >
> >
> >
> >
>
>

[Dave Butler]

Ross Magnaldo wrote:
> 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?

Sorry to confuse you, Ross.

Stating the disclaimer again: I've never flown with a KAP140, so my information
is based on autopilots that I *assume* operate similarly.

Set the heading bug on the desired radial. The track will be slightly off the
radial if you have a crosswind. This is usually good enough.

As a refinement, you can offset the heading bug in the direction into the wind,
and the aircraft will track closer to the desired radial.

Remove SHIRT to reply directly.

Dave

[Peter R.]

Dave Butler ) wrote:

> Set the heading bug on the desired radial. The track will be slightly
> off the radial if you have a crosswind. This is usually good enough.

True, in my experiences.

> As a refinement, you can offset the heading bug in the direction into
> the wind, and the aircraft will track closer to the desired radial.

With the KAP140 in this case, changing the heading bug even a few degrees
to match your current course (after the AP had been tracking the course)
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.

This also occurs after the DG precesses and the pilot attempts to reset it
to the compass.


--
Peter












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[Dave Butler]

Peter R. wrote:
> Dave Butler ) wrote:
>
>
>>Set the heading bug on the desired radial. The track will be slightly
>>off the radial if you have a crosswind. This is usually good enough.
>
>
> True, in my experiences.
>
>
>>As a refinement, you can offset the heading bug in the direction into
>>the wind, and the aircraft will track closer to the desired radial.
>
>
> With the KAP140 in this case, changing the heading bug even a few degrees
> to match your current course (after the AP had been tracking the course)

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

> 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.

Remove SHIRT to reply directly.

Dave

[Peter R.]

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.

--
Peter












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[Peter R.]

I wrote:

<snip>
> 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.

Whoops, left off a bit...

In my example, if the pilot moves the heading bug to the right to match the
desired heading, the KAP140 will momentarily turn to the right, off course.
If the pilot moves the heading bug to the left to place it over the desired
heading, the KAP140 momentarily turns to the left, again off course.

--
Peter












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[Tarver Engineering]

"Peter R." > wrote in message
...
> 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.

Think "selected heading", as "selected course" is a different thing in
autopilot land. (gyro vs radio)

[Peter R.]

Peter ) wrote:

> Peter R. > wrote
>
> >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.
>
> I take it you don't have an HSI - with an HSI it is the course pointer
> (not the heading bug) which is set to the desired track. The heading
> bug is ignored completely.

That's correct. I do not have experience flying behind an HSI. With about
500 hours in a C172, I am getting the itch to move up.


--
Peter












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[Dave Butler]

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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[Tarver Engineering]

"Peter R." > wrote in message
...
> Peter ) wrote:
>
> > Peter R. > wrote
> >
> > >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.
> >
> > I take it you don't have an HSI - with an HSI it is the course pointer
> > (not the heading bug) which is set to the desired track. The heading
> > bug is ignored completely.
>
> That's correct. I do not have experience flying behind an HSI. With about
> 500 hours in a C172, I am getting the itch to move up.

Then you need to think like the airplane.

[Tarver Engineering]

"Dave Butler" > wrote in message
...
> 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.

So far you have set up to fly a ground station based on radio magnetic
course angle.

> 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.

Now you use the tern "track", which is Long Range Nav; such as GPS, et al.
Track is a position displacement system and different from an angle.

[Dave Butler]

Tarver Engineering wrote:
> "Dave Butler" > wrote in message

<snip>

> Now you use the tern "track", which is Long Range Nav; such as GPS, et al.
> Track is a position displacement system and different from an angle.

Oh, OK, thanks for that refinement in terminology.

Dave

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[Tarver Engineering]

"Dave Butler" > wrote in message
...
> Tarver Engineering wrote:
> > "Dave Butler" > wrote in message
>
> <snip>
>
> > Now you use the tern "track", which is Long Range Nav; such as GPS, et
al.
> > Track is a position displacement system and different from an angle.
>
> Oh, OK, thanks for that refinement in terminology.

I fear a silibus for some autopilot awareness is being sifted here, so we
must be careful.

No slight intended as to your skills.

jt

[Michael]

"Ross Magnaldo" > wrote
> 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?

Contrary to other answers you are getting, the correct procedure
(where it matters) is to set the heading bug to the estimated course
made good. The reason is as described - when there is a crosswind, if
you set the radial you will actually fly an offset course.

Whether it matters or not is another story.

Every autopilot is, at the core, a wing leveler. If attitude based,
it uses an attitude gyro with pickups to establish wings level. If
rate based, it uses a TC with pickups to establish zero turn/roll.
That zero is the setpoint - if you're not banking you shouldn't be
turning (if trimmed). Note that a T&S is NEVER used - the response
time is too slow, and a significant bank can be established before the
autopilot knows it. For that reason, I always find it amusing when
people tell me they prefer the old needle and ball for partial panel
flying, but that's another discussion.

Heading hold does not eliminate the basic wing leveler function - it
alters it. The heading offset is used to change the setpoint of the
wing leveler. A common setting is 1 degree of heading per degree of
roll, with a 25 degree limit. That means that if the heading bug is 5
degrees to the right of the heading being flown, then instead of
trying to maintain wings level the autopilot will try to maintain a 5
degree bank to the right. But if the heading bug is 45 degrees to the
right of the heading being flown, the autopilot will try to maintain a
25 degree bank (the limit) to the right.

If a rate based wing leveler is used, then it is usually set to treat
a standard rate turn as being equivalent to anywhere from 15 to 30
degrees of roll, and limits the turn rate to standard rate. For
general reference, the Century I and derivatives are rate-based, as
are the S-TEC's. Most everything else is attitude based. To the
user, it hardly matters (other than knowing which gyro to monitor to
ensure the autopilot isn't doing something ugly). From here on in I'm
going to assume an attitude based autopilot, but it all carries over
to rate based ones.

When you add in nav tracking, it gets interesting. Some autopilots
use the CDI to directly drive the attitude set point, just like
heading is used. What this means to you is that if the CDI is right
of center, the plane will stay in a right bank until the CDI centers,
with a bank proportional to CDI deflection (up to some limit, like 25
degrees bank or standard rate). The built-in assumption is that if
the needle is centered, you are on the correct heading, and if it's
not, you're not. Now, if I had a buck for every time I told an
instrument student that just because the needle is centered DOESN'T
mean you're on the right heading... Still, it tends to work OK
tracking a LOC because the response is so quick - because the LOC
needle is VERY sensitive. It also tends to work OK tracking a GPS,
because the needle is almost as sensitive - if you don't mind the
constant wing wobble in turbulence.

If tracking a VOR in turbulence, it just doesn't work. Therefore most
of the older autopilots (designed before everyone had GPS) don't use
the CDI output directly - they use it to modify the heading error
output. In other words, it flies like you would. For example, say
the heading is dead on the bug, the wings are level, but the CDI is a
dot left. This will generally be treated the same as if the bug was 5
degrees left of heading. The plane will bank 5 degrees left and start
to turn. As the heading deviation increases to 1 degree left, the
bank will be reduced to 4 degrees. In other words the autopilot will
make an asymptotic turn - a technique that you would be well advised
to master if you are ever going to fly partial panel in something fast
and slippery. As the CDI approaches center, the plane will turn back
to the bug - again, asymptotically.

Some autopilots (the older Century II and III come to mind) do it both
ways - depending on how you set the nav converter, they will or won't
use the heading data in tracking the nav signal. Generally, they will
not use the heading data in approach mode (because the CDI is presumed
to be sufficiently sensitive, and the response sufficiently quick) but
WILL use it for enroute nav.

OK, now how do you figure out which kind you have? Easy.

Get the plane on course and on heading, set the heading bug to the
heading you are flying, make sure the CDI is centered, engage
autopilot in nav track mode. Once you are satisfied it is tracking,
give the heading bug a big ol' twist - say 45 degrees. If the plane
tries to follow the bug initially, you know the heading data is being
used in that mode. If it doesn't twitch, it's not.

Michael

Please do not send email replies to this posting. They are checked
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address in the usual way.

[Peter R.]

Dave Butler wrote:

<snip>
> 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.

No, now that I read your example, perhaps we are describing the same
event.

You have piqued my curiosity, though. When my aircraft comes out of its
annual late this week I am going to experiment with this more closely.

--
Peter

[Ross Magnaldo]

Thanks Michael for the clarity of the explanation.
The autopilot I'm using (when it wants to work) is rate based - I'll check
it next time for the VOR heading.

Fly safely,
Ross
"Michael" > wrote in message
om...
> "Ross Magnaldo" > wrote
> > 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?
>
> Contrary to other answers you are getting, the correct procedure
> (where it matters) is to set the heading bug to the estimated course
> made good. The reason is as described - when there is a crosswind, if
> you set the radial you will actually fly an offset course.
>
> Whether it matters or not is another story.
>
> Every autopilot is, at the core, a wing leveler. If attitude based,
> it uses an attitude gyro with pickups to establish wings level. If
> rate based, it uses a TC with pickups to establish zero turn/roll.
> That zero is the setpoint - if you're not banking you shouldn't be
> turning (if trimmed). Note that a T&S is NEVER used - the response
> time is too slow, and a significant bank can be established before the
> autopilot knows it. For that reason, I always find it amusing when
> people tell me they prefer the old needle and ball for partial panel
> flying, but that's another discussion.
>
> Heading hold does not eliminate the basic wing leveler function - it
> alters it. The heading offset is used to change the setpoint of the
> wing leveler. A common setting is 1 degree of heading per degree of
> roll, with a 25 degree limit. That means that if the heading bug is 5
> degrees to the right of the heading being flown, then instead of
> trying to maintain wings level the autopilot will try to maintain a 5
> degree bank to the right. But if the heading bug is 45 degrees to the
> right of the heading being flown, the autopilot will try to maintain a
> 25 degree bank (the limit) to the right.
>
> If a rate based wing leveler is used, then it is usually set to treat
> a standard rate turn as being equivalent to anywhere from 15 to 30
> degrees of roll, and limits the turn rate to standard rate. For
> general reference, the Century I and derivatives are rate-based, as
> are the S-TEC's. Most everything else is attitude based. To the
> user, it hardly matters (other than knowing which gyro to monitor to
> ensure the autopilot isn't doing something ugly). From here on in I'm
> going to assume an attitude based autopilot, but it all carries over
> to rate based ones.
>
> When you add in nav tracking, it gets interesting. Some autopilots
> use the CDI to directly drive the attitude set point, just like
> heading is used. What this means to you is that if the CDI is right
> of center, the plane will stay in a right bank until the CDI centers,
> with a bank proportional to CDI deflection (up to some limit, like 25
> degrees bank or standard rate). The built-in assumption is that if
> the needle is centered, you are on the correct heading, and if it's
> not, you're not. Now, if I had a buck for every time I told an
> instrument student that just because the needle is centered DOESN'T
> mean you're on the right heading... Still, it tends to work OK
> tracking a LOC because the response is so quick - because the LOC
> needle is VERY sensitive. It also tends to work OK tracking a GPS,
> because the needle is almost as sensitive - if you don't mind the
> constant wing wobble in turbulence.
>
> If tracking a VOR in turbulence, it just doesn't work. Therefore most
> of the older autopilots (designed before everyone had GPS) don't use
> the CDI output directly - they use it to modify the heading error
> output. In other words, it flies like you would. For example, say
> the heading is dead on the bug, the wings are level, but the CDI is a
> dot left. This will generally be treated the same as if the bug was 5
> degrees left of heading. The plane will bank 5 degrees left and start
> to turn. As the heading deviation increases to 1 degree left, the
> bank will be reduced to 4 degrees. In other words the autopilot will
> make an asymptotic turn - a technique that you would be well advised
> to master if you are ever going to fly partial panel in something fast
> and slippery. As the CDI approaches center, the plane will turn back
> to the bug - again, asymptotically.
>
> Some autopilots (the older Century II and III come to mind) do it both
> ways - depending on how you set the nav converter, they will or won't
> use the heading data in tracking the nav signal. Generally, they will
> not use the heading data in approach mode (because the CDI is presumed
> to be sufficiently sensitive, and the response sufficiently quick) but
> WILL use it for enroute nav.
>
> OK, now how do you figure out which kind you have? Easy.
>
> Get the plane on course and on heading, set the heading bug to the
> heading you are flying, make sure the CDI is centered, engage
> autopilot in nav track mode. Once you are satisfied it is tracking,
> give the heading bug a big ol' twist - say 45 degrees. If the plane
> tries to follow the bug initially, you know the heading data is being
> used in that mode. If it doesn't twitch, it's not.
>
> Michael
>
> Please do not send email replies to this posting. They are checked
> only sporadically, and are filtered heavily by Hotmail. If you need
> to email me, the correct address is crw69dog and the domain name is
> this old airplane dot com, but remove the numbers and format the
> address in the usual way.