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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"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.

"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.

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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"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

"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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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

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.