Autocontrol IIIB

[PaulaJay1]

I have the original Autocontrol IIIB in my 79 Archer. Lately it has been
flying one dot to the right of course when in GPS/NAV mode. I've set the CDI
scale to 1.0nm full scale on the GPS so the error is only about 0.2 nm. If I
set the heading but to the right a little, we fly right down the line. This
says to me that the autopilot is putting out a small bias to the left that the
GPS/NAV has to constantly correct. Is there a simple adjustment in the
autopilot to correct this? Maybe I should just vent to the newsgroup and leave
good enough alone!

Chuck

[Ben Jackson]

In article >,
PaulaJay1 > wrote:
>
>I have the original Autocontrol IIIB in my 79 Archer. Lately it has been
>flying one dot to the right of course when in GPS/NAV mode.

Wild guess: Is your rudder trim set right?

--
Ben Jackson
>
http://www.ben.com/

[[email protected]]

On 21-Oct-2003, (PaulaJay1) wrote:

> I have the original Autocontrol IIIB in my 79 Archer. Lately it has been
> flying one dot to the right of course when in GPS/NAV mode. I've set the
> CDI scale to 1.0nm full scale on the GPS so the error is only about 0.2
> nm.
> If I set the heading but to the right a little, we fly right down the
> line.
> This says to me that the autopilot is putting out a small bias to the left
> that
> the GPS/NAV has to constantly correct. Is there a simple adjustment in
> the
> autopilot to correct this? Maybe I should just vent to the newsgroup and
> leave good enough alone!


We have an Autocontrol IIIB in our '79 Arrow but it is not coupled to a GPS,
just CDIs for the two VOR/LOC receivers. My understanding of the
Autocontrol IIIB nav coupling is that it steers to capture and center the
CDI, using the heading bug on the DG as a reference for direction (and
correcting with a "crab angle" as necessary to intercept and hold course as
defined by the CDI). Assuming it works the same way coupled to a GPS-driven
CDI, it seems that what you are saying is that your Autocontrol IIIB wants
to track with the needle slightly off center. This I understand. It could
be due to a maladjustment in the CDI or in the Autocontrol IIIB. You can
probably determine which by seeing if the same thing happens when tracking
on a VOR radial and/or when coupled to a second CDI (if you have one).
Actually, from your post I infer that you have verified CDI accuracy against
aircraft position as shown on a moving map display. That would point to the
A/P as the culprit. But then you say that if you "set the heading but to
the right a little" you fly "right down the line". Do you mean that you are
slightly adjusting the DG's heading bug? This really shouldn't make a
difference, because to the autopilot this should just appear as a slight
change in the crosswind component.

One other thing: In our plane, the A/P mode switch has been a bit flaky in
that it stays in HDG (track heading bug) mode when we think we have switched
to NAV (track CDI) mode. It's easy to check when using a VOR by spinning
the OBS and seeing if the airplane rolls in response to the CDI needle
movement.

--
-Elliott Drucker

[James M. Knox]

(PaulaJay1) wrote in
:

> I have the original Autocontrol IIIB in my 79 Archer. Lately it has
> been flying one dot to the right of course when in GPS/NAV mode. I've
> set the CDI scale to 1.0nm full scale on the GPS so the error is only
> about 0.2 nm. ... Maybe I should just vent to the newsgroup and leave
> good enough alone!

That's all it's off! Heck, you may have one of the best units out there!
<G>

Seriously... All the IIIB does is sum the voltage from the CDI and the
"created" voltage from the sine wave off of the heading bug (and the AH).
In a cross-wind situation the autopilot WILL track off of the course line
by an amount that is proportional to the cross-wind component. The IIIB
operators manual (which may still be downloadable from Century) mentions
this, and simply suggests to do what you do - move the hdg bug a little off
to compensate.

The only IIIB adjustment for "centering" is probably not relevant. Set the
IIIB in HDG mode (no NAV tracking). Does it track with the DG heading bug
under the lubber line? If so, it's adjusted properly.

-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------------------------------------------

[Michael]

(PaulaJay1) wrote
> I have the original Autocontrol IIIB in my 79 Archer. Lately it has been
> flying one dot to the right of course when in GPS/NAV mode. I've set the CDI
> scale to 1.0nm full scale on the GPS so the error is only about 0.2 nm. If I
> set the heading but to the right a little, we fly right down the line. This
> says to me that the autopilot is putting out a small bias to the left that the
> GPS/NAV has to constantly correct. Is there a simple adjustment in the
> autopilot to correct this? Maybe I should just vent to the newsgroup and leave
> good enough alone!

If you have a burning desire to fix this, there is an adjustment on
your nav converter (the box that has the multiposition switch with
labels like Omni, Loc, Loc Rev, Hdg, and Nav). Personally, I wouldn't
mess with it.

Michael

[PaulaJay1]

In article >, "James M. Knox"
> writes:

>The only IIIB adjustment for "centering" is probably not relevant. Set the
>IIIB in HDG mode (no NAV tracking). Does it track with the DG heading bug
>under the lubber line? If so, it's adjusted properly.
>

Interesting. I've not looked at the heading bug tracking that closely. I'll
look next time and see if the heading but is on the lubber line or off to the
right a little.

Chuck

[PaulaJay1]

In article >, "James M. Knox"
> writes:

>. Maybe I should just vent to the newsgroup and leave
>> good enough alone!
>
>That's all it's off! Heck, you may have one of the best units out there!
><G>

Like I said, "Maybe I should leave good enough alone." :-)

[PaulaJay1]

In article >,
writes:

>It could
>be due to a maladjustment in the CDI or in the Autocontrol IIIB. You can
>probably determine which by seeing if the same thing happens when tracking
>on a VOR radial and/or when coupled to a second CDI (if you have one).
>Actually, from your post I infer that you have verified CDI accuracy against
>aircraft position as shown on a moving map display. That would point to the
>A/P as the culprit. But then you say that if you "set the heading but to
>the right a little" you fly "right down the line". Do you mean that you are
>slightly adjusting the DG's heading bug? This really shouldn't make a
>difference, because to the autopilot this should just appear as a slight
>change in the crosswind component.

I've pretty well determined that it is the autopilot. That is, the everything
acts the same in GPS or VOR and from either CDI. I was hoping that there was a
simple adj in the autopilot for "offset".

Chuck

[Jeff]

Are you in GPS or NAV mode?
GPS - the CDI would be reading the GPS course
NAV - it would be reading what you had your OBS set to and your NAV receiver set to
the freq of the navaid
what GPS/ NAV do you have, the garmin 430 and the NAV that you can get with it?

I know that when I am having my auto pilot track a VOR it may be off by a dot
sometimes, this is basically because the VOR is not that sensitive when your a ways
from it. But when I shoot a localizer, the AP tracks it perfect. Have you tried
tracking a localizer with your AP ?

Jeff


PaulaJay1 wrote:

> I have the original Autocontrol IIIB in my 79 Archer. Lately it has been
> flying one dot to the right of course when in GPS/NAV mode. I've set the CDI
> scale to 1.0nm full scale on the GPS so the error is only about 0.2 nm. If I
> set the heading but to the right a little, we fly right down the line. This
> says to me that the autopilot is putting out a small bias to the left that the
> GPS/NAV has to constantly correct. Is there a simple adjustment in the
> autopilot to correct this? Maybe I should just vent to the newsgroup and leave
> good enough alone!
>
> Chuck

[PaulaJay1]

In article >, Jeff > writes:

>Are you in GPS or NAV mode?
>GPS - the CDI would be reading the GPS course
>NAV - it would be reading what you had your OBS set to and your NAV receiver
>set to
>the freq of the navaid
>what GPS/ NAV do you have, the garmin 430 and the NAV that you can get with
>it?
>
>I know that when I am having my auto pilot track a VOR it may be off by a dot
>sometimes, this is basically because the VOR is not that sensitive when your
>a ways
>from it. But when I shoot a localizer, the AP tracks it perfect. Have you
>tried
>tracking a localizer with your AP ?

When I am flying a GPS course (with the garmin 430) straight and narrow, the
CDI is constantly about one dot to the left. Same for an ILS, though the
Garmin goes into approach mode and the full scale CDI is 0.2 nm. Thus the
"error" is only a couple hundred feet. I generally fly the last part of the
approach so, no problem at all.

I've decided to take the advice of several and leave it alone and say that it
is the nature of the beast. Thanks to all for the comments.

Chuck

[Jeff]

Both my old cherokee 180 and my current 1978 Turbo Arrow do the same as yours is
doing. I have to put the heading bug a couple of drgrees to the left of my
heading. I think thats just the way the heading bug is, when I turn the AP to NAV
or LOC, it reads my HSI and is right on the money.

Jeff

PaulaJay1 wrote:

> In article >, Jeff > writes:
>
> >Are you in GPS or NAV mode?
> >GPS - the CDI would be reading the GPS course
> >NAV - it would be reading what you had your OBS set to and your NAV receiver
> >set to
> >the freq of the navaid
> >what GPS/ NAV do you have, the garmin 430 and the NAV that you can get with
> >it?
> >
> >I know that when I am having my auto pilot track a VOR it may be off by a dot
> >sometimes, this is basically because the VOR is not that sensitive when your
> >a ways
> >from it. But when I shoot a localizer, the AP tracks it perfect. Have you
> >tried
> >tracking a localizer with your AP ?
>
> When I am flying a GPS course (with the garmin 430) straight and narrow, the
> CDI is constantly about one dot to the left. Same for an ILS, though the
> Garmin goes into approach mode and the full scale CDI is 0.2 nm. Thus the
> "error" is only a couple hundred feet. I generally fly the last part of the
> approach so, no problem at all.
>
> I've decided to take the advice of several and leave it alone and say that it
> is the nature of the beast. Thanks to all for the comments.
>
> Chuck

[James M. Knox]

(Michael) wrote in
om:

> (PaulaJay1) wrote
> If you have a burning desire to fix this, there is an adjustment on
> your nav converter (the box that has the multiposition switch with
> labels like Omni, Loc, Loc Rev, Hdg, and Nav). Personally, I wouldn't
> mess with it.

There's left and right intercept (R13 and R20), plus radio gain. I don't
think any of those would affect the situation he is seeing (since it is
consistent, not wandering around within a window). There are two more that
are only internally adjustable, mostly setting the hysterisis. But since
he doesn't have an alignment rig...

-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------------------------------------------

[CriticalMass]

"Michael" > wrote in message
om...
> If you have a burning desire to fix this, there is an adjustment on
> your nav converter (the box that has the multiposition switch with
> labels like Omni, Loc, Loc Rev, Hdg, and Nav). Personally, I wouldn't
> mess with it.

If this a/p is anything like the one I had in my prior PA-28, and my current
PA-24 before I installed the Stec system, the adjustments I remember are
behind the faceplate on the a/p head, not at the coupler.

There are two variants of this system, and there will be either 3 or 4
adjustment screws behind that faceplate, depending on whether it has the
push-button switches or the rocker type switches.

As I recall (and my recall is a little fuzzy), two of them are left and
right roll limit adjustments, one sets heading center, and I think the
fourth was for sensitivity of some kind. It's been a long time.

I used to be anal about getting them set right, and I kept a set of
jeweler's screwdrivers in the glove box to use in flight whenever I got
bored.

[James M. Knox]

"CriticalMass" > wrote in
:

>
> "Michael" > wrote in message
> om...
> If this a/p is anything like the one I had in my prior PA-28, and my
> current PA-24 before I installed the Stec system, the adjustments I
> remember are behind the faceplate on the a/p head, not at the coupler.

Both have adjustments. The NAV Coupler basically sits in series with
the DG (HDG) and, if in anything but HDG mode, combines the DG error
with the CDI error (from VOR or LOC receiver, or from GPS) into a single
signal that the AP "computer" sees as a DG heading bug offset.

> As I recall (and my recall is a little fuzzy), two of them are left
> and right roll limit adjustments, one sets heading center, and I think
> the fourth was for sensitivity of some kind. It's been a long time.

Actually, you remember pretty accurately, long time or not. That last
one is the hysterisis adjustment. It sets the "dead band" on the servo
motor (yes, in effect, making the unit more sensitive to error sooner).
Set it too low and the aircraft will wander either side of the course
line. Set it too high and you will get wing rock. In theory, there is
a correct setting in between the two. <G>

The "B" units have this fourth trimpot. The earlier units just used a
couple of diodes... no adjustment.

-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------------------------------------------