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