STEC-30 Autopilot HDG mode off 10 degrees

[Radpilot]

I have an issue with my STEC-30 AP on which I was hoping someone might have some insight.

Issue:
Works fine in NAV, GPSS, and steer mode. Follows the GPS magenta in lo and high threshold modes appropriately.

On HDG mode, however, about 6 months ago, it began flying 10 degrees to the LEFT of the heading bug, regardless of the direction of flight. I recently tested this on NW and return SE legs in low wind/turbulence conditions and it was 10 degrees off for both legs. DG is Sigma-Tek.

History:
2 years ago I had the DG overhauled by mid-continent and reinstalled. I had just purchased the plane at that time and the DG was already failing and the HDG function didn't work at all with the AP.

After overhaul of the DG and a service of the AP, all functions worked (although on the initial flight on all modes there was a very small oscillation/correction of the ailerons which went away on the next flight and has not returned). Over a year later, this HDG issue arose. I asked the avionics person who installed the DG whether it could just be a calibration issue with the heading bug spinner. Paraphrasing (I don't remember his exact words), he said that the DG has no easily accessible calibration and that the problem was the DG, not the AP. He didn't seem like he wanted to deal with it. Since I had the DG overhauled only 100 hours ago, I am not excited about the prospect of removing the DG and facing unknown trouble shooting and/or overhaul costs just for the hassle of having to point the heading bug 10 degrees to the right of my intended heading. But, it is annoying. If someone has dealt with this exact issue and knows if it is easily repairable/corrected (and who they know that can fix it), please let me know.

 

[AggieMike88]

We have had a similar issue in the club Skylane and has been there ever since I have joined. Both the gyro instruments have been overhauled in the past year or two. But to use HDG mode correctly, you set the bug so the left edge of the bug is on the desired course.

So far, our avionics tech hasn’t developed a long term solution. So we just live with what we got and fly on.

We do have plans on a major upgrade in the next 24 months. Likely whatever we choose will eliminate the bug variance.

 

[Ravioli]

I think your correction is the left edge of desired HEADING, n'est pas?

 

[AggieMike88]

whatever.... I just set it so the airplane does what I want, and cross check against the compass

 

[denverpilot]

What does the manual for the AP day the signal is from the DG?

If it’s a voltage based system, something adding resistance external to the DG could cause an “offset” like that.

Why not just have someone measure it? The signals on most of these ancient APs aren’t digital or difficult to measure, they’re usually an offset from a “Center voltage” and can be checked at the output of the DG with a multimeter in five minutes.

If the DG is outputting the correct voltages then the downstream hunt begins. Check at the other end of the wire going to the brain box.

But once inside the brain box, you need a good description of what it does, schematic is helpful, and a shop that knows how to work on that model or has solid documentation to follow for alignment.

The good news is, if the AP is doing everything else correctly, hunting one input having a problem should be workable for anyone with a decent sense of electronics. For most of these old systems there’s nothing that’s “rocket science” about them. It’s usually something as simple as voltage goes up, heading bug is right of course, voltage goes down it’s left of course.

Specific to the AP, obviously, and some send some oddball signals, but they’re usually much more expensive AP systems, and that particular problem shouldn’t be that hard to chase.