Detuning Autopilot?

A number of people have complained of errors in their autopilot, with various resolutions.
I wonder if we could benefit by ‘detuning’ or biasing our autopilots slightly. There was just a thread on AvSig discussing the virtues of flying airways shifted 1 mile right of centerline. There have been proposals to have the FAA add this feature to FMS’s. The problem is the high accuracy of GPS puts everybody right on the centerline, so if someone is at the wrong altitude, splatto!

Of course the FAR’s require pilots to fly the centerline of airways under IFR, but still, leaving some margin of bias seems sensible. That goes for cruise, but in the approach you want to be right down the middle since tolerances are tighter.

So what exactly does that little adjustment screw in the front of the 55X do?


Has anyone else noticed that your A/P tend to fly just to the right (up to 1/2 mile) of the center of a direct to course line?

As a matter of fact, I am sure you can ‘detune’ your A/P to track to the left or right of centerline because I just got mine adjusted to stop doing that. It took about 35-45 minutes in a smooth, higher altitude cruise to get it just right with a homemade adjustment tool given to us by our service center. It takes a while to turn the screw, then wait, turn, wait, turn , wait, etc. The reason we did not like it off centerline is because it affects ALL course tracking, including ILS and VOR approaches. I see your point about other traffic, but you only need to worry about it on airways. I am always fearful I will come zinging up on some 'ole Bonanza or Mooney plodding along 10-20kts slower, and heaven forbid if a 172 is trying to float over to another airport in MY airway ;-). If you are GPS direct, you will probably have no problem. If you wanted to do this on an airway, you could set up your A/P on the airway as usual using the NAV mode on the 55X and make a note of the wind correction angle. Then, switch to heading mode, steer over a mile or so and put your original heading worked out by the A/P back in. You can monitor your distance off centerline extremely precisely with the CDI on the NAV page of the Garmin. It’s not as slick as using the NAV mode, but it will get the job done. Of course, there is always the flight following option for assistance.

Yep, that’s what the centering screw on the front of the S-Tec will fix.

Now my problem is that the a/p flies right down the middle in NAV or GPSS mode but flies five degrees left of the bug in HDG mode (preferable.) Couldn’t find anything in the Sandel setup pages to adjust this (it is just software after all) but at least it’s consistent, I’m used to it, and I’d rather have it pop out of the clouds with the runway directly in front of me…

mine flies 3 miles to the right!


With the 3mile bias in cruise, how does the A/P behave in approach mode?



Yah, you’ve described exactly what I do now. I try to stay out of the center of airways especially around the VOR’s using HDG mode. And the last thing I want is anything that puts me off course in APP mode. So, I’ll be stuck with manual adjustments, until aviation starts believing in Open Source[:)].