A new feature here at Smart Aviator is the Avionics Troubleshooter—a place to noodle avionics gremlins and a way to better prepare for a visit to the shop by doing some of your own simple troubleshooting. This might ultimately cut down on shop time and with any luck, send the techs on the right path of troubleshooting to keep your invoice in check. We’re often our own worst enemies by not describing the problem correctly or not doing some basic troubleshooting of our own before handing the aircraft over to the shop.
This latest issue comes from the owner of a new-to-him Cessna with an S-TEC autopilot that wants to parallel the commanded course and heading. It could be a straightforward if not tedious adjustment.
Parallel Track
“I recently bought an older Skyhawk that has an S-TEC 30 dual-axis autopilot. On the long trip home after I closed the deal, I found that the autopilot would consistently track the GPS (it has a Garmin GNS 430W GPS and a King KX155 radio) left of the course. The CDI shows a one-dot course deflection to the right and the moving map display on the Garmin checks—showing the airplane paralleling the course. When I select the heading mode on the autopilot, it also flies left of the bugged heading. The owner bragged that this airplane had GPSS digital steering and it flies a lateral course like it’s on rails. But even in GPSS mode (with an S-TEC GPSS unit), it flies the lateral segment of an LPV approach left of the final. What’s going on with this system?”
A Potential Fix
Based on your description of seeing the same steering error in all lateral modes, it sounds like this system might benefit from a roll centering adjustment—not an uncommon adjustment on aging and even newer systems. Remember, these are analog autopilots. The roll centering adjustment may or may not want to be something you attempt.
The in-flight adjustment for this system is done via a three-turn adjustment potentiometer located under the lower right-hand panel attaching screw on the turn coordinator. Find a screw stick that fits the notch on the pot before you go flying. If you have the plastic panel overlay, it will have to be removed to access the pot. Bring a safety pilot along and fly the airplane to smooth air, trimming it for level flight and establishing it on a desired course for tracking (a GPS waypoint or a VOR radial can work) and once it’s established on the course, engage the autopilot in Lo Track mode and make fine adjustment turns with the potentiometer using very gradual increments. You can center the heading bug on the DG or EHSI and adjust for it, too. Regardless, allow for sufficient time between adjustments for the system to stabilize.
Note that clockwise rotation of the pot induces a right turn and counterclockwise turns left. The spec in the S-TEC manual calls for a +/- 3° centered value. This is easier to see with an EHSI than on a vacuum-driven mechanical gyro, but with either you will need exceptionally smooth air to be successful. If this adjustment doesn’t make it fly true, there could be an issue with the roll servo motor or even a problem in the flight computer. But this roll centering calibration is the first step to try. Don’t forget to reinstall the attachment screw before putting the false panel back on. This general concept works for nearly all S-TEC rate-based autopilots, so get the manual for your specific system or ask your shop to help.
Got a problem that needs some noodling? Drop us a note and we’ll see if we can help.
Proud of you Larry for taking your talent and dignity away from a once great publication turned toxic and refused to be chattel despite still having to make a living.
Larry, I have a 1976 Cardinal RG with an original-issue ARC-300A autopilot. Once I ditched my mechanical DG and AI for an Aspen unit, I found that the included digital-analog computer suddenly made the heading bug on the autopilot start working again.
However, as with this 172, the 300A tracks 1/2 to 1 dot left of centerline. But in GPSS mode it will track a course and make the correct turns, albeit a bit sluggishly.
And you know what, I figure that for a nearly 50-year-old autopilot that was a piece of junk when it rolled out of the factory, that’s CLOSE ENOUGH FOR GOVERNMENT WORK! 😉
Thanks for the article, Larry.
I looked around the website, and don’t see an email to contact you or Russ. Is the only way at present to contact you by posting a comment?
I have a similar panel in my Bonanza, 430W and KX-155, both overhauled just before Garmin ran out of parts, Garmin transponder, KN62 with new display, GDL88 with FlightStream 210, Aspen EFD1000 Pro Max, S-Tec 60-2 with yaw damp. My autopilot a couple years ago started tracking parallel to the desired course. Doing the roll centering adjustment didn’t help. I don’t have the sequence of things done written out, but the S-Tec turn coordinator got overhauled, the programmer sent in, the roll computer sent in, and it still didn’t track correctly. Ended up being a broken wire,, and given the onset of problems, was probably broken by a different shop when they installed the LoneStar 12 to 24 V converter that sits right by the Sted programmer and Aspen. That was a costly and lengthy repair.
BTW, as you may know, AvWeb posted one of your old Aviation Consumer articles at the bottom of today’s AvWeb, “ LSA Step-Downs: Training Is A Must” and at the bottom says “ Larry Anglisano is a regular AVweb contributor and the Editor in Chief of sister publication Aviation Consumer magazine. He’s an active land, sea and glider pilot, and has over 30 years experience as an avionics tech.”.
If my article and video reposted by AVweb saves someone from breaking an airplane (or worse) by getting some decent training, it’s all good by me.
Larry- Glad to see you at this site. Can you give a recommendation for an avionics repair shop to troubleshoot a DFC 90 autopilot? I am in the north central FL area and the shop I am using is perplexed by the issue.