S-Tec 20 Non-Assertiveness

[iamtheari]

My plane has an S-Tec 20 single-axis autopilot. Its flaw is that it is not very assertive, so if I hit a bump and a wing is low, it will correct to level flight very slowly, leading to being a few degrees off course. Then it will very slowly slalom and never get back to the magenta line.

Is this a common complaint? Is it an adjustment that was just not done right a few owners back when the autopilot was installed? I don’t want to take the plane to an avionics shop if there isn’t a good chance of real improvement.

 

[chemgeek]

Are you in wing level mode or tracking mode? If in wing leveling mode you should not expect it to stay on track, especially in turbulence. In tracking mode, it may get knocked off course temporarily in turbulence, but will correct back to on course in time. That's what mine does. There are two tracking modes, one designed for VOR which is more damped to prevent wandering, and one for GPS which is more responsive. If turbulence knocks you more than 10 degrees off course, it may be necessary to put it in leveling mode and turn to re-intercept the course to prevent scalloping. Rate based autopilots take a while to respond to the bumps, so the ride can be pretty wiggly. I can usually hand fly a bit smoother in turbulence.

 

[Kenny Phillips]

Clutch slippage?

 

[iamtheari]

Are you in wing level mode or tracking mode? If in wing leveling mode you should not expect it to stay on track, especially in turbulence. In tracking mode, it may get knocked off course temporarily in turbulence, but will correct back to on course in time. That's what mine does. There are two tracking modes, one designed for VOR which is more damped to prevent wandering, and one for GPS which is more responsive. If turbulence knocks you more than 10 degrees off course, it may be necessary to put it in leveling mode and turn to re-intercept the course to prevent scalloping. Rate based autopilots take a while to respond to the bumps, so the ride can be pretty wiggly. I can usually hand fly a bit smoother in turbulence.

This is in tracking mode. Even the higher-sensitivity mode intended for approaches seems to be lacking in assertiveness. I've found that it's better to hand-fly if there is any degree of turbulence.

Clutch slippage?

Based on how well it does eventually turn the plane, I don't think it's slippage. But I'll make a note of that possibility. My annual is in the spring and that would be a better time to have someone look at it.

 

[chemgeek]

In my plane, it's extremely difficult to overcome the clutch with the autopilot engaged. I find the "VOR" mode pretty sloppy, but it's designed to deal with a wiggly VOR needle. When tracking via GPS I always set the STEC-20 to precision mode. The unit does require you to be within +/- 10 degrees to intercept a course without doing S-curves. Mine will track in light turbulence but it wanders quite a bit. If I can't stand the wandering I hand fly for a bit or put it in heading mode and issue corrections as needed. But it will definitely head back in the right direction if it diverts a dot or two from the assigned track.