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Autopilot Pitch Not Behaving (Read 1782 times)
Jun 14th, 2017 at 11:17am

Ricksol   Offline
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I love flying!

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I have a new Skyview HDX system in a new RV-10.  For 6 weeks now, several things are occurring and seem to be getting worse:
- on glide slope, the a/p will decide to go into altitude hold mode, or to begin a climb with no mode annunciation.
- in cruise and altitude hold, the a/p will randomly decide to climb.
- when commanding a climb or descent with VS, either the a/p will respond correctly while the flight director shows an incorrect suggestion, or the flight director will show the proper guidance while the autopilot ignores it.

Any thoughts??  Dynon has been given several diagnostic files, but to date has made no suggestions on how to fix the problem.

Thanks in advance!
 
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Reply #1 - Jun 14th, 2017 at 4:49pm

Dynon Avionics   Offline
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Dynon Technical Support

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The first thing that I'd check is that the pitch servo shear screw hasn't been broken. One way to check that - engage the AP on the ground. When you overpower the servo, you should see the "slip" annunciation on the display. If you're able to move the stick appreciably and not see the the slip annunciation, that may indicate that the servo is not actually driving the control surface.

A couple other thoughts:
-On the glide slope issues.
-The cruise/altitude hold issue is also consistent with a shear screw or other slop in the control system that is inhibiting the servo's ability to control the axis
-The AP is always trying to get to the flight director bars when it's engaged. At a deep level, it's not possible for the autopilot to ignore the flight director bars.

All of the above could be caused by either a broken shear screw, by a servo that is slipping (do you see the slip annunciation when you're not seeing the performance that you deserve?), or a servo that isn't able to successfully drive the axis for another reason. Among those could be a torque setting that is too low (you'd see slip annunications in this case too), slop in the linkage between the control surface and servo, or perhaps a defective servo. If you have your AP settings set drastically wrong, lag could possibly look like this too. Did you use the AP tuning guide to try to dial the performance in for your aircraft?
« Last Edit: Jun 14th, 2017 at 4:49pm by Dynon Avionics »  

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Reply #2 - Jun 21st, 2017 at 8:37am

Ricksol   Offline
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It was the shear screw!  Replaced it and all is well.
 
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Reply #3 - Aug 9th, 2018 at 10:18am

Warren   Offline
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I love flying!
Minneapolis

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After 200+ hours of trouble free operation of my SkyView autopilot, I discovered a broken pitch servo shear screw this week after a trip from Minneapolis to Wash and Oregon.

The symptoms I noted on this trip included
     Intermittent/frequent autopilot disconnects
     Failure to hold altitude, though not all the time
     Occasional pitch oscillation observable in smooth air

Even with the broken shear screw, alt hold worked much of the time; apparently there was enough friction between the servo arm and the servo shaft collar to move the servo arm

I replaced the shear screw, and all symptoms disappeared.


Warren
Plymouth, MN
RV-7
N671CW
260 hrs
 
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