Autopilot and coupled LPV approach

F

flyindoorman

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Joined
Jan 10, 2023
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I have a new to me RV14 with dual Dynon Touch installed along with the Dynon autopilot and panel control panel.

I had a Garmin 175 WASS GPS installed last week. When I fly the coupled approach, it starts out great brings the plane down and centered up. The last mile it starts to drift to the right hand side about one dot by missed approach DH.

Can anyone tell me if that is an autopilot adjustment?
 
flyindoorman, I have the same setup with the 175. Is your auto-trim continuing to work AFTER your intercept the glide slope? I flew two LPV approaches this afternoon. It trims fine until it intercepts. It stops auto-trim and ultimately has servo slip once it can’t handle the load. Odd issue.
 
I think that's the same thing I'm having. Servo slip message today when shooting a couple of approaches
 
Hey now, that's interesting - I hadn't noticed that before, I'll have to go fly some approaches to verify.

Dynon - care to comment? Is auto-trim disabled upon VNAV capture?
 
I shot another coupled approach this afternoon, same result. Ultimately had to disengage the AP because it was bucking so much. It needed trim up, but for whatever reason, the AP wasn’t commanding the trim to actuate. After I disengaged, I actuated the trim manually to confirm that I hadn’t run out of trim, but there was plenty trim remaining. I’m stumped by this one.
 
Speeddog8 said:
I shot another coupled approach this afternoon, same result. Ultimately had to disengage the AP because it was bucking so much. It needed trim up, but for whatever reason, the AP wasn’t commanding the trim to actuate. After I disengaged, I actuated the trim manually to confirm that I hadn’t run out of trim, but there was plenty trim remaining. I’m stumped by this one.
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Why not just trim manually with the AP engaged, rather than disengage it?
 
Auto-trim is enabled anytime the a/p is engaged, this includes approaches, vnav, etc. This sounds more like the amount of trim needed exceeds auto-trim's ability to keep up. The first place I would expect to find this problem would be with the speed / trim speed settings. If the trim speed is too slow to make a large trim adjustments during power changes, descents, etc. the servo will slip and trim will stop until the servo slip is eliminated. This is due to the lack of force being reported by the servo during a slip event and without that force reading, the system has no idea what is needed in regards to trim.
 
djones said:
Auto-trim is enabled anytime the a/p is engaged, this includes approaches, vnav, etc. This sounds more like the amount of trim needed exceeds auto-trim's ability to keep up. The first place I would expect to find this problem would be with the speed / trim speed settings. If the trim speed is too slow to make a large trim adjustments during power changes, descents, etc. the servo will slip and trim will stop until the servo slip is eliminated. This is due to the lack of force being reported by the servo during a slip event and without that force reading, the system has no idea what is needed in regards to trim.
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Thanks Don. I have trim speed currently set to 100% below 135 kts. I plan to increase trim motor pulse rate to see if that helps the issue.
 
This issue still isn’t resolved. Once I intercept the glideslope/vertical guidance, the aircraft starts pitch bucking. I’m not sure what to make of it.
 
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