Autopilot doesn't maintain altitude

[ploucandco]

Hello, I have the following setup in my Europa monowheel:
- D10A EFIS
- D10A HSI (DSAB master)
- AP74
- HS34 connected to GNS480 and AVMAP IV
- SV32 pitch
- SV32 roll
All running v5.6

Very good setup. Pretty happy. I have the following autopilot parameters that allow to maintain +-50ft altitude in all conditions.

I am facing one strange issue: from time to time (happens every 10-20 flight hours), it seems that the the pitch servo is not commanding (even if it is announced as commanding) the plane that loose altitude. When that happens, I disconnect and reconnect and everything is back OK.

That started to happen when I modified the pitch parameters. In the beginning, I had huge oscillations with the smallest turbulence. Pitch was really not stable. Then, I really spent time to find an optimal set of parameters. But now I get this issue of pitch servo not controlling the aircraft from time to time. If I remember correctly, that started to happen when I modified the pitch and alt gain parameters. But that could be a coincidence.

What could that be?

 

[vlittle]

I'd check your pitch servo shear screw.
Vern

 

[Dynon]

Ditto the shear screw advice.

Also, with your minimum speed set to 75 knots, the AP will push the nose down to maintain that min speed if you're getting that slow. So make sure that's not the case either.

 

[William_Ince]

Ditto the shear screw advice

Speaking of shear screws . . . is there any news from Dynon engineering on the development on a beefed up shear screw?
We last heard they were seriously considering it.
It can't come soon enough.

 

[Dynon]

Nothing today, but we'll have more to share shortly!

 

[William_Ince]

Nothing today, but we'll have more to share shortly!

Great.
Thank you.

 

[Dynon]

An update here: Our updated shear screw design is shipping now, but we haven't made any big announcements yet. If you've had past issues with shear screws breaking (particularly more than once) do fill out a request at http://www.dynonavionics.com/shearscrew. If you haven't had issues with broken shear screws and yours isn't broken now, there is no need to replace yours.