Both shear screws on servos broken

[Janekom]

RV10 with dual SV, SV42 with capstan for pitch and SV52 for roll. The owner asked me to look at his AP not working in Pitch. When I checked it out, I realised that both the shear screws are broken.

I would really like to hear from someone who has experienced more or less the same. how is it possible. Mike at Dynon says the torque being set at 90% cannot do it.

I really want to learn from this and understand what could have caused this.

 

[Dynon]

A broken shear screw is definitely not routine, but we do design them to be conservative. Did the autopilot work and then not one day? Was there flight into moderate or severe turbulence before the shear screws broke?

 

[Janekom]

I am not the pilot but he reported that it did work one day and then he noticed pitch stopped working. He cannot remember going thorugh severe turbulence. Maybe moderate.

I have noticed some play in roll and on investigating found the roll one broken as well. It was still working sort of as there as some friction.

 

[jc2da]

I had my SV42 roll servo fail mid-flight while servo was on and tracking. It was a smooth flight and no explanation for why the shear screw would fail on this flight. One minute it was tracking, the next minute it was not.

The scary part was there was no indication of the failure. The plane started to gradually roll and bank to one side. i figured i must have accidentally bumped the AP mode or something. i disconnected AP, levelled wings, and re-engaged AP again. Again same story until i realized it must be a failure of some sort, even though there was no indication of a failure. It took me a few minutes to realize what was going on.

 

[plea1949]

RV10 with dual SV, SV42 with capstan for pitch and SV52 for roll. The owner asked me to look at his AP not working in Pitch. When I checked it out, I realised that both the shear screws are broken.

I would really like to hear from someone who has experienced more or less the same. how is it possible. Mike at Dynon says the torque being set at 90% cannot do it.

I really want to learn from this and understand what could have caused this.

Jan I just had the same experience this week with the same setup as what you have mentioned. The pilot unknowingly engaged the CWS button on the ground and when "testing" the controls against the servo's sheared BOTH shearpins in the process. The pilot although current on the RV10 panicked when he thought that the controls were jamming!!!

 

[rv4brown]

I had a shear pin break on the roll on my RV-4.

I might visually check the servo installation for alignment with the torque arms and the limits of servo travel. This may be hard to visualize based on my description. It is possible to install such that the limit of the servo arc is reached before hitting the control stops. If this is the case, the servo will be flexing. I would lay eyes on the servo while moving the controls from stop to stop and make sure the servo travel arc is adequate for the control arm travel.
I might also take a look at how the torque arms are aligned relative to the servo. It's possible to apply the torque force at non parallel angles. Also hard to describe but the force applyied by the control system on the servo torque arm inceases as the servo arm approaches the limit of the arc travel. That is the force moves closer to the rotational centerline.
Your milleage may vary.