PDA

View Full Version : Lesson learned on shear screws


Carl Froehlich
06-26-2016, 03:11 PM
My Dynon SkyView autopilot has been flawless for 350 hours. On a hop from south Texas to the Phoenix area we hit a hard pocket - enough so that I hit my head on the cabin top.

Long story short the autopilot altitude hold stopped working. It took some time to figure this out as once the RV-10 is in trim it tends to hold altitude - and after the turbulence I was hand flying. Looking back I suspect I jerked the stick hard enough when we hit the pocket that I broke the shear screw. In other words the shear screw functioned as designed.

So - don't assume the nice smooth ride will stay smooth as you cross the western mountain range and keep the belts tight.

Dynon sent me a replacement screw and start to finish it took 60 minutes to remove the servo, replace the screw and reinstall the servo. Here is the link to watch how it is done: https://www.youtube.com/watch?v=4wFEY3kT97g&list=PLjU8CSiBHFbYBa4F1eagGA2q9l3_S4u-F

Carl

aerhed
06-26-2016, 05:41 PM
Shouldn't the servo disengage before the screw shears?

Carl Froehlich
06-26-2016, 06:38 PM
Shouldn't the servo disengage before the screw shears?

Depends on the circumstances. The shear screw is there to make sure you can always overpower the servo for control - for example there is a mechanical jam. In this case the servo was doing just fine, and I did the control stick jerk on the turbulenc bump.

Carl

PerfTech
07-06-2016, 12:33 PM
Depends on the circumstances. The shear screw is there to make sure you can always overpower the servo for control - for example there is a mechanical jam. In this case the servo was doing just fine, and I did the control stick jerk on the turbulenc bump.

Carl

...This same thing happened to us! We found ourselves suddenly in very violent turbulence at a speed far above the appropriate (200mph). The first hit slammed both of us into the canopy with enough force to brake it. After regaining control and landing to address the obvious problems created by this encounter we returned to home for repairs. After complete inspection etc. in the next few flights I became aware of the autopilot (tru-trac) having trouble with aileron control. I went in and tried a few times to adjust this out but it always seemed to come back but not consistent. While on route to Independence Or. for the homecoming, in bad weather, on top where I really appreciated having an auto-pilot, it signed off. Upon our return and further investigation I found the sheer pin on the aileron servo had failed in our encounter with the violent turbulence a few weeks prior. We replaced the pin, returned to our previous auto-pilot settings and all is well. The failure doesn't always show up immediately. Should the pin fracture at an angle the outcome can be intermittent, erratic auto-pilot operation. I hope this may help someone else that is seeing this kind of issue. Thanks, Allan...:D