Trutrak auto pilot pitch problem

has any one had the pitch servo shear pin break and know why it happen screw was tight on the torque enhancer wheel ???

We had a pin shear on Louise's RV-6 a year or so back. Never did figure out why - it just went. Very easy to replace (once youtake the access panels out to get to the servo.....)

Paul

glenn72pc said:

has any one had the pitch servo shear pin break and know why it happen screw was tight on the torque enhancer wheel ???

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My recollection of a recent phone call to TruTrak on this subject: the shear pin will break at 75 in-lb static load (+/- 2 in lb). Less torque is required to shear the pin under an impulse load. The servos are supposed to be capable of only about 45 in-lb. So a sheared pin indicates an impulse load was applied.

Apparently I inadvertently applied an impulse (jerk) to my roll servo in ground testing -- the pin sheared. I backed off the controller's torque value after that.

I had TruTrak send me a few spare shear pins.

I've seen a total of 2 sheer pins break in all of my experience with auto pilot servos. One was a pitch servo and one was a roll, and in different planes. I wouldn't be surprised if it was just a fluke occurrence, maybe the screw was over-tightened or something like that. Either way, it's an easy fix, and I'm personally glad they had the sheer screw, because if it wasn't there and there was a more serious problem (servo locking up or something like that), it'd be the end, very likely, for pilot and passengers.