Sheared Propeller Bolts
in General Tech Discussion
10 years ago
Yesterday, shortly after takeoff, the 912UL in my Pipistrel Sinus began to run roughly. Half way through my turn back to the runway, the glider began to shake similarly to a stall. I pushed the nose down and rolled out on final then, at idle RPM, there was a "bang" that came from the front. I immediately turned the ignition switches and key off, closed the fuel valves, and pulled the feather knob. The propeller continued to windmill.
After rollout, I pushed the glider off the runway and inspected the prop. All four bolts had sheared off behind the gearbox flange. The prop was retained only by the feathering rod and lever! I was surprised to see that there is no mechanical "key" to lock the prop to the gearbox flange and that all of the engine torque is reacted through shear loads on the four bolts.
I had installed new nuts and bolts and torqued them to the value specified in the Heavy Maintenance Manual and I'm mystified as to why they sheared. Is it possible to install the propeller in the wrong "clock" position? I did not find guidance on the angle of the propeller with respect to the prop flange.
A final question: would it be OK to use higher strength bolts to reinstall the propeller (after inspection/repair)?
After rollout, I pushed the glider off the runway and inspected the prop. All four bolts had sheared off behind the gearbox flange. The prop was retained only by the feathering rod and lever! I was surprised to see that there is no mechanical "key" to lock the prop to the gearbox flange and that all of the engine torque is reacted through shear loads on the four bolts.
I had installed new nuts and bolts and torqued them to the value specified in the Heavy Maintenance Manual and I'm mystified as to why they sheared. Is it possible to install the propeller in the wrong "clock" position? I did not find guidance on the angle of the propeller with respect to the prop flange.
A final question: would it be OK to use higher strength bolts to reinstall the propeller (after inspection/repair)?
