What happened
During an instructional flight, the pilot experienced vibration in the right engine and subsequently shut it down, feathering the propeller. The crew continued the flight to the intended destination without further incident. Witnesses on the ground observed the right propeller still rotating while the aircraft was on final approach, despite the crew's attempt to feather it.
After landing, the airplane owner, who held an A&P certificate, began troubleshooting the vibration issue. During the inspection process with the engine running at 2000 RPM, one blade from the right propeller separated. The detached blade penetrated a hangar structure and struck the windshield of a parked aircraft.
The investigation
Examination of the failed propeller revealed a series of fatigue cracks on the hub, originating at or near the hub surface near the hub spider. Records indicated that the propeller had been exposed to a ground strike 217 hours prior to the failure. Despite this damage, the propeller had been returned to service following an inspection and overhaul.
Findings
The separation of the propeller blade was caused by fatigue cracks in the hub. These cracks originated from a previous ground strike that occurred 217 hours before the incident. The propeller was deemed airworthy after the prior inspection, but the latent damage eventually led to catastrophic failure during engine operation.
Safety message
Propellers subjected to ground strikes require rigorous inspection for internal or surface defects that may not be immediately visible. Fatigue cracks can initiate from such impacts and propagate over time, potentially leading to blade separation even after the component has been returned to service.