What happened
Shortly after liftoff, one of the propeller blades detached from the hub. The separation occurred near the outboard end of the associated ferrule fitting. This event induced a catastrophic failure of the crankshaft. Unable to maintain flight, the pilot executed a forced landing. During the landing roll, the aircraft overran the end of the runway and subsequently nosed over.
The investigation
A metallurgical analysis was conducted on the failed components to determine the root cause of the mechanical failure. The examination indicated that the failure originated from a fatigue fracture characterized by multiple origin points. These fractures were caused by a scratch in the material.
Further analysis revealed discrepancies in the maintenance history of the propeller. All threads except the first three had been reworked using a post-manufacture hand grinding process. This procedure was performed in accordance with a McCauley service bulletin, which specified a hand grinding rework to remove sharp threads from the ferrule. Previous data had shown that such sharp threads could produce prior fatigue fractures.
In the specific area of the fatigue fracture, investigators found that the first three threads of the ferrule retained sharp edges. This suggested that the required rework was not fully applied to these initial threads. Additionally, maintenance records did not show the total time on this propeller since its major overhaul.