What happened
While pulling up out of a field and initiating a turn, the pilot heard a loud bang and experienced an abrupt reduction in engine power. During the ensuing emergency landing, the aircraft nosed over.
Following the event, the pilot reported that the entire propeller assembly was missing from the engine, having broken off at the crankshaft flange. A single propeller blade was recovered more than one month after the accident.
The investigation
An examination of the recovered propeller blade revealed that it had broken off at the blade root. Prior to the accident, the pilot had noted that the propeller had been leaking oil. To address this, the propeller had been sent to a maintenance shop for an inspection regarding leaks and cracks. The shop's examination found no cracks, and the seals were replaced before the propeller was returned to service. The pilot had flown only seven hours with the propeller after its service.
A metallurgical laboratory analysis determined that the failure was caused by a transverse fracture in the threads located at the base of the blade.