Incident Overview
During the takeoff ground roll, shortly after the pilot applied full power, one propeller blade failed and separated from the hub assembly. The engine subsequently broke free of its engine mounts.
Metallurgical Findings
Post-accident metallurgical examination of the propeller blade and hub assembly revealed that the failure was due to fatigue. The point of origin of the fatigue crack was identified as the first thread of the threaded hub.
Propeller Maintenance History
The propeller had been installed on July 11, 1975, and had accumulated 1,227.57 hours of operation at the time of the incident. It had not been overhauled. The manufacturer recommends propeller overhaul at intervals not to exceed five calendar years.
Subsequent Airworthiness Directive
An airworthiness directive, effective 33 days after the accident, requires a penetrant inspection of the propeller blade threaded retention area and a modification of the propeller assembly to enable early detection of failure.