Accident Sequence
During the initial climb, the propeller separated from the right engine. Witnesses reported that the engine cowling was torn. The aircraft then entered a right turn and a dive, impacting the ground in a near inverted attitude. The pilot, the sole occupant, was killed.
Wreckage Examination
Examination of the wreckage revealed that the propeller hub fracture resulted in one of the three propeller blades detaching from the hub. The remainder of the propeller hub then separated and struck the right front of the fuselage. Oil was spread across the aircraft nose and windshield. Damage to the right side of the fuselage increased aerodynamic drag.
Metallurgical Findings
Metallurgical examination of the failed propeller hub showed metal fatigue emanating from the threaded hole for the grease fitting. The threads had been deformed by shot peening, which resulted in increased stress concentrations at the threads.
Probable Cause
The official probable cause was determined to be: Failure of the right propeller hub due to metal fatigue which resulted in catastrophic separation of the propeller. Contributing to the accident was damage done to the aircraft airframe in flight by the separating propeller making the aircraft uncontrollable.