Incident Overview
A certificated flight instructor with 1,911 hours of flight experience reported that the propeller separated from the engine while the aircraft was in cruise flight. Moments before the separation, the instructor observed a light oil spray covering the windscreen. He promptly took control of the airplane and executed a forced landing to a nearby airport. The landing was completed without further incident.
Aircraft Owner's Statement
The aircraft owner stated that he was not aware of any anomalies that could have led to the propeller assembly failure. He noted that there were no pre-existing vibrations, no propeller changes, and to his knowledge, no prior indications or incursions related to the occurrence.
Examination Findings
An examination of the engine revealed that the crankshaft had separated aft of the propeller flange. The fractured area was subjected to a metallurgical examination.
Metallurgical Analysis
Metallurgical examination of the fractured crankshaft area determined that the fracture was consistent with fatigue within the nitrided layer of the crankshaft. The fatigue crack originated from a small circumferential crack that was consistent with overload. The crack then progressed to the identification stamp. The origin of the initial circumferential crack was not determined.