Incident Overview
A commercial pilot was operating an airplane in cruise flight at an altitude of 1,500 feet mean sea level (MSL) when the engine experienced an overspeed condition. The pilot reduced the engine RPM and subsequently executed a forced water landing. The airplane was recovered for post-accident inspection.
Post-Accident Findings
Inspection of the airplane revealed that the propeller had separated from the crankshaft flange. Three propeller bolts were recovered and sent to the NTSB Materials Laboratory in Washington, D.C. for metallurgical examination.
Metallurgical Examination Results
Examination of the propeller bolts determined that they fractured in a mode consistent with high vibratory tensile stress. The bolts failed due to insufficient torque on the bolts, which led to the observed fracture pattern.
Conclusion
The forced water landing resulted from an engine overspeed condition, which was itself a consequence of the propeller separation. The separation was caused by the failure of propeller bolts due to insufficient torque, resulting in high vibratory tensile stress.