Incident Overview
A flight instructor and a student pilot were returning to their home base after practicing landings at a local airport. Approximately eight miles from the destination, the engine began to run roughly. Shortly thereafter, the propeller became unattached and the engine completely stopped. The pilot executed a forced landing on a field, with no further incident reported.
Post-Accident Examination
A post-accident examination revealed that the crankshaft had completely fractured. Detailed analysis of the crankshaft indicated that the fracture was caused by a fatigue failure that initiated from an oil transfer tube hole. The origin hole was found to have an enlarged diameter portion above the specified value.
Aircraft History
Review of the airplane's logbook disclosed that the aircraft had been involved in a propeller strike four years and 1,220 flight hours prior to this accident.
Conclusion
The engine failure was the result of a fatigue fracture of the crankshaft originating from an oil transfer tube hole with an oversized diameter.