Accident Overview
An aircraft encountered engine problems while approaching Des Moines Airport. The pilot reported the issue to approach control and asked about the closest airport for a precautionary landing. An attempt was made to land at an airstrip three miles east of the aircraft's position, but landing there proved impossible. Consequently, the aircraft was landed in a field, where it sustained substantial damage.
Pilot Actions
Upon realizing the engine problems, the pilot communicated with approach control to determine a suitable nearby airport. The decision to attempt landing at the airstrip three miles east was made, but when that was unsuccessful, the pilot chose a field for the landing. The specific circumstances that prevented the airstrip landing are not detailed in the source.
Aircraft and Engine Inspection
Subsequent inspection of the engine revealed a broken crankshaft idler gear. Examination of the fracture surfaces showed evidence of fatigue progression. Additionally, an elliptical deformation of the threaded portion of the retaining bolt was noted, present below the bolt head. This deformation matched the thickness of the bolt flange and locknut plate. Visual inspection of the fracture surfaces gave no evidence of unusual contact or wear patterns prior to failure.
Conclusion
The source provides no explicit probable cause for the accident, only the findings from the engine inspection. The engine failure originated from a fatigue crack in the crankshaft idler gear, with associated deformation of the retaining bolt. No pre-existing unusual wear or contact was observed.