Event
The pilot leveled off at the cruise altitude of 6,500 feet when the engine began to vibrate and oil was observed leaking from the engine cowling. The vibration and oil flow increased in intensity, prompting the pilot to shut off the engine and execute a forced landing to a field. During the ground roll, the airplane struck a berm approximately 2 feet high.
Examination
Post-accident examination of the engine revealed that the number 2 connecting rod had failed between the wrist pin hole and the crankshaft yoke. A metallurgical examination of the fracture surface identified fatigue as the failure mechanism. Additionally, evidence of gauling was found in the area of the connecting rod bolt holes. Gauling was also observed on the number 6 connecting rod bolt holes.
Background
According to the engine manufacturer, gauling could occur from high loading, such as that produced during overspeed or excessive manifold pressure operation. The operator reported that the airplane was flown by company pilots on charter flights and was also available for rental.