What happened
While operating in cruise flight at 3,500 feet, approximately 17 miles from the destination airport, the pilot noticed a slight decrease in engine RPM and oil pressure. The pilot initiated a descent toward the destination airport while searching for an appropriate area for a forced landing. As the RPM continued to decrease, the pilot elected to perform a forced landing in a grassy field.
During the approach, the pilot executed a base leg, turned final, and extended the flaps to the full down position. Upon touchdown, the landing gear struck ruts that were not visible from the air, causing the aircraft to nose over. There were no injuries reported in the accident.
The investigation
An examination of the engine assembly revealed that three cylinders and three connecting rods had failed. The crankshaft journals exhibited heavy scoring and burn marks. During the inspection of the oil sump, portions of two connecting rod bolts were recovered; these bolts showed severe necking and signs of plasticization.
Further investigation involved removing the oil sump to inspect the oil pump and lubrication pick-up screen. The screen was found to be free of significant debris. Additionally, the oil pump showed no evidence of failure or contamination that would have prevented proper oil supply to the engine. Examination of the pressure regulator and pressure relief valve also revealed no evidence of failure.