Incident Summary
During a cruise at 3,500 feet approximately 17 miles from the destination airport, the pilot noticed a slight decrease in engine rpm and oil pressure. He initiated a descent toward the airport while searching for a suitable forced landing area. As engine rpm continued to drop, he chose to conduct a forced landing in a grassy field. The pilot flew a base leg, turned final, and extended the flaps to the full down position. Upon touchdown, the landing gear encountered ruts that were not visible from the air, causing the airplane to nose over.
Engine Examination
Disassembly of the engine revealed that three cylinders and three connecting rods had failed. The crankshaft journals were heavily scored and showed signs of burning. Two connecting rod bolts were recovered from the oil sump; these bolts exhibited severe necking and plastic deformation. The oil sump was removed to inspect the oil pump and lubrication pick-up screen. The screen was free of significant debris. The oil pump was examined and showed no signs of failure or contamination that would have prevented proper oil supply to the engine. The pressure regulator and pressure relief valve were also inspected, with no evidence of failure in either component.
Conclusion
Despite the examination, the reason the engine ceased oiling could not be determined.