Engine Vibration Leads to Forced Landing and Nose-Over (N5961J)

Event Summary

During cruise flight, the pilot reported the onset of a severe engine vibration accompanied by a reduction in engine RPM to approximately 1500. Attempts to remedy the situation were unsuccessful, and the engine produced insufficient power to maintain altitude. The pilot subsequently attempted to land on an interstate highway but deemed the vehicle traffic too heavy for a safe landing. Instead, the pilot executed a landing in a dirt field adjacent to the highway. During the landing roll-out, the aircraft contacted a large berm, causing it to nose over onto its back.

Engine Examination

Post-accident examination of the engine revealed heavy deposits on both the intake and exhaust valves. The exhaust valve for the number 3 cylinder was found in the closed position and could not be moved. Further detailed examination of the number 3 cylinder disclosed that the rocker arm pivot stud for the exhaust valve was broken, with shear signatures evident on the fracture surfaces.