Engine Failure Sequence
While the aircraft was in en route cruise, the number four cylinder exhaust valve guide separated from its associated cylinder head. When the aircraft was approximately 12 miles from the airport, the head of the number four cylinder exhaust valve separated from the valve stem. The exhaust valve head then dropped into the number four cylinder, where it repeatedly impacted the piston dome and the cylinder head ceiling. This was followed by approximately 30 percent of the piston dome failing inward, resulting in the pressurization of the crankcase through the open exhaust valve port.
As a result of the crankcase pressurization, one of the number six cylinder pushrod-to-crankcase seals was forced off its crankcase mounting surface, allowing the majority of the oil to escape from the engine. Soon thereafter, due to thermal stress, the number one connecting rod separated from the crankshaft and punched holes in the crankcase near the number one and number two crankshaft journals.
Forced Landing Attempt
After experiencing a total loss of power, the pilot attempted to land on the uneven rolling terrain, but due to the darkness of the night, the aircraft collided with the terrain.