Accident Overview
The airplane experienced a catastrophic engine failure during cruise flight. While receiving VFR flight following from an air route traffic control center (ARTCC), the facility lost radar and radio contact with the aircraft. The pilot attempted a forced landing in mountainous terrain, during which the airplane collided with multiple trees. The debris path was along the eastern edge of an elliptical meadow, approximately 1 mile long and 1/4 mile wide, oriented east-west. The meadow was surrounded by trees over 100 feet tall. The airplane came to rest near the meadow's edge. Multiple large trees exhibited broken branches and major impact marks on their trunks.
Engine Examination
Examination of the engine revealed a 3-inch diameter hole in the crankcase above cylinder number 3. The crankshaft had fractured and separated at the rear of the number 4 cheek, with the fracture located at the forward radius of the number 2 main bearing journal. Smeared metal was present on the fracture surfaces. The fractures of the crankshaft caused additional damage, including separation of the number 3 and 4 connecting rods from their respective journals, which would have led to a complete loss of engine power.
Metallurgical Findings
A metallurgical examination determined that a large portion of the fracture surface exhibited smooth crack arrest markings typical of fatigue propagation. The failure of the crankshaft was caused by a fatigue crack in the cheek between the number 3 rod journal and the number 2 main bearing journal. The origin of the fatigue fracture was in the transition between the number 2 main bearing journal surface and the forward fillet radius, approximately 0.036 inches below the journal surface. No metallurgical anomalies or manufacturing process errors were found to explain the fatigue initiation. This failure is similar to others investigated by the Safety Board's Materials Laboratory, where fatigue cracks initiated just below the hardened case layer without identifiable material or microstructural defects. The crankshaft met design material specifications.