Incident Overview
The flight was approximately 3 miles east of the airport, returning to land at an altitude of about 2,000 feet, when the engine ceased operation. The pilot was unable to restart the engine and, after observing that a nearby highway was congested, elected to land in a patch of small trees. The aircraft collided with the trees and subsequently fell to the ground.
Engine Examination
A post-accident examination of the engine revealed that the crankshaft had failed at the rear crankshaft cheek between the connecting rods for cylinders #1 and #2. Metallurgical analysis of the failed crankshaft identified both low-cycle and high-cycle fatigue. The evidence indicated that the fatigue was caused by vibrations generated when the three dynamic counterweights came into contact with the adjacent camshaft lobes. This contact occurred because the counterweight bushing was worn.
Maintenance History
Review of the engine logbooks disclosed that a major overhaul had been performed in 1969, approximately 793 hours of engine operation prior to the failure. Subsequently, three top overhauls were carried out. The first and second top overhauls were performed at one-year intervals after the major overhaul. The third top overhaul was performed 10 years after the second.