Incident Overview
During a cross-country flight, the pilot reported a loss of engine power while in cruise flight. An emergency was declared, and the pilot initiated a forced landing to an open field. During the landing roll, the airplane collided with a ditch.
Engine Examination
An examination of the engine revealed that the crankshaft was broken between the number two main journal, extending to the number two main journal cheek and number two connecting rod journal.
Metallurgical Findings
A metallurgical examination identified occluded material within the crankshaft, which appeared to be oxides or other non-metallic inclusions produced during the manufacture of the metal prior to forging. The metallurgical report stated that these oxides most likely provided stress risers which concentrated stresses in the crankshaft to develop cracks under cyclic loading. The cracks propagated until failure occurred.