Accident Summary
During an en route flight, the pilot experienced an engine backfire, which was immediately followed by a complete loss of engine power. The pilot initiated a forced landing and attempted to reach a nearby airport but was unable to make the runway. The aircraft struck trees and brush approximately 200 yards short of the runway, coming to rest in the vegetated area.
Engine Examination
A post-accident examination of the engine revealed a fractured crankshaft. The fracture occurred through the forward side of the number four journal. A metallurgical analysis of the fracture surface determined that the crankshaft failed due to fatigue. The fatigue crack initiated at a subsurface inclusion located 0.023 inch below the journal surface.
Metallurgical Findings
Scanning electron microscopy confirmed that the fatigue crack originated at a single subsurface site. At the origin, a pit measuring approximately 0.002 to 0.003 inch in diameter was observed. The pit was mostly empty but contained remnants of material. Energy dispersive X-ray spectroscopy identified the remnant material as primarily aluminum and calcium.
Crankshaft Age and Manufacturing
The age of the failed crankshaft could not be determined; however, it appeared to be an older component, manufactured before the transition to vacuum arc remelted steels. Such steels offer higher fatigue strength and virtually eliminate the presence of inclusions that can serve as fatigue initiation sites.