Introduction
An accident occurred during the initial climb phase of a flight when the engine experienced a complete failure due to a crankshaft fracture. The flight instructor, who was at the controls, managed the emergency by performing a forced landing into a swamp.
Accident Details
The flight was in the initial climb after takeoff when the engine lost power. The flight instructor took control and executed a forced landing, which ended in a swampy area. The aircraft sustained damage from the landing.
Investigation Findings
Post-accident examination of the engine revealed that the crankshaft had fractured between the number three bearing journal and the number three connecting rod journal. The National Transportation Safety Board Materials Laboratory Factual Report provided a detailed analysis of the fracture surface. The report described the forward piece of the crankshaft as having a "relatively smooth" surface with "curving crack arrest lines," which are characteristic of fatigue. The fatigue cracking originated at the surface of the aft radius of the third main journal and propagated through more than 80 percent of the crankshaft cross-section before final separation occurred.
Further examination of the bearings showed that the inner and outer diameter surfaces of the number three main bearing halves were circumferentially rubbed. One edge of each of these bearing halves was worn in the approximate shape of the journal radius, and the antirotation tabs were worn away from the outside diameter surface. The number four main bearing halves were deformed, with gouging on the inner diameter surface. Slight circumferential rubbing marks were observed on the inner diameter surfaces of the number one and two main bearing halves. No evidence of heat tinting was noted on any of the bearing halves.
Probable Cause
Fatigue cracking of the engine crankshaft, originating at the aft radius of the third main journal, leading to crankshaft failure and subsequent engine failure.