In-flight engine failure, 13 km west-south-west of Young, New South Wales

During a flight from Essendon to Armidale, the left engine of a Piper PA31P-350 (VH-IGW) failed during cruise at 17,000 feet. Examination of the engine revealed that the crankshaft had fractured in two locations: through the web between the No.4 main bearing journal and the No.4 connecting rod journal; and through the web between the No.3 main bearing journal and No.3 connecting rod journal. It is evident that the event that initiated the multiple fractures of the crankshaft and the subsequent engine failure, was the creation of surface damage in the No.4 main bearing journal fillet radius through rubbing contact between the main bearing insert and the fillet radius. The factors that contribute to this event may be related to the retention of the main bearing insert in its housing and the crankshaft loading conditions that act to displace the bearing insert from its location in the bearing housing.

The movement of main bearing inserts during engine operation is a function of the magnitude of the forces that resist movement (created by establishing an interference fit) and the magnitude of forces acting to move the insert (crankshaft bending moments).

One factor that lowers the resistance of an insert to movement, the inclusion of material between the parting faces of the main bearing housings during engine assembly, was identified. However, other factors that may contribute to bearing insert movement, such as the magnitude of crankshaft bending moments, could not be established from an examination of the physical evidence.

The restoration of the surfaces of the main bearing housings indicated that main bearing insert movement was not an isolated case.