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.
