The PA31-350 aircraft was in cruise flight, at 8,000 ft, when
the flight crew noticed that the propellers went out of
synchronisation. Adjustments were made to correct the problem but
were unsuccessful. Following right engine RPM fluctuations, the
crew shut the engine down, feathered the propeller and conducted a
single engine landing.
During the subsequent disassembly of the engine, the crankshaft
was noted to have fractured at the number-6 connecting rod
journal.
The engine components were sent to the ATSB for further
technical analysis. Analysis of the fracture indicated that fatigue
cracking had initiated below the surface of the journal, associated
with a discontinuity in the nitrided surface zone, at the
transition from the journal to the forward fillet radius. The
number-6 connecting rod bearing inserts had been destroyed during
operation; the remnants included flattened fragments of steel
backing material. In addition, fatigue crack growth had commenced
at the centre of the connecting rod cap, most probably after the
bearing inserts had failed.
Because of the small amount of bearing debris available for
testing, the reason the number-6 bearing inserts failed could not
be determined.
This engine failure is one of a number of events being used in a
detailed safety study of failures to high powered piston engines.
On completion, the results will be available on the ATSB website
www.atsb.gov.au or from the Bureau on request.