On 21 August 2003, during approach to Bankstown aerodrome, the
left engine of a Piper PA31-350 (Chieftain), registered VH-OCF,
malfunctioned. The pilot first became aware of the malfunction
through a series of 'explosions' followed by severe vibration and
the observation of flames. The pilot shut down the engine
immediately and feathered the propeller, at which stage the fire
extinguished.
Subsequent examination of the engine revealed that the number-6
connecting rod little end had fractured and separated from the
piston. The separated end of the connecting rod had collided with
the underside of the number-6 piston, driving it into the cylinder
head and fracturing the cylinder attachment fasteners. The force of
the collision and the flailing of the connecting rod fractured the
camshaft and extensively fractured the crankcase. Both upper engine
mounts separated from the crankcase during that
sequence.
Number-6 connecting rod little end housing
fracture
Examination of the fracture surfaces in the number-6 connecting
rod little end housing revealed that fracture initiated from a
region of fatigue cracking. Fatigue cracking initiated on the inner
surface of the housing. Examination of both the housing inner
surface and the piston pin revealed that galling had occurred
between the pin and the housing inner surface. Galling is a term
used to describe surface damage created by adhesive wear. In this
process sliding contact between two surfaces results in localised
welding, fracture of localised welds and transfer of material from
one surface to the other.
The surface damage created by galling lowers the fatigue
resistance of a component, making the initiation of fatigue cracks
more likely under normal loading. For galling to occur between the
piston pin and the inner surface of the housing, the bronze bush
normally fitted to the housing must not be present. It is evident
in this case that the bronze bush had been destroyed during engine
operation.
An examination of other connecting rods from the engine revealed
that the bushes were in various states of destruction. Subsequent
examination of a connecting rod from the right engine of the
aircraft (following engine overhaul at a time after the occurrence
involving the left engine) revealed that the little end bush was
being destroyed progressively.
The issue of little end bush destruction is being addressed in
detail in a comprehensive ATSB technical investigation
(BO/200305443), titled Aircraft Reciprocating Engine Structural
Failure - An Analysis of Failure in a Complex System.