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Summary

Summary

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.

 
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