What happened
On 5 July 1998, a Luscombe 8A, registration G-BSWA, was conducting a private flight from Cranfield to Compton Abbas. While cruising at 2,300 feet near Marlborough, the pilot noticed an unusual engine note. After attempting various checks, the engine began to misfire severely. The pilot declared a Mayday and prepared for a forced landing in a nearby field.
During the approach, the pilot identified a suitable field to the left. However, upon closer inspection, the pilot realized the landing strip featured a severe downslope that would likely prevent the aircraft from stopping before the end of the field. Attempting to execute a 180-degree turn to land upwind and uphill, the aircraft entered a stall and spun into the ground from approximately 150 to 200 feet. The two occupants sustained minor injuries, though the aircraft was destroyed.
The investigation
Investigators examined the Continental A65-8 engine to determine the cause of the power loss. The analysis revealed that the No 4 piston and its gudgeon pin had suffered a catastrophic failure. Metallurgical testing showed that the gudgeon pin had fractured due to fatigue. The failure was traced back to the manufacturing process of the pin, which was made from a drawn tube of free-machining mild steel containing manganese sulphide inclusions. This material choice led to axial fatigue origins and scoring within the pin bore.
Further enquiries with the engine manufacturer revealed that the pins used in this engine did not meet the required standard. While the pins in the accident aircraft were made from drawn tube, the manufacturer's updated drawings specified that pins should be manufactured from machined mild steel bar stock and be case-hardened. The pins in the engine were found to have a higher surface hardness but lower core hardness than the required specification.
Findings
- The engine failure was caused by the fatigue fracture of a gudgeon pin.
- The gudgeon pin was manufactured from sub-standard free-machining mild steel rather than the specified machined bar stock.
- The use of drawn tube material introduced manganese sulphide inclusions and surface scoring that facilitated fatigue.
- The engine had operated for approximately 900 hours in the UK since its last overhaul without any indications of component fatigue during internal inspections.