What happened
On 17 May 1998, a Piaggio FW P149D, registration G-BPWW, was participating in an air race near Lydd Airport, Kent. During the event, the aircraft's Lycoming GO-480-B1A6 engine suffered a sudden loss of power, began running roughly, and subsequently ceased operation. The pilot was forced to perform an emergency landing in a nearby field. While there were no fatalities and no injuries to the two occupants, the impact caused mechanical damage to the engine, the propeller, the underside of the fuselage, and the wing.
The investigation
The investigation involved a review of the engine's maintenance history and a detailed strip examination of the engine at the AAIB headquarters. Records showed the engine had undergone a significant rebuild in August 1996 and had only accumulated 38 hours of flight time since that work. Notably, the owner and maintenance organisation had previously reported an unusual noise when the engine was running with an unloaded propeller, though no defect was identified during subsequent inspections.
Upon examination, investigators discovered that the No 5 cylinder connecting rod had failed due to overstressing. This failure was caused by extreme overheating of its big-end bearing, which subsequently caused severe secondary damage to the adjacent crankcase and the No 5 cylinder. Further inspection of the crankshaft bearings revealed that most big-end bearings showed signs of minimal oil wetting, suggesting a lack of oil supply. While the No 2 bearing appeared normal, the No 3 big-end bearing showed clear evidence of overheating and material loss.
Findings
- The primary cause of the engine failure was the failure of the No 5 cylinder connecting rod due to overstressing.
- This overstressing was a direct result of gross overheating within the big-end bearing.
- Evidence from the crankshaft bearings suggested that a lack of oil supply to the bearings was the most likely cause of the overheating in both the No 3 and No 5 bearings.
- No specific mechanical obstruction or defect in the oil delivery system, such as the oil pump or relief valve, was identified to explain the lack of oil supply.