History of the Flight
The helicopter departed Bishops Waltham for Goodwood. After a 10-minute engine run, it took off. About 5 minutes later, while climbing, the pilot felt a vibration and then heard a loud bang. Smoke filled the cabin, and the helicopter yawed right. The pilot lowered the collective, turned into wind away from built-up area, transmitted a mayday, and performed a run-on landing in a large field. No injuries occurred.
Examination of the Engine
The engine was removed and examined at the AAIB facility. The No. 2 connecting rod big end had failed, breaking out of the crankcase. Both cap bolts failed. Metallurgical analysis found high cycle fatigue in the cap section and high strain low cycle fatigue in the rod and one bolt. The sequence indicated primary failure of the cap due to tensile fatigue.
Service History
The engine had a history of previous incidents, including an overspeed of >10% in December 1998, after which a full overspeed inspection was performed. It had accumulated 128 hours since then prior to the failure.
Discussion
The manufacturer found galling on the big end cap near the fatigue origin. Galling is associated with bearing shell movement and can result from overspeeding. The investigation suggested that an unreported overspeed may have caused the galling, which could have initiated the fatigue.