What happened
On 4 July 2006, a Piper PA-23-250, registration G-BBHF, was conducting a private flight between Lands End and St Mawgan. While cruising at approximately 2,000 feet, the aircraft's left engine failed. The pilot immediately feathered the propeller and performed the necessary engine failure procedures.
During the incident, the aircraft's hydraulic system became inoperative. Because the hydraulic system is responsible for both the flaps and the retractable landing gear, the pilot was forced to perform an orbit on the final approach to manually extend the gear using the emergency hand pump. Despite the loss of hydraulics, the pilot completed a successful flapless landing at St Mawgan. There were no injuries to the two occupants, though the left engine sustained damage.
The investigation
The AAIB examined the aircraft and the left engine through a strip examination and detailed metallurgical analysis. Investigators discovered that a connecting rod had been ejected through the crankcase of the left engine, though the pieces were caught by the engine cowling.
Upon inspecting the oil filter, investigators found a significant amount of metallic debris. Further examination of the oil pump revealed it had seized due to additional metal particles. The engine sump also contained a large volume of similar debris. Metallurgical analysis using a Scanning Electron Microscope confirmed that the debris was consistent with crankshaft bearing shell material. While the crankshaft and main bearings showed wear from the circulating debris, the other connecting rods were found to be correctly torqued.
Findings
- The failure of the left engine was caused by the break-up of the No 4 crankshaft bearing.
- The destruction of the bearing led to the uncontained failure of the connecting rod.
- Evidence suggests that metallic debris was already circulating in the engine prior to the failure, as a small amount of alloy particles had been noted during a 50-hour inspection 30 hours before the accident.
- The mechanical failure of the bearing was likely triggered by the presence of this pre-existing contamination.