What happened
On 10 May 2012, an EC2TR225 LP Super Puma, registration G-REDW, was performing a scheduled passenger flight from Aberdeen Airport to an offshore platform in the North Sea. While cruising at 3,000 feet, the flight crew received indications of low pressure in both the main and standby oil lubrication systems of the main gearbox. This was accompanied by a chip indication on the Vehicle Monitoring System and rising oil temperatures.
The commander responded by reducing speed, turning back toward the coast, and initiating a descent. The crew activated the emergency lubrication system and, following established procedures, performed a controlled ditching. The helicopter remained upright due to its emergency flotation gear. Following the impact, the crew and 12 passengers evacuated into life rafts. Six occupants were rescued by a search and rescue helicopter, while the remaining eight were recovered by an RNLI lifeboat. There were 2 minor injuries reported among the passengers, and the crew remained uninjured.
The investigation
The investigation focused on the failure of the bevel gear vertical shaft within the main gearbox. Examination of the shaft's fracture surface revealed three distinct cracks. Analysis determined that approximately 99% of the fracture surface had failed due to fatigue, with the remaining 1% resulting from overload.
Investigators examined the 4.2 mm diameter hole used in the electron beam welding of the shaft. While the diameter met design specifications, the internal geometry of the countersinks showed variability, and the surface exhibited tooling marks and a spiral scratch. Furthermore, a small gap was found to exist between the PTFE plug and the side of the countersink, which could potentially allow for the accumulation of corrosive elements.
Findings
- The primary cause of the incident was a fatigue failure of the bevel gear vertical shaft.
- The failure originated from a small corrosion pit within the 4.2 mm diameter hole at the weld site.
- The breakdown of the shaft caused the lower part of the shaft to move, damaging the oil pump drive gears and generating debris that triggered the lubrication warnings.
- Manufacturing variations in the countersink geometry and the presence of surface scratches contributed to the crack initiation.
- The investigation noted that the emergency lubrication system had not been previously operated in-service during this aircraft's operations.