What happened
On 22 October 2012, an EC2 and25 LP Super Puma, registration G-CHCN, was performing a commercial passenger flight from Aberdeen International Airport to the West Phoenix drilling rig. While cruising at 3,000 feet in the North Sea, approximately 32 nm southwest of the Shetland Islands, the crew observed the transmission caption illuminate on the Central Warning Panel, accompanied by a drop in main gearbox oil pressure to zero.
Following the loss of pressure, the crew attempted to activate the emergency lubrication system (EMLUB). However, within one minute, a warning indicated that the emergency system had failed to activate. The crew subsequently executed an emergency landing procedure, successfully ditching the aircraft in the sea near a vessel. There were no injuries to the 17 passengers or the 2 crew members on board.
The investigation
The AAIB examined the wreckage and the main gearbox (MGB) to determine the cause of the oil pressure loss. The investigation focused on the failure of the bevel gear vertical shaft. Physical analysis of the shaft revealed a fracture in the area of the electron beam weld.
Engineers compared the findings with a previous accident involving a similar aircraft, G-REDW, which had suffered a similar failure in May 2012. The investigation looked into the manufacturing processes of the 32CDV13 steel alloy used for the shaft and the impact of the welding process on the material's integrity. Additionally, the investigation reviewed Health and Usage Monitoring System (HUMS) data, which showed trend indicators for the shaft had exceeded learned thresholds prior to the loss of oil pressure.
Findings
- The primary cause of the incident was a high cycle fatigue crack in the bevel gear vertical shaft.
- The crack originated in the area of the weld and propagated through the parent material.
- The failure was likely driven by the shaft bending or flexing as it rotated.
- The investigation identified that the shaft was manufactured from 32CDV13 steel alloy, which was subject to different stresses than previous variants.
- On G-CHCN, there was evidence of a bearing slippage in the upper roller bearing, though the lower roller bearing appeared normal.