What happened
On 22 October 2012, an EC225 LP Super Puma, registration G-CHCN, was performing a commercial passenger flight from Aberdeen International Airport to the West Phoenix drilling rig in the North Sea. While cruising at 3,000 feet with approximately 81% torque, the crew observed a transmission caption illumination on the Central Warning Panel. This was followed by several warnings indicating a loss of main gearbox oil pressure, including the main and standby oil pump pressure captions.
In response to the loss of oil pressure, the crew initiated the 'Total Loss of MGB Oil Pressure' checklist, which involved activating the emergency lubrication system. However, within one minute, a warning appeared indicating that the emergency lubrication system had also failed. The crew then executed the 'Emergency Landing – Power ON' checklist, which resulted in a controlled ditching in the sea near a vessel. All 17 passengers and 2 crew members evacuated the aircraft into life rafts and were rescued without any reported injuries.
The investigation
The AAIB, with assistance from the BEA, examined the aircraft and its components. Investigators recovered the Digital Voice and Data Recorder (DVDR) and a memory card from the Vibration Health Monitoring (VHM) system. While the VHM card did not contain data from the actual accident flight, it provided critical vibration signatures from the two preceding sectors flown on the same day.
An engineering inspection of the drained main gearbox revealed a 360-degree circumferential crack on the bevel gear vertical shaft, specifically near the weld joining two sections of the shaft. This crack resulted in the main and standby oil pump gears no longer being driven.
Findings
- The primary cause of the incident was a crack in the bevel gear vertical shaft at the weld location.
- Vibration monitoring data from the two sectors prior to the accident showed increasing trends and exceedances of alert thresholds (MOD 45 and MOD 70 indicators).
- The failure of the vertical shaft led to the loss of lubrication in the main gearbox and the subsequent failure of the emergency lubrication system.