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. While cruising at 3,000 feet in the North Sea, approximately 32 nm southwest of Sumburges, the crew observed a transmission warning on the Central Warning Panel. This was followed by several indicators showing a loss of main gearbox oil pressure and temperature fluctuations.
Within one minute of the initial warning, the crew received an alert indicating that the emergency lubrication system had also failed. Following the established emergency landing procedures, the crew performed 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 injuries.
The investigation
The AAIB investigation focused on the failure of the main gearbox (MGB) lubrication system. Investigators examined the recovered aircraft and analyzed data from the Digital Voice and Data Recorder (DVDR) and the Vibration Health Monitoring (VHM) system.
Preliminary engineering analysis involved draining the main gearbox, which revealed a 360-degree circumferential crack on the bevel gear vertical shaft. This crack was located near the weld where two sections of the shaft are joined. The failure of this shaft meant that the main and standby oil pumps were no longer being driven.
Findings
- The primary cause of the incident was a crack in the bevel gear vertical shaft near its welded junction.
- This mechanical failure led to the loss of oil pressure and the subsequent failure of the emergency lubrication system.
- Vibration data from previous flight sectors showed increasing trends in indicators monitoring the meshing frequency of the bevel gear and oil pump wheels.
- The specific shaft involved was not covered by previous Airworthiness Directives regarding similar component failures.