What happened
On 10 May 2012, an EC225 LP Super Puma, registration G-REDW, was performing a scheduled passenger flight from Aberdeen Airport to the Maersk Resilient oil platform. While cruising at 3,000 feet, the flight crew received several simultaneous warnings regarding the main gearbox (MGB) lubrication system. These included a red warning light, an aural gong, and indications of pressure drops in both the main and standby lubrication systems. Additionally, the MGB oil pressure gauge showed zero, and the oil temperature began to rise.
In response to these indications, the commander reduced the aircraft speed to approximately 80 KIAS and initiated a turn back toward the coast. Although the crew activated the emergency lubrication system, a subsequent warning indicated that the emergency lubrication system had also failed. Following established procedures for such an event, the commander performed a controlled ditching in the North Sea, approximately 20 nm east of Aberdeen. The aircraft remained upright due to its emergency flotation gear. All 12 passengers and 2 crew members evacuated into a life raft, where 2 passengers sustained minor injuries. The occupants were eventually rescued by a search and rescue helicopter and an RNLI lifeboat.
The investigation
The AAIB investigation focused on the mechanical failure within the main gearbox. Upon draining the gearbox, investigators found approximately 14 litres of fluid, consisting of oil and some glycol. A visual inspection of the internal components revealed a 360° circumferential crack located on the bevel gear vertical shaft, specifically near a manufacturing weld. This crack caused the drive to both the main and standby mechanical oil pumps to disengage. Furthermore, the lower portion of the vertical shaft was found to have displaced downwards by 6 mm.
Investigators also reviewed data from the digital voice and data recorder (DVDR) and the Health and Usage Monitoring System (HUMS). The HUMS vibration sensors had recorded increasing vibration levels during the hours preceding the accident flight, which had been noted and monitored according to maintenance manuals.