An oil pump drive failure which lead to a fall in gearbox oil pressure appears to be behind the 11 May ditching of a Eurocopter EC225 (G-REDW) in the North Sea with 14 people onboard.
The detail is contained in an European Aviation Safety Agency emergency airworthiness directive, which says: "The preliminary findings of the investigation have shown a full circumferential crack of the lower vertical shaft of the MGB bevel gear. As a result, the vertical shaft ceased to drive the main and back-up oil pumps.
"The vertical shaft failed after a low number of accumulated flight hours and although the investigation is still in progress, at this early stage a manufacturing defect of the part must be considered. The investigation has also determined that prior to the flight during which the helicopter ditched, the Vibration Health Monitoring system (VHM) installed on the helicopter had identified a rising trend in certain monitoring parameters associated with the MGB oil pump drive system."
UK MoD/Crown Copyright
The EC225 ditched in the sea off Aberdeen but the 14 occupants were winched to safety
Operators are required to check EC225 VHMs at least every three flight hours, and if a VHM is not fitted must operate the aircraft only in daylight visual meteorological conditions.
The UK Air Accidents Investigation Branch says it is continuing its investigation into the successful ditching.
The deployment of flotation devices when the aircraft was set down on the water enabled the helicopter and all on board to be recovered. The medium twin-engined aircraft, operated by Bond Offshore Helicopters, was on its way from Aberdeen to offshore drilling platforms.