According to the report the root cause of the incidents, which occurred in May and October 2012, was a loss of main rotor gearbox oil pressure. This caused the crews of the G-REDW and G-CHCN helicopters to activate an emergency lubrication system that would provide 30 minutes of alternative cooling and lubrication – so giving the helicopters the time needed to reach a nearby airport. However, in both cases a warning light indicated that the emergency lubrication system had failed, which required the crews to ditch their helicopters immediately in the North Sea.
The AAIB concluded that the loss of oil pressure on both helicopters was caused by a failure of the bevel gear vertical shaft in the main rotor gearbox, which drives the oil pumps. The shafts failed as a result of a circumferential fatigue crack in the area where the two parts of the shaft are welded together.
Meanwhile, the report said, the emergency lubrication system's warning light illuminated because of an incompatibility between the helicopter's wiring and the pressure switches. This meant that the warning light would always illuminate after the crew activated the emergency lubrication system.
Consequently, the AAIB has made 10 safety recommendations while the organization noted that the helicopter's manufacturer, Eurocopter (now called Airbus Helicopters), has carried out several safety actions and is redesigning the bevel gear vertical shaft, taking into account the findings of the investigation.
Commenting on the AAIB report, Oil & Gas UK Health & Safety Director Robert Paterson said:
"Today's AAIB report into 2012's Super Puma ditchings has made a number of safety recommendations. We are aware that the parties to which the relevant recommendations have been directed have been actively involved in the compilation of this report, and have been taking action. We welcome it, and the progress that has already been made towards addressing these recommendations, such as the redesign, approval and introduction of a replacement vertical shaft.
"Airbus Helicopters has been very open and engaged with industry and workforce representatives throughout the whole investigation into the underlying causes of these incidents, and this high level of engagement has been hugely welcomed.
"The Civil Aviation Authority (CAA) will be monitoring the responses to the recommendations, and we remain fully engaged and will support all parties to ensure all recommendations are acted on as appropriate."
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