The Norwegian Accident Investigation Board (AIBN) has issued another preliminary report into the helicopter crash on 29 April that killed 13 people.
The crash happened after the main rotor head and mast detached from an Airbus Helicopters H225 that was returning from an oil platform to Bergen, in Norway. The helicopter fell onto a small island.
‘At this stage of the investigation, the AIBN finds that the accident most likely was a result of a fatigue fracture in one of the eight second stage planet gears,’ the report says. ‘It appears that the fracture has propagated in a manner which is unlikely to become detected by existing mandatory or supplementary systems for warning of an imminent failure.’
The agency said it had not determined what initiated the fracture on the planet gear’s inner surface, which acts as a bearing race. The crack had grown in a way not anticipated by the gearbox’s designers. ‘The observed failure mode in this investigation seems to differ from what was expected or foreseen during certification,’ is said.
In an unsettling statement for helicopter operators who rely on inspection, replacement and health and usage monitoring systems (HUMS) to detect and eliminate mechanical failure, the report said no debris had been discovered by inspections since the gearbox was installed in January 2016. Also, the aircraft’s HUMS (which the investigation found was working properly) had found no anomalies. The gearbox had had 1340 hours in service.
The report noted the gearbox was involved in a road accident during transport in 2015. ‘Whether there is a link between this event and the initiation and growth of a fatigue fracture, is being investigated,’ it said.
The report ruled out other possible failure modes, such as failure of a suspension bar attachment, or failure of the main gearbox conical housing. ‘The investigation activities since the previous report do not suggest that either of these scenarios was the initiating event,’ it said, but added that further sea search for components ‘will be considered’.
CASA has grounded the EC 225 LP and AS 332 L2 helicopters.