Norwegian investigators have reiterated their belief that metal fatigue in a gearbox component was behind the fatal crash of an Airbus Helicopters H225 near Bergen in late April.

The SHT agency also warns that mechanisms installed in the Super Puma to provide advance warning of a gearbox failure may be inadequate.

The rotorcraft (LN-OJF) came down after the main rotor assembly broke away from the fuselage at around 2,000ft. All 13 people on board were killed.

Investigation agency SHT says it is “most likely” that the crash “was the result of a fatigue fracture in one of the second-stage planet gears”.

This supports an interim report issued in early June that had identified failure of that component as the probable cause of the accident.

It considers as “unlikely” the possibility that fatigue cracks found on recovered second-stage planet gears could have “propagated as a consequence of a structural break-up of another component”.

The agency again points to similarities with the fatal loss of an AS332 L2 (G-REDL) in 2009: “Even though some differences are observed when comparing the LN-OJF accident with the G-REDL accident, the fatigue fractured planet gears, however, show clear similarities,” it says.

In addition, the SHT rejects alternative hypotheses that suggested the failure of either the main gearbox conical housing or the gearbox lift-strut bars could have caused the accident.

“The investigation activities since the previous report do not suggest that either of these scenarios were the initiating event,” it says.

CT scans of the recovered planet gear revealed a number of sub-surface cracks in the bearing race – effectively the inner part of the gear – which had propagated without causing significant break-up, or spalling, of the bearing surface.

This would have created metallic particles that would have been detected by the H225’s magnetic chip detection system.

However, no debris had been discovered by inspections since the gearbox was installed in January 2016, it says. In addition, no anomalies were recorded by the aircraft's health and usage monitoring system (HUMS).

“It appears that the fracture of the failed second stage planet gear on LN-OJF 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 says.

In addition, the fatigue cracks propagated inward towards the web of gear teeth, rather than outwards as expected.

“The observed failure mode in this investigation seems to differ from what was expected or foreseen during certification,” says SHT.

Further studies will be conducted to help understand how the fatigue crack propagated, it says. In addition, it notes that the gearbox was involved in a road accident during transportation.

It was then “inspected, repaired and released for flight by the manufacturer” prior to installation. SHT says it will look for any possible links between the two events.

Airbus Helicopters has, meanwhile, released an emergency service bulletin to support customers – both civil and military – that continue to fly the H225 or AS332 L2. This calls for the “short-term withdrawal from service of a specific type of second-stage planet gears”, to be managed through a retrofit programme.

“This is aimed at improving the in-service behaviour of epicyclic modules and will provide increased reliability and safety across the fleet,” it says.

H225s and AS332 L2s are subject to a grounding order issued by European safety regulators.