By Nerijus Adomaitis and Camilla Knudsen
OSLO (Reuters) - The fatal crash of an Airbus helicopter in Norway last year was the result of metal fatigue in the aircraft's gearbox, the country's Accident Investigation Board (AIBN) said in a report on Friday, confirming its initial findings last June.
All 13 people on board were killed when the Super Puma's main rotor blades separated from the aircraft as it was ferrying passengers from a Norwegian offshore oil platform operated by Statoil in April 2016.
The flight was operated by Canada-based CHC Helicopter, owned by U.S. private equity firm First Reserve.
"The investigation has shown that the accident was a result of a fatigue fracture in one of the eight second-stage planet gears in the epicyclic module of the main rotor gearbox," the investigators said.
"The crack initiation appears to be a surface micro-pit," they said in a report released on the first anniversary of the accident.
AIBN said the reason was unclear and no material conformity issues or discrepancies in the manufacturing process had been found during the investigation.
The main rotor gearbox had been involved in a road accident during its transportation in 2015. It was inspected, repaired and installed 260 flight hours prior to the accident.
However, the investigators said they had found no physical evidence that could connect the road accident to the subsequent initiation and growth of the fatigue cracks in the gearbox.
The fracture also propagated in a manner which was unlikely to be detected by maintenance procedures or by the monitoring systems on the helicopter at the time of the accident, it added.
AIBN said it would continue to investigate the accident, similar to one in Scotland in 2009, but it could not provide any completion date.
"We have found the direct cause but we will look for the underlying causes for the accident," AIBN deputy director Kaare Halvorsen told reporters.
The European Aviation Safety Authority (EASA) temporarily grounded H225 LP and AS332 L2 Super Puma helicopters last year after the AIBN's report in June.
Norway and Britain still have flight bans in place.
The Norwegian Civil Aviation Authority (NCAA) said on Friday it would not lift the ban for the time being but would consider its next step in May.
"The Civil Aviation Authority will not lift the aircraft ban until we are completely confident that the helicopter types are safe to fly," the authority's head Lars Kobberstad told Reuters.
CHC said in an email it welcomed AIBN's findings and would review its position "as the output from the investigation dictates."
Airbus said the latest report did not alter the action it had already taken to avoid similar accidents in the future.
The Super Puma has come under intense media and regulatory scrutiny in the UK after a series of incidents linked to gearbox problems including a 2009 crash off Peterhead, Scotland, in which the rotor also flew off and 16 people died.
Airbus said on Friday it did not have the full picture of the reasons behind the 2009 accident because significant parts were lost, but the information from Norway enabled it to take the protective measures.
"We are using lessons learnt from the ongoing investigations into the 2016 accident to set new standards for the helicopter industry," it added.
(Additional reporting by Ole Petter Skonnord and Gwladys Fouche in Oslo, Tim Hepher in Paris and Victoria Bryan in Berlin; Editing by Edmund Blair)