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Bergen, 20160429: A helicopter has crashed west of the Norwegian city of Bergen with 13 people on board. A rescue vessel lifts up parts of the helicopter. NTB Scanpix/Torstein Boe/via REUTERS

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OSLO (Reuters) - Norwegian investigators looking into the cause of a fatal North Sea helicopter crash have sent a recommendation to European air safety authorities about a possible safety issue with the model's gearbox, they said on Wednesday.

An Airbus H225 Super Puma helicopter ferrying passengers from a Norwegian oil platform operated by Statoil went down on April 29, killing all 13 people on board as the main rotor blades separated from the aircraft.

The Super Puma, a workhorse of the oil industry, has since been grounded for commercial flights in Norway and Britain. Investigators have ruled out human error, saying the crash was caused by a technical fault.

Results of the investigation have potential implications for the Super Puma programme following earlier accidents.

On Wednesday, the Accident Investigation Board Norway said it had found metallurgical evidence "strongly consistent with fatigue" in a part of the gearbox.

It urged the European Aviation Safety Agency, or EASA, to take immediate action to ensure the safety of the helicopter's main gearbox.

Early on Thursday, EASA issued an emergency bulletin, however, focusing on a different problem.

It said checks on the rest of the Super Puma fleet had revealed problems with struts that fix the rotors, including bolts tightened incorrectly or washers in the wrong position.

It ordered all attachments replaced before the next flight.

It was not immediately clear whether EASA also planned another bulletin responding to Norway's gearbox findings.

Investigators have been focusing simultaneously on several different scenarios for the crash.

"We are still investigating and keeping options open," AIBN General Director William Bertheussen told Reuters.

Previous Super Puma incidents linked to gearbox problems included a 2009 crash off Peterhead, Scotland, in which the rotor also flew off and 16 people died.

Airbus Helicopters told operators in a bulletin this week that there were "significant elements" differing from the 2009 crash, according to a copy seen by Reuters.

It placed most emphasis on a possible failure of one of the struts holding the rotors to the aircraft, saying that could have been caused by either faulty bolts or missing safety pins.

A company spokesman said that did not rule out any of three areas for investigation cited by AIBN, including the struts or suspension bars and two parts linked to the gearbox - the "epicyclic module" and the main gearbox housing.

(Reporting by Gwladys Fouche in Oslo, Cyril Altmeyer in Paris, Victoria Bryan and Tim Hepher in Dublin; Editing by Mark Heinrich and Peter Cooney)

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