(Corrects this of July 5 story to remove CHC ownership reference in paragraph three.)

OSLO (Reuters) - The crash of an Airbus Super Puma helicopter that killed all 13 people aboard was the result of metal fatigue in its gearbox, Norway’s Accident Investigation Board (AIBN) said in its final report on Thursday, confirming past findings.

The 2016 crash occurred after the Super Puma’s main rotor blades separated from the helicopter as it was ferrying passengers from a Norwegian offshore oil platform operated by Equinor, then known as Statoil.

The flight was operated by Canada-based CHC Helicopter [CHCEL.UL].

“The accident was a result of a fatigue fracture in a second stage planet gear in the epicyclic module of the main rotor gearbox,” investigators reiterated on Thursday.

“Cracks initiated from a micro-pit at the surface and developed subsurface to a catastrophic failure without being detected.”

Investigators said they had excluded material unconformity and mechanical failure, as well as maintenance actions by the helicopter operator, as causes for the crash. Nor was there any connection between the crew handling and the accident, said the report.

Among the 12 recommendations they made, investigators said Airbus should take another look at the design of the main gearbox of the Super Puma.

“The Accident Investigation Board Norway recommends that Airbus Helicopters revise the type design to improve the robustness, reliability and safety of the main gearbox in AS 332 L2 and EC 225 LP (helicopters).”

Airbus Helicopters said it welcomed the conclusion of the investigation and took note of the findings in the report.

“Airbus Helicopters is committed to the continuous improvement of the H225 MGB with the objective of increasing robustness, reliability and safety, as highlighted in the AIBN recommendation,” said an Airbus spokesman.

“This includes, among other initiatives, a roadmap for the development of new vibration monitoring methods which will be shared with industry in due course.”

The Super Puma has come under intense media and regulatory scrutiny in Britain 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.