Norway helicopter crash gearbox 'in road accident'
A gearbox in a helicopter which crashed off Norway in April had previously been involved in a road accident, Norwegian investigators have revealed.
A fatigue crack below the surface of the metal has been identified as the "most likely" cause of gearbox failure.
They are examining whether the road accident while the gearbox was being transported by lorry in 2015 played a role in the tragedy.
Manufacturer Airbus stressed that no link had been established.
The Super Puma 225 came down near the small island of Turoey, near Bergen, while it was returning from a Norwegian oil field.
Iain Stuart 41, from Laurencekirk, Aberdeenshire was one of 13 people who died in the crash.
The interim report from the Air Accident Investigation Board Norway said the most likely cause was a fatigue fracture within the helicopter gearbox which resulted in the main rotor becoming detached.
It noted that the "design philosophy" of the gearbox components assumed that fatigue would produce tiny amounts of debris that would be instantly detected by onboard sensors
The investigators believe a crack developed in such a way that no debris was produced, and was therefore not picked up by existing safety systems.
The report notes: "The gearbox was involved in a road accident in 2015. The gearbox was inspected, repaired and released for flight by the manufacturer before it was installed in LN-OJF in January 2016.
"Whether there is a link between this event and the initiation and growth of a fatigue fracture, is being investigated".
The report also identifies "clear similarities" with the crash off Peterhead in 2009 in which 16 people died.
The earlier accident involved a different model of Super Puma helicopter.
Both types of aircraft, manufactured by Airbus, have been grounded by the European Aviation Safety Agency.
Airbus Helicopters later confirmed that a lorry carrying the gearbox had been involved in an accident but stressed that no link had been established between this event to the subsequent helicopter crash.
A spokesperson said: "The report states that no link has been established yet and tests are ongoing to assess the potential impact of this road accident on the second stage planet gears."
The statement added: "We continue to focus our efforts on providing assistance to the investigation team and the authorities as they work toward the identification of the accident root cause.
"In parallel, we are putting precautionary measures in place to support our global customers and address potential initiating events."