A probe by the Air Accidents Investigation Branch found that a Bond Super Puma suffered a "catastrophic failure" of its main rotor gearbox before it crashed into the North Sea, killing 16 men.
The AAIB report said that the gearbox failure caused the main rotor on the AS332-L2 model to break away and its "tail boom" was severed from the fuselage.
A Fatal Accident Inquiry, which will publish its findings today, focused largely on maintenance carried out on the helicopter's gearbox in the weeks and months before the tragedy.
More top news
Amber Rudd feels the heat and a summer visit for Donald Trump raises fears of unrest.
Experts called on the Government to immediately tighten up fire regulations without waiting for the conclusion of the Grenfell inquiry.
Tottenham Hotspur is one of the first clubs to declare its commitment to phasing out single-use plastics at its new stadium.