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
Turning colder and brighter towards the end of the the week.
Detectives have launched a murder investigation after a man was fatally stabbed in Kensington.
Stars appeared in black as they called for an end to harrassment at the UK's film awards night.