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.
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Armed police have carried out a raid apparently linked to Wednesday's deadly terror attack in Westminster.
Cheryl Grimmer vanished from inside a beach shower block in Wollongong, Australia, in 1970, when she was just three.
Pc Keith Palmer was on the front line and because of his actions, the Westminster attacker was unable to kill anyone else.