Initial investigation has revealed that the Eurocopter AS332L2 Super Puma helicopter lost in the North Sea on 1 April suffered a catastrophic failure of its main rotor gearbox, causing the main rotor head to detach.
The main rotor blades struck the pylon and tail boom, severing it from the fuselage, and the aircraft crashed into the sea, about 17km off the Scottish coast, killing all 16 on board.
In a preliminary report the UK Air Accidents Investigation Branch advises Eurocopter to issue a service bulletin ordering additional inspection and monitoring of the epicyclic module of the main gearbox.
The investigation, assisted by Health and Usage Monitoring System (HUMS) data, has shown that the gearbox experienced a "major failure" within the epicyclic module. But the AAIB says the failure is "not yet fully understood" and the inquiry is trying to piece together the accident sequence.
Operated by Bond Offshore Helicopters, the Super Puma had been transporting oil workers from the BP Miller platform to Aberdeen. Cockpit-voice and flight-data recorder information showed no operational abnormalities.
Fifty-one minutes after leaving the platform, with the helicopter travelling at 2,000ft, the co-pilot made a routine call giving an estimated time of arrival. Just 12sec later, one of the pilots transmitted a brief 'Mayday' and a similar call followed from the other pilot.
Radar information shows the helicopter climbed briefly to 2,200ft, before turning right and descending rapidly. It was seen to strike the surface of the sea by witnesses on a supply vessel 2nm (3.7km) away, with the four main rotor blades - still connected at the hub - hitting the water immediately afterwards.
Recovery teams retrieved the helicopter's fuselage, the separated rotor head and its blades, and the separated tail boom, by 6 April.
The AAIB notes that the epicyclic gearbox module's magnetic chip detector had undergone daily inspection since 25 March, after the discovery of a particle on the detector, with HUMS data analysed each time the helicopter returned to its Aberdeen base, but no further abnormalities were identified.
Investigators state that the right-hand engine casing was also found to have ruptured, but believe that this is a "secondary feature" of the accident.
In its initial report the AAIB is also recommending that Eurocopter improve the gearbox monitoring and warning systems on the AS332L2 in order to "identify degradation and provide adequate alerts".
BP has temporarily suspended its offshore transport contract with Bond while it conducts a review of operational safety at the helicopter company.