Gearbox failure caused N.Sea helicopter crash-probe

LONDON, April 10 (Reuters) - The North Sea helicopter crash that killed 16 people last week was caused by a "catastrophic failure" that led to the aircraft's main rotor breaking away, British air accident investigators said on Friday.

In an initial report, the Air Accidents Investigation Branch (AAIB) said the failure occurred in part of the helicopter's main gearbox.

The rotor then broke off, hitting and severing the aircraft's tail boom, the AAIB said, adding there was also a rupture in the helicopter's right-hand engine casing.

The 14 passengers and two crew were returning from a BP oil platform when the Super Puma helicopter crashed in the sea in fine weather on April 1.

The AAIB said the Puma's manufacturer Eurocopter should instruct all operators to increase main rotor gearbox inspections.

It also said Eurocopter should "improve the gearbox monitoring and warning systems on the (Puma) helicopter so as to identify degradation and provide adequate alerts".

It was the second Super Puma crash in the North Sea in six weeks. In February, all 18 on board a Super Puma travelling to a BP oil rig were rescued after it went down into the sea after hitting a fog bank.

Both aircraft were operated by Bond Offshore Helicopters, which runs a fleet of Super Puma twin-engined helicopters to supply sea-based oil rigs. (Reporting by Frank Prenesti; Editing by Alison Williams)