A CATASTROPHIC failure of its gearbox caused the loss of helicopter flight 85N and 16 men on board, it was revealed today.

An initial report from the accident investigators said a major malfunction in the gearbox caused the main rotor to detach from the helicopter.

The spinning blades then tore through the helicopter’s tail, severing it and sending the fuselage plummeting more than 2,000ft to the North Sea below.

The report also revealed daily inspections of the helicopter’s gearbox were being carried out after a “particle” was discovered, one week before the crash.

Now a wave of inspections has been ordered for all Super Puma helicopters of the same type as the one involved in the tragedy.

But investigators admitted that exactly what caused the helicopter gearbox failure remains a mystery.

A major inquiry was launched by the Air Accidents Investigation Branch within hours of the helicopter crash on Wednesday, April 1.

The team of experts tasked with uncovering the cause of the tragedy has now revealed its initial findings.

The report stated: “Examination of the wreckage indicates that the accident occurred following a catastrophic failure of the main rotor gearbox.

“This resulted in the detachment of the main rotor head from the helicopter and was rapidly followed by main rotor blade strikes on the pylon and tail boom, which became severed from the fuselage.

“It is apparent that there was also a rupture in the right-hand engine casing.”

But the report revealed the failure of the gearbox was “not yet fully understood”.

Bill Munro, managing director of Bond Offshore Helicopters, said: “We believe that the AAIB report confirms that we have been undertaking everything required of us under current systems and procedures.”

groberts@ajl.co.uk