UK Air Accidents Investigation Branch (AAIB) investigators say an accident that bought down a Super Puma helicopter in the North Sea killing 16 was caused by a 'catastrophic' main gearbox failure.

The Bond Offshore Helicopters AS332L2 Super Puma (G-RED) crashed into the sea on April 1 as it returned from a oil platform.

In an interim report released April 10, the AAIB said that investigation of the wreckage revealed that the accident occurred following a catastrophic failure of the main rotor gearbox (MGB).

As a result of the gearbox failure, the main rotor head detached from the helicopter and was rapidly followed by main rotor blade strikes on the pylon and tail boom, which was severed from the fuselage. There was also a rupture in the right hand (No2) engine casing, in the plane of the second stage power turbine.

"The investigation has so far revealed that the MGB had suffered from a major failure within the epicyclic module. This is supported by HUMS data; however, this is not yet fully understood. The examination of the MGB is continuing in conjunction with detailed analysis of the HUMS and other recorded information" the AAIB preliminary report stated.

Based on the accident evidence uncovered to date, the AAIB made three safety recommendations:

Safety Recommendation 2009-048. It is recommended that Eurocopter issue an Alert Service Bulletin to require all operators of AS332L2 helicopters to implement a regime of additional inspections and enhanced monitoring to ensure the continued airworthiness of the main rotor gearbox epicyclic module.

Safety Recommendation 2009-049. It is recommended that the European Aviation Safety Agency (EASA) evaluate the efficacy of the Eurocopter program of additional inspections and enhanced monitoring and, when satisfied, make the Eurocopter Alert Service Bulletin mandatory by issuing an Airworthiness Directive with immediate effect.

Safety Recommendation 2009-050. It is recommended that Eurocopter improve the gearbox monitoring and warning systems on the AS332L2 helicopter so as to identify degradation and provide adequate alerts.