Investigators examining the wreckage of the Bond Offshore Eurocopter Super Puma AS332L2 that crashed off north-east Scotland in the UK on 1 April have issued an additional urgent inspection requirement for all operators of the type, including the EC225LP variant.
The UK Air Accidents Investigation Branch has now issued the following information: "The investigation has determined that a failure within the epicyclic reduction gearbox module of the main rotor gearbox resulted in the rupture of the gearbox case, which allowed the main rotor head to separate from the helicopter. Examination of the remains of the epicyclic gearbox, and associated areas of the helicopter, continues apace with the aim of establishing as soon as possible the sequence of the failure and initiating cause."
The associated recommendation reads: "It is recommended that Eurocopter, with the European Aviation Safety Agency, develop and implement an inspection of the internal components of the main rotor gearbox epicyclic module for all AS332L2 and EC225LP helicopters as a matter of urgency to ensure the continued airworthiness of the main rotor gearbox. This inspection is in addition to that specified in EASA emergency airworthiness directive 2009-0087-E, and should be made mandatory with immediate effect by an additional EASA emergency airworthiness directive."
The Air Accidents Investigation Branch had already issued an inspection regime that required frequent checks of the aircraft's gearbox metallic chip detectors and magnetic plugs, but it now says: "This procedure was in effect being carried out on the helicopter involved in the accident. This resulted from the discovery of a small chip of metallic debris on the epicyclic gearbox module chip detector some 34 flying hours before the failure. However, during the period between the discovery of the chip and the accident, no signs of an incipient gearbox failure were detected."
The AAIB says it was issuing the instruction for detailed checks on the epicyclic gearbox components because "the cause of the failure has yet to be identified, and the failure occurred without apparent warning to maintenance or flightcrews".