On 13 October 2006 a Bristow Helicopters Eurocopter AS332L Super Puma prepared for take off from Runway 24 at Aberdeen airport with 13 passengers on board bound for North Sea oil platforms, when something went wrong.
According to the just-published UK Air Accidents Investigation Branch final report, 5s into its take-off there was a loud bang followed by heavy vibration, and the crew landed the machine back on the runway and shut down.
The AAIB investigation has found that one main rotor blade spindle had fractured through the lower section of its attachment yoke on the leading side of the spindle. This could have led to the separation of a main rotor blade.
The investigation found that there was no history of this type of damage to this component, and no indication in the aircraft's health and usage monitoring system (HUMS) readings that would have warned the crew or operators of the impending failure.
A formal recommendation from the AAIB calls on the European Aviation Safety Agency to "extend the scope of HUMS into the rotating systems of helicopters".
At the time of the incident Eurocopter sent out an emergency telex advising a visual inspection regime, and EASA formalised the requirement shortly afterwards.
According to the investigators, the probable causes were stresses owing to wear in the flapping hinge inner bearing race, and excessive clamping pre-load across the yoke caused by over-torqueing of the tie bolt during maintenance. A contributory factor was evidence of the regular use of high-speed level flight in the cruise, which increased stresses.
Still under investigation by the AAIB is the 1 April 2009 fatal accident to an AS332L2 Super Puma, caused by the failure of its epicyclic gearbox module leading to the total separation of the rotor head from the fuselage during cruise.
Source: Flight International
