The final report into the accident involving a Bond Helicopters Eurocopter AS332L2 Super Puma over the North Sea two years ago has confirmed a sudden main gearbox failure caused the main rotor to separate from the airframe at 2,000ft (610m), resulting in an unsurvivable descent into the sea.
The UK Air Accident Investigation Branch's report on the 1 April 2009 accident, which resulted in the death of all 16 people on board, said: "The catastrophic failure of the main rotor gearbox was a result of a fatigue fracture of a second stage planet gear in the epicyclic module."
The AAIB said indications in the flight data recorder of metal chips accumulating in the oil occurred 3min before the gear failure, but the first indication the crew had of a problem was a sudden fall in the main gearbox oil pressure only 20s before the main rotor separated from the airframe. The report said "neither the weather nor the crew's actions were factors in the accident".
"Actions taken following the discovery of a magnetic particle on the epicyclic module chip detector on 25 March 2009, 36 flying hours prior to the accident, resulted in the particle not being recognised as an indication of degradation of the second stage planet gear which subsequently failed," the report added.
"After 25 March, the existing detection methods did not provide any further indication of further degradation of the second stage planet gear."
Referring to a particular aspect of the design of the chip detection systems in the engine, the AAIB stated: "The ring of magnets installed on the AS332L2 and EC225 main rotor gearboxes reduced the probability of detecting released debris from the epicyclic module."
The AAIB said no manufacturing defects were discovered in any of the components.
There were epicyclic chip indications recorded in the aircraft's health and usage monitoring system (HUMS) that could have been an advance indicator of gear wear, the report observed, but they were not picked up by the HUMS ground analysis system because the HUMS data card had not been closed down correctly. The AAIB has recommended that procedures for ensuring card closure are reviewed.
The investigation also noted that a possible opportunity for the manufacturer to have recognised an abnormal degradation in the gearbox was missed as a result of a communications misunderstanding between Eurocopter and Bond, which led Eurocopter to believe Bond was reporting magnetic chip results following inspection of the ring of magnets on the oil separator plates in the epicyclic module, but the operator had not done so.
The AAIB has made 17 recommendations, most of which call for additional maintenance procedures or future research that might make the detection of such an impending failure more likely.