What happened
Two significant accidents involving the Eurocopter EC2 and 25 LP Super Puma occurred in the North Sea within five months of each other. The first incident involved aircraft G-REDW, which crashed 34 nm east of Aberdeen, Scotland, on 10 May 2012. The second accident involved aircraft G-CHCN, which went down 32 nm southwest of Sumburgh, Shetland Islands, on 22 October 2012.
Both aircraft were performing offshore operations at the time of their respective accidents. The investigation focused on the mechanical integrity of the transmission systems and the failure of critical components within the main rotor gearbox (MGB).
The investigation
The AAIB conducted a detailed examination of the wreckage from both aircraft, specifically focusing on the condition of the main rotor gearbox and the bevel gear vertical shafts. Investigators analyzed the fracture surfaces of the shafts to determine the nature of the failure.
Laboratory testing was performed on various bevel gear vertical shafts to simulate the conditions leading to the failures. This included examining the impact of corrosion pits on the fatigue life of the metal. The investigation also reviewed the manufacturer's fatigue tolerance evaluations and the effectiveness of the emergency lubrication system. Research was conducted into how surface roughness, corrosion, and humidity influenced the initiation of cracks in the steel components.