All 16 people aboard a Eurocopter AS 332L2 Super Puma operated by Bond Offshore Helicopters died when the medium twin hit the surface of the North Sea on April 1, after what the UK’s Air Accidents Investigation Branch (AAIB) called a catastrophic failure of the main gearbox.
According to investigators, the failure took place in the epicyclic module. “This is supported by HUMS [health and usage monitoring system] data; however, this is not yet fully understood,” the report states. The failure resulted in the detachment of the main rotor head from the helicopter and was rapidly followed by main rotor blade strikes on the pylon and tail boom, which became severed from the fuselage.
The helicopter had been undergoing a daily inspection of the epicyclic module magnetic chip detector since March 25, when a particle was discovered on that part. In addition, the HUMS data was downloaded and analyzed each time the helicopter returned to its base at Aberdeen for the next 25 flying hours. According to the report, the operator noticed no further abnormalities in the week preceding the accident.
The AAIB recommended that operators implement “a regime of additional inspections and enhanced monitoring to ensure the continued airworthiness of the main rotor gearbox epicyclic module.”
It requested a subsequent airworthiness directive (AD) from the European Aviation Safety Agency (EASA). The investigative board also recommended that Eurocopter improve the AS 332L2’s gearbox monitoring and warning systems “to identify degradation and provide adequate alerts.” Eurocopter on April 10 issued an Alert Service Bulletin (ASB) to that effect. The EASA mandated it for AS 332L2s and EC 225s on April 17.
On that day, the AAIB recommended that Eurocopter and the EASA develop an inspection of the module’s internal components on the AS 332L2 and EC 225. On April 20, Eurocopter answered, “Current maintenance and check procedures, provided they are correctly applied, are fully satisfactory.” The manufacturer said 70 percent of the AS 332L2 fleet in the North Sea would fly again by April 23.
Trade association Oil & Gas UK insisted none of the 25 Super Puma AS 332L2s and EC 225s affected would fly until the AD and the ASB were fully implemented.
The AAIB recovered and downloaded the CVFDR, which contained 24 hours of flight data and one hour of cockpit voice recording. Twelve seconds after the copilot made a routine call on the company operating frequency, one of the pilots made a brief Mayday call on the ATC frequency. This was followed by a similar call from the other pilot that included some position information.
The radar controller at Aberdeen acknowledged the Mayday call and tried unsuccessfully to contact the crew. An eyewitness, working on a supply vessel approximately two nautical miles from the accident site, heard the helicopter and saw it descend rapidly before it hit the surface of the sea. Immediately after impact, he saw the four main rotor blades, still connected to their hub, strike the water.
The helicopter was operating a return scheduled passenger flight from Aberdeen to the Miller Oil Platform, situated in the North Sea approximately 145 nm northeast of Aberdeen. The accident took place during the day, at 12:55 UTC. Weather conditions were “benign.”
A similar accident (albeit with still undetermined causes) occurred six weeks earlier. On February 18, all 16 occupants survived when their EC 225 Super Puma, also operated by Bond on behalf of BP, ditched in the North Sea.