What happened
On 1 April 2009, a Eurocopter AS332L2 Super Puma, registration G-REDL, was conducting a scheduled passenger flight from the Miller Oil Platform to Aberdeen. The aircraft had previously arrived from the Bruce Platform and had undergone a 'rotors-running' crew change. After refueling and boarding 14 passengers, the helicopter departed the platform at 1040 hrs.
While flying at approximately 2,000 ft, the aircraft began a rapid descent. Radar tracking showed the helicopter climbing briefly to 2,200 ft before turning right and descending sharply. At 1254 hrs, the crew transmitted a MAYDAY call on the ATC frequency, followed by a second call containing position information. Shortly thereafter, the aircraft struck the sea. An eyewitness on a nearby supply vessel observed the helicopter descend rapidly, followed by the main rotor blades striking the water and two loud bangs. All 16 fatalities (2 crew and 14 passengers) were confirmed.
The investigation
The AAIB investigation involved wreckage recovery from the seabed, including the fuselage, engines, and the main rotor gearbox. Investigators recovered the Cockpit Voice and Flight Data Recorder (CVFDR), which provided 24 hours of flight data and one hour of cockpit audio. Analysis of the CVFDR showed the crew were engaged in routine tasks prior to the emergency, with no operational abnormalities noted before the distress calls.
Technical examination of the wreckage revealed that the accident was caused by a catastrophic failure of the main rotor gearbox (MGB). This failure led to the detachment of the main rotor head and subsequent strikes by the rotor blades against the pylon and tail boom, which eventually severed the tail boom from the fuselage. Additionally, a rupture was identified in the right-hand engine casing, though this is currently believed to be a secondary effect of the primary failure.
Findings
- The primary cause of the accident was a major failure within the epicyclic module of the main rotor gearbox.
- The failure of the MGB caused the main rotor head to detach, leading to the structural disintegration of the tail boom.
- A rupture in the No2 engine casing occurred, likely as a secondary consequence of the mechanical failure.
- HUMS data had previously identified a particle on a magnetic chip detector in the epicyclic module on 25 March, though no further abnormalities were noted in the 25 hours of flight following that discovery.
Safety action
- Safety Recommendation 2009-048: It is recommended that Eurocopter issue an Alert Service Bulletin for all AS332L2 operators to implement enhanced inspections and monitoring of the epicyclic module.
- Safety Recommendation 2009-049: It is recommended that EASA evaluate the effectiveness of these inspections and, if necessary, issue a mandatory Airworthiness Directive.
- Safety Recommendation 2009-050: It is recommended that Eurocopter improve the gearbox monitoring and warning systems on the AS332L2 to better identify degradation.