What happened
On 1 April 2009, a Eurocopter AS332L2 helicopter, registration G-REDL, was performing a scheduled commercial passenger flight from the Miller Oil Platform to Aberdeen. The flight was proceeding normally until approximately 1254 hrs, when the crew transmitted a MAYDAY call. Shortly thereafter, the aircraft experienced a rapid descent and struck the sea surface approximately 11 miles north-east of Peterhead, Scotland. The impact resulted in 16 fatalities, including both the crew and all 14 passengers. An eyewitness on a nearby supply vessel observed the helicopter descend abruptly, followed by the main rotor blades striking the water and two loud bangs.
The investigation
The investigation, involving the AAIB, BEA, and EASA, focused on the mechanical integrity of the main rotor gearbox (MRG). Recovered flight data and engine control units showed that the crew was engaged in routine operations with no operational abnormalities prior to the sudden loss of oil pressure. Wreckage recovery efforts identified the main rotor head, mast, and the upper section of the MRG had separated from the airframe during the incident.
Investigators examined the epicyclic reduction gearbox module and found significant damage. While the engines themselves showed no signs of pre-existing distress, the investigation identified that a failure within the epicyclic module caused the gearbox case to rupture. This rupture allowed the rotor head and upper gearbox components to detach from the helicopter.
Findings
- The primary cause of the accident was a failure within the epicyclic reduction gearbox module of the MRG.
- A section of a second-stage planet gear became jammed between the ring gear and a planet gear, which triggered the failure of the ring gear.
- The rupture of the gearbox case led to the separation of the main rotor head and upper MRG from the aircraft.
- Evidence suggests that the ring gear burst open due to the mechanical stress of the gear failure.
- Although the helicopter's monitoring system (HUMS) had detected a metallic chip 34 flying hours prior to the accident, the crew was not alerted to the developing issue as the detection occurred within the gearbox module and was not immediately visible to the pilots.
Safety action
Following the investigation, the EASA and Eurocopter implemented several measures. This included the issuance of Emergency Airworthiness Directives to mandate inspections of the internal components of the epicyclic module for the AS332L2 and EC225LP fleets. Additionally, modifications were made to the chip collector within the gearbox to enhance the ability of magnetic plugs to detect early signs of contamination.