What happened
On 1 April 2009, a Eurocopter AS332L2 helicopter, registration G-REDL, was performing a scheduled passenger flight from the Miller Oil Platform to Aberdeen. The flight commenced at 1203 hrs under benign weather conditions with good visibility. After a period of routine operation, the aircraft began a rapid descent. Radar data indicated the helicopter climbed briefly before turning and descending sharply. At 1254 hrs, the crew issued a MAYDAY call, followed shortly by a low oil pressure warning from the main rotor gearbox (MRG). The aircraft struck the sea approximately 11 miles north-0east of Peterhead. The impact resulted in 16 fatalities, including both the crew and all 14 passengers.
The investigation
The AAIB investigation, supported by international partners including the BEA and EASA, focused on the mechanical integrity of the transmission system. Examination of the wreckage revealed that a failure within the epicyclic reduction gearbox module had caused the gearbox case to rupture. This structural failure allowed the main rotor head and the upper section of the MRG to separate from the helicopter due to the intense loads of the impact.
Investigators analyzed data from the Cockpit Voice and Flight Data Recorder (CVFDR) and the Helicopter Usage Monitoring System (HUMS). While the HUMS had detected a metallic chip 34 flying hours prior to the accident, the investigation found that the crew was not alerted to this specific detection. Detailed metallurgical analysis of the recovered components, including the ring gear and planet gears, was conducted to trace the failure sequence.
Findings
- A section of a second stage planet gear became jammed between the ring gear and another planet gear, which triggered the failure of the ring gear.
- The failure of the ring gear led to the rupture of the gearbox case and the subsequent separation of the rotor head.
- A failure within the epicyclic reduction gearbox module was the primary cause of the catastrophic structural breakup.
- The discovery of a metallic chip 34 hours earlier had been identified by the HUMS, but the specific nature of the chip was not communicated in a way that prompted the removal of the gearbox for inspection.
Safety action
Following the investigation, the EASA issued Emergency Airworthiness Directives to mandate inspections of the internal components of the main rotor gearbox for the AS332L2 and EC225LP fleets. These actions included modifications to the chip collector to enhance the early detection of metallic particles.