What happened
On 1 April 2009, an AS332 L2 helicopter, registration G-REDL, was conducting a scheduled passenger flight from the Miller Platform in the North Sea to Aberdeen. While cruising at 2,000 ft amsl, the aircraft experienced a catastrophic failure of its Main Rotor Gearbox (MGB). The failure caused the main rotor and a portion of the epicyclic module to separate from the fuselage. Following the loss of MGB oil pressure, the helicopter began a rapid descent and the main rotor blades struck the tail boom, severing it from the airframe. The aircraft struck the sea with high vertical speed, resulting in all fatalities.
The investigation
The investigation, involving the AAIB, BEA, and EASA, focused on the mechanical failure of the transmission system. Investigators examined the recovered fragments of the epicyclic module and identified that a second stage planet gear had suffered a fatigue fracture. Analysis of the gear's outer race suggested the crack likely originated near the loaded area of the bearing.
Crucially, the investigation looked into maintenance history, discovering that a metallic particle had been found on the epicyclic chip detector 36 flying hours before the accident. However, this finding was not recognized as an indicator of impending component degradation. The investigation also reviewed the effectiveness of the magnetic chip detectors and the impact of the magnetic ring design on debris detection capabilities.
Findings
- The primary cause of the accident was a fatigue fracture of a second stage planet gear within the epicyclic module.
- A metallic particle found during maintenance 36 hours prior to the crash was the only warning of the impending failure, but it was not identified as a sign of degradation.
- The separation of the main rotor was caused by the conical housing separating from the MGB, which subjected the lift struts to forces beyond their design limits.
- The existing magnetic chip detector system was less effective at detecting released debris due to the presence of a ring of magnets in the gearbox.
- The flight crew was properly qualified, and the aircraft was maintained according to regulations; weather conditions were not a factor.