Investigation Summary
The accident was caused by a fatigue crack in one of the eight second stage planet gears within the epicyclic module of the main rotor gearbox. This crack initiated from a surface-connected pit on the gear's inner race. As the crack propagated, it eventually led to the gear's structural failure, which caused the epicyclic module to seize and the gearbox casing to rupture. The resulting forces caused the entire main rotor assembly to detach from the helicopter. The investigation highlighted that on March 25, 2009, a magnetic chip detector had captured a metallic particle. Maintenance personnel, following the manufacturer's guidelines, did not identify this as a sign of imminent gear failure. Furthermore, the design of the gearbox included a ring of magnets intended to capture debris, but this inadvertently reduced the likelihood of significant particles reaching the chip detectors where they could be monitored by the crew or maintenance staff.
Final Conclusions
The AAIB Formal Report 20/2011 concluded the cause was a catastrophic failure of the Main Rotor Gearbox due to a fatigue fracture of a second stage planet gear. Contributory factors included the failure to recognize a magnetic particle found 36 hours prior as an indication of gear degradation, the lack of further indications from existing detection methods after that date, and the presence of internal magnets that reduced the probability of debris reaching the chip detector.
Video Analysis
The Helicopter That Fell From The Sky | Bond Offshore 85N — A detailed look at the mechanical failure and maintenance oversights that led to the 2009 Super Puma crash off the coast of Scotland.
If playback is blocked, the owner has disabled embedding — use the link above.
Crew Experience
Captain
Paul Burnham, 31
Co-pilot
Richard Menzies, 24
Systems & Failure Modes
Main Rotor Gearbox (MGB)
The central transmission system that reduces engine RPM to drive the main rotor; its epicyclic module failed due to a gear fatigue fracture.
Magnetic Chip Detector
A sensor designed to attract and hold metallic particles from the oil system to provide early warning of component wear or failure.
Epicyclic Module
A stage of the gearbox using planet gears to achieve high torque reduction; a second-stage planet gear in this module failed.
Interesting Facts
- 01The main rotor detached completely from the fuselage in flight.
- 02Metallic debris was detected in the gearbox 34 to 36 flying hours before the crash but was not recognized as a failure warning.
- 03The wreckage was located on the seabed at a depth of approximately 100 meters.
- 04The combined CVR/FDR was recovered from the seabed by the Diving Support Vessel Bibby Topaz.
- 05The accident led to an urgent global inspection of AS332L2 and EC225LP gearbox modules.
- 06The victims included 15 UK residents (12 from Scotland, 3 from England) and one Latvian national.
Safety Actions & Advisories
EASA Emergency Airworthiness Directive
Ordered urgent inspections of the epicyclic modules on all AS332L2 and EC225LP helicopters following the initial AAIB findings.
Review of Magnetic Particle Detection
The AAIB recommended that EASA review manuals regarding magnetic particle detection and planetary gear inspection to improve the identification of degradation.
AAIB Formal Report Recommendations
The AAIB issued 17 safety recommendations in the final report, including calls for improved gearbox monitoring systems and changes to the design of internal magnets to prevent debris trapping.
Known Controversies & Unanswered Questions
Fatal Accident Inquiry Findings
Reference ↗The 2014 FAI concluded the crash was preventable, specifically criticizing the decision-making process after the discovery of the metal particle on March 25, which Bond Offshore Helicopters disputed.