Investigation Summary
The investigation by the Accident Investigation Board Norway (AIBN) focused on the epicyclic module of the Main Gearbox. Metallurgical analysis revealed that a fatigue crack initiated from a micro-pit on the surface of a planet gear's bearing race. This crack propagated subsurface, a failure mode that bypassed the helicopter's chip detection system because it produced minimal metallic debris (spalling) before the final catastrophic fracture. The investigation also noted that the MGB had been involved in a road transport accident in 2015 where it fell off a truck, though it was inspected and returned to service by the manufacturer.
Final Conclusions
The AIBN concluded the accident was caused by a fatigue fracture in a second-stage planet gear. The failure mode—subsurface crack growth with limited spalling—was not anticipated during certification and rendered the existing Health and Usage Monitoring Systems (HUMS) and magnetic chip detectors ineffective. The report highlighted 'clear similarities' to the 2009 G-REDL accident, suggesting that previous safety actions were insufficient to prevent a recurrence of this specific gearbox failure.
Video Analysis
No video analysis linked for this case file yet.
Systems & Failure Modes
Main Gearbox (MGB) Epicyclic Module
The component containing the planet gears that reduce engine RPM to rotor speed; a gear within this module suffered a fatigue fracture.
Health and Usage Monitoring System (HUMS)
A system designed to detect vibrations and mechanical anomalies, which failed to provide any warning of the impending gear failure.
Magnetic Chip Detectors
Sensors in the oil system meant to catch metallic debris; they were ineffective as the crack stayed subsurface until the final break.
Interesting Facts
- 01The time between rotor detachment and ground impact was approximately 11 seconds.
- 02The helicopter fell 640 meters (2,100 ft) during the uncontrolled descent.
- 03The CVFDR (Black Box) stopped recording abruptly at the moment of rotor separation.
- 04The planet gear failure mode involved subsurface fatigue that did not trigger magnetic chip detectors.
- 05The accident aircraft was manufactured in 2009 and had the registration LN-OJF.
Safety Actions & Advisories
EASA Research Commission
Source ↗AIBN recommended that EASA commission research into crack development in high-loaded case-hardened bearings to better predict service-life reduction from micro-pits.
MGB Design Review
Source ↗Safety recommendations were issued to Airbus Helicopters to consider a complete redesign of the EC225LP main rotor gearbox.
Unusual Event Protocol
Source ↗Recommendation to amend regulatory requirements for critical parts subjected to unusual events (like the 2015 transport accident) to ensure design integrity is maintained.