What happened
On April 29, 2016, an Airbus Helicopters EC 225 LP, registration LN-OJF, was performing a scheduled transport flight for oil workers from the Gullfaks B platform to Bergen Airport Flesland. The flight was proceeding normally until the aircraft had descended to 2,000 feet and established a cruise speed of 140 knots.
Without any prior warning to the crew, the engine torque dropped abruptly and the main rotor began to tilt erratically. The aircraft briefly climbed before the main rotor assembly completely detached from the fuselage. Following the separation, the helicopter entered a ballistic descent, impacting a small island near Turøy. The impact destroyed the aircraft and ignited a fire fueled by spilled fuel. The main rotor traveled a significant distance from the crash site, landing on the island of Storskora. There were 13 fatalities in total, including the two crew members and 1-1 passengers.
The investigation
The Norwegian Safety Investigation Authority (NSIA) conducted an extensive technical analysis of the wreckage and components. Investigators focused on the mechanical integrity of the main rotor gearbox (MGB) and the flight data recorded during the final moments of the flight. The investigation involved international cooperation with agencies from France, the UK, and Germany to examine the metallurgy of the failed components and the certification standards for large rotorcraft.
Findings
Technical analysis determined that the accident was caused by a fatigue fracture in a second stage planet gear within the epicyclic module of the main rotor gearbox. The failure originated from a microscopic pit on the surface of the bearing's upper outer race. This flaw propagated subsurface through the material, eventually reaching the gear teeth and fracturing the rim of the gear.
Crucially, the investigation found that this specific failure mode—driven by a combination of material properties, design, and operational loading—was not previously anticipated or assessed during the aircraft's certification. The fracture developed in a manner that was unlikely to be detected by the existing maintenance procedures or the onboard monitoring systems. Furthermore, the investigation noted that the failure was similar to a previous accident involving an AS 322 L2 helicopter, suggesting that previous post-accident safety actions had been insufficient to prevent a recurrence.