The fatal accident of an Airbus Helicopters H225 (EC225LP), registered as LN-OJF, on April 29 off the coast of Norway, near Turoey outside Bergen, was “most likely” the result of a fatigue fracture in one of the main gearbox's second-stage planet gears. According to AIBN, Norway's accident investigation bureau, the fatigue phenomenon appears to have its origin on the inner surface of the gear, propagating toward the web of the gear teeth.
The failed component shows “clear similarities” with that of G-REDL, an AS332L2 Super Puma that crashed in 2009. At the time, indirect fatigue detection was already in place and measures (using magnetic sensors) were taken to detect the spalling caused by a crack. However, the failure mode seen on LN-OJF “seems to differ from what was expected during certification.”
Norway's AIBN believes that a subsurface crack propagated without creating a significant amount of magnetic debris from spalling. It is now looking for what initiated the fatigue fracture. The gearbox was involved in a road accident in 2015 and, although a repair was conducted before installation on CHC's helicopter, the investigators have not ruled out the event as a factor.