AIBN Says Gear Failure Most Likely Reason For Main Rotor Separation

The Accident Investigation Board of Norway (AIBN) has determined that the most likely cause of an accident involving a Super Puma helicopter in April was a fatigue fracture in one of the second stage planet gears. What initiated the fatigue fracture has not yet been determined.

Thirteen people were fatally injured in that accident in which the main rotor of the helicopter separated from the aircraft.

According to the report, the main gearbox (MGB) was received from Airbus Helicopters after modification, inspection and repair before it was installed in LN-OJF 15 January 2016. At the time of installation, the MGB had accumulated 1,080 hrs since new. At the time of the accident, it had accumulated approximately 1,340 hrs since new.

Two pieces of the recovered parts have been of particular interest. Together they make up approximately half of a second stage planet gear. Examinations of these parts show that one of the fracture surfaces can be described as being close to 100% fatigue.

The AIBN is aware that the gearbox was involved in a road accident during transport in 2015. The gearbox was inspected, repaired and released for flight by the manufacturer before it was installed in LN-OJF in January 2016. Whether there is a link between this event and the initiation and growth of a fatigue fracture, is being investigated.

According to the report, scenarios under consideration as part of this investigation have included failure of a suspension bar attachment or failure of the MGB conical housing as the initiating event. The investigation activities since the previous report do not suggest that either of these scenarios were the initiating event.

(Images provided with AIBN report)

FMI: Full report