Accident investigators led by Norway don't know a Super Puma's main gearbox disintegrated in flight a year ago or why safeguards failed to detect a gear's pending degradation before it led to the deaths of the 13 people onboard the aircraft.
In a new preliminary report on the April 29, 2016 crash of the CHC Helicopter Airbus Helicopters EC225LP, the Accident Investigation Board Norway (AIBN) said its investigation points to a fatigue fracture in the main rotor gearbox as the cause of the crash, but it is unknown at this point what caused the fracture.
“The investigation has shown that the accident was a result of a fatigue fracture in one of the eight second stage planet gears in the epicyclic module of the main rotor gearbox. The fatigue had its origin in the upper outer race of the bearing (inside of the gear), propagating towards the gear teeth,” the report said. “The crack initiation appears to be a surface micro-pit. However, the reason for formation of the micro-pit and the underlying driving mechanisms are not currently understood, i.e. how and why the cracks continued to grow sub-surface. No material conformity issues or discrepancies in the manufacturing process have been revealed during the investigation.”
Focus is being put on design and subsequent certification. The AIBN said the fatigue fracture was unlike anything expected during the design and certification process and happened in a way that was unlikely to be found during maintenance.
“The certification process and [Europe's] Certification Specifications for Large Rotorcraft related to catastrophic failure and requirements for safety barriers will be subject to further investigation,” the AIBN said.
Also under further investigation is the 2009 crash of an AS33L2 off the coast of Scotland. The AIBN noted that accident, and the one seven years later, have “clear similarities.” The two incidents involved a “near identical” main rotor gearboxes. And in both accidents, one of the eight second stage planet gears in the epicyclic module acquired a fatigue fracture. However, a warning of possible gear failure was deployed in the 2009 incident.
The lingering mystery about the cause of the 2016 accident would seem to make it unlikely that Norwegian or U.K. regulators will lift their ban on Super Puma flights, which were imposed after the crash.
The AIBN was not able to say when the full investigation would be completed.