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The information was released in the Accident Investigation Board of Norway’s (AIBN’s) latest preliminary report — its fourth since the investigation began — in which it said the fracture was in one of the aircraft’s second stage planet gears, but said the cause of the fracture had not yet been determined.
The aircraft (LN-OJF), operated by CHC Helicopter, had been returning to Bergen Airport Flesland from the Gullfaks B platform in the North Sea when a catastrophic failure resulted in its main rotor head and mast detaching in flight. The aircraft crashed into a small island near Turoy, exploding on impact, and killing all 13 on board.
The latest report drew comparisons to the fatigue fracture found in the second stage planet gear in the epicyclic module of an Airbus Helicopters AS332 L2 (G-REDL) that crashed off the coast of Peterhead, Scotland, in 2009. In that instance, a catastrophic failure of the helicopter’s main rotor gearbox caused the main rotor and part of the epicyclic module to separate from the fuselage, and all 16 on board were killed as the aircraft hit the sea at high speed. After an extensive investigation, the main rotor gearbox failure was found to be caused by a fatigue fracture of a second stage planet gear in the epicyclic module.
Since the last report, released on June 1, investigators said they have completed further examinations into the three different failure modes under consideration for the cause of the crash — suspension bar (lift strut) attachment, the MGB, and conical housing — and, at this stage, it believed the fatigue fracture of the planet gear to be the most likely cause of the loss of the main rotor. “It is considered unlikely that this fatigue crack propagated as a consequence of a structural break-up of another component,” the report states.
The AIBN said the accident aircraft’s gearbox had been involved in a road accident in 2015 — though it had been inspected, repaired and released for flight by Airbus Helicopters before it was installed in the aircraft in January 2016. “Whether there is a link between this event and the initiation and growth of a fatigue fracture, is being investigated,” the report states.
The report said the AIBN has completed detailed metallurgical examinations on various components from the accident aircraft, with the participation of Airbus Helicopters, and said two pieces “have been of particular interest.” These pieces represent about 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 percent fatigue,” the report states.
“An essential design philosophy regarding a possible failure inside the epicyclic module has been that propagation of a crack would be suppressed by the compressive surface stress,” the report states. “Thus a crack in the surface area should grow outboard and create spalling that would produce magnetic debris, which will be detected on the magnetic plugs (chip detectors). The optional HUMS [health and usage monitoring system] is an additional means for detecting developing degradation.”
The AIBN notes that this issue was discussed in connection with the crash of G-REDL, and that measures were taken to improve the detection of spalling.
“The observed failure mode in this investigation seems to differ from what was expected or foreseen during certification. AIBN believes that a sub-surface crack has propagated without creating a significant amount of magnetic debris from spalling. Also, the HUMS appears unable to identify symptoms of such degradation in the epicyclic module.”
Going forward, the AIBN said it will seek to determine the origin of the fatigue fracture and the mechanisms behind its growth. It said an extensive sea and land search for missing components had located parts key to the investigation, but that were still some important components missing, and it will consider a further sea search.
While the investigation continues, the global fleet of 179 Super Pumas remains largely grounded, with CHC, Bristow and Era among the operators who have grounded the type, while the Civil Aviation Authority of Norway and European Aviation Safety Agency have banned all flight with the H225/EC225LP and AS332L2.