Norwegian investigators seeking causes of the fatal April 29 CHC EC225 crash are focused on a main gearbox fatigue failure that certification processes may not have been foreseen and maintenance procedures may have been unable to detect.
The Accident Investigation Board of Norway said the most likely cause of the Airbus Helicopters Super Puma's main rotor separation in flight was fatigue cracking that led a second-stage planet gear to disintegrate and rip the main gearbox apart. The resulting crash killed 11 passengers and two pilots on the CHC Helikopter Service aircraft.
The board has considered other causal scenarios, such as a main gearbox suspension bar attachment or conical housing failure. But investigative efforts to date "do not suggest that either of these scenarios were the initiating event," the board said in a June 28 update.
Furthermore, the board said, "it is considered unlikely that this fatigue crack propagated as a consequence of a structural break-up of another component."
The failure of the second-stage planet gear, which is in the main gearbox's epicycle module, is similar to the one cited by the U.K. Air Accidents Investigation Branch's investigation of the 2009 crash of a Bond Offshore AS332 L2 Super Puma. While there are differences in the evidence gathered in the two investigations, the Norwegian board said, "the fatigue-fractured planet gears, however, show clear similarities."
Gearbox components generally are designed to fail gradually enough that small chips would break free and collect in magnetic chip detectors, triggering a warning, inspection and maintenance long before a component like the second-stage planet gear could suffer a sudden, catastrophic failure.
"The observed failure mode in this investigation seems to differ from what was expected or foreseen during certification," the Norwegian board said. It appears that the second-stage planet gear fracture “propagated in a manner which is unlikely to become detected by existing mandatory or supplementary systems for warning of an imminent failure.”
The board added that it found no evidence of malfunctions of the accident helicopter's magnetic debris detection or a failure by CHC “to follow procedures for visual inspection and checks before flight.” It said it also found no record that magnetic debris had been found in inspections of the gearbox since it was installed on the EC225 on Jan. 15, after Airbus returned it to CHC following modification, inspection and repair. When it was installed, the main gearbox had 1,080 hr since its original manufacture date, the board said. At the time of the accident, it had accumulated about 260 more hours.
The board also said the EC225 health and usage monitoring system (HUMS) “appears unable to identify symptoms of such degradation in the epicyclic module.”
The board cited metallurgical examinations performed for it by Qinetiq in the U.K. of two pieces of the failed gear, which together make up about half of the gear. (While some components of the crashed helicopter have not been accounted for yet, the board said, “parts key to the investigation have been recovered.”)
The second-stage planet gear serves two roles. Its face consists of gear teeth that mesh with the second stage’s ring gear. Its inner face forms the outer race of a roller bearing in the second-stage assembly.
The board said Qinetiq’s examinations indicate the fatigue originated in this outer race (on the interior of the gear) and propagated out toward the gear teeth. X-ray computed tomography (CT) scans found several subsurface cracks on this bearing race.
The board said it believes “that a sub-surface crack has propagated without creating a significant amount of magnetic debris” (in a process called spalling).
The board is continuing to investigate how the fatigue first began and to determine the loading cycles that contribution to cracks’ growth and how quickly they grew, or propagated. It said the gearbox was involved in a road accident during transport in 2015 and is assessing whether there is a link between that road accident and the initiation and growth of a fatigue fracture.
Photo courtesy of CHC Helicopter