A new preliminary report into a helicopter crash which killed 11 offshore workers and two pilots has pointed to a fatigue fracture in the aircraft's gear box as the most likely cause.
The Airbus Helicopters H225, operated by CHC Helicopters. crashed suddenly and fatally near Bergen, Norway, on 29 April this year when the main rotor head and mast suddenly detached. The aircraft had been returning from Statoil's Gullfaks B platform to Bergen Airport Flesland.
"At this stage of the investigation, the AIBN finds that the accident most likely was a result of a fatigue fracture in one of the eight second stage planet gears," the Accident Investigation Branch Norway (AIBN) said. "It appears that the fracture has propagated in a manner which is unlikely to become detected by existing mandatory or supplementary systems for warning of an imminent failure. What initiated the fracture has not yet been determined."
The main gearbox had been installed on the aircraft in January this year after modifications, inspection and repair at Airbus Helicopters. At the time, it had accumulated 1080hrs operation since new. At the time of the accident, it had reached 1340hrs.
The investigation into the report has included detailed metallurgical examinations, including CT scans of the parts, at QinetiQ in Farnborough, UK.
The report says that usually, if there is a failure in the epicyclic module, containing the planet gears, crack propagation would be suppressed by compressive surface stress and result in spalling, which would be detected by chip detectors. However, there were no records of debris from the inspections on the gearbox, the report says. "AIBN believes that a sub-surface crack has propagated without creating a significant amount of magnetic debris from spalling," and a detection system "appears unable to identify symptoms of such degradation in the epicyclic module."
The European Safety Agency (EASA) also temporarily grounded all H225 Super Puma flights following a 1 June preliminary report from the AIBN. That report highlighted that the current means to detect a failure in this area were not effective.
Statoil described the crash as: "One of the most severe helicopter accidents in the history of the Norwegian oil industry."
Of the 13 killed, 11 worked for Halliburton, Aker Solutions, Schlumberger, Welltec, Karsten Moholt and Statoil. Two pilots were employed by CHC.
Following the incident, UK and Norwegian authorities restricted use of the Super Puma types EC225L and AS332LP for commercial flights. The ban was later extended to search and rescue aircraft.
The week following the disaster, CHC filed for Chapter 11 relief in the US. The company was also delisted from the New York Stock Exchange earlier this year after it failed to keep a market capitalization of US$15 million.
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Initial crash findings released
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