The Accident Investigation Board of Norway (AIBN) issued another preliminary report on April 28 about the fatal crash of a CHC Airbus Helicopters EC225LP (LN-OJF) near Turoy on April 26 last year that killed all 13 people on board. The helicopter was cruising at 140 knots at 2,000 feet when the main rotor separated from the aircraft.
Throughout last summer, most of the worldwide fleet of EC225s remained grounded. While the EASA cleared the type for return to flight in October contingent on a more stringent inspection regime and component replacement, it remains grounded by authorities in Norway and the UK, and several operators elsewhere have declined to return the type to service. The latest AIBN report is not final and the investigation is continuing. However, it concludes that the cause of 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 AIBN's findings and conclusions to date will provide little comfort for Airbus, which is facing litigation and continuing public relations fallout from the accident as it attempts to rebuild market confidence in the EC225. The accident has significantly hurt resale values, and energy workers' unions have expressed concerns about their members flying in the aircraft.
The AIBN concluded that the planet gear failed without warning and that onboard systems were unable to detect its degradation before failure. The board noted, “The observed failure mode in this accident (crack initiation and propagation with limited spalling) seems to differ from what was expected or foreseen during the design and certification of the main rotor gearbox. The fracture propagated in a manner that was unlikely to be detected by the maintenance procedures and the monitoring systems fitted to LN-OJF at the time of the accident. The certification process and certification specifications for large rotorcraft related to catastrophic failure, and requirements for safety barriers will be subject to further investigation.”
Similarities to Previous Accident
The board said there are marked similarities between the crash of LN-OJF and the 2009 crash of an Airbus AS332L2 off the coast of Scotland (G-REDL). “The crack propagation path in the retrieved second-stage planet gear in this accident (LN-OJF) appears to be very similar to the estimated crack growth in G-REDL. The AIBN will continue the investigation into how and why two similar catastrophic accidents could happen to nearly identical helicopters only seven years apart. Further assessment of the follow-up on the G-REDL safety recommendations and the continuing airworthiness of the gearbox after 2009 is a relevant issue.”
After the release of the report, Airbus Helicopters CEO Guillaume Faury issued this statement: “In the course of the AIBN investigation, additional elements brought by the 2016 accident showed that there were mechanical similarities between the 2016 accident and an earlier one in 2009. We were not aware of any related issue at the time of the 2016 accident. The information available to us from the 2016 accident has allowed us to take protective measures that we could unfortunately not have put in place in 2009 based on the knowledge and evidence available at the time, and also because significant parts from the 2009 accident were never recovered.”
However, the AIBN noted the crash of a Super Puma-series helicopter with similarities to LN-OJF's—an Aerospatiale SA330J on Dec. 16, 1980 in Brunei, in which the entire main rotor system detached in flight. Investigators in that accident pointed at “planetary gear failure in the second stage of the two-stage epicyclic main gearbox reduction gear” as the likely cause. Between the 2009 and 2016 accidents, Airbus reported that only two second-stage planet gears were removed from epicyclic gearboxes as a result of spalling or micro-pits on the outer race—similar to the damage observed from LN-OJF. None of the damaged gears was subjected to laboratory analysis.
While the EC225LP has a chip detection system, the AIBN noted that only the mast bearing chip detector, the epicyclic module chip detector and the sump chip detector are connected to a flight crew warning circuit; the oil cooler chip detector is not connected to any warning system and must be inspected visually. Four days and 15 flight hours before the Turoy accident, visual inspections of LN-OJF's chip detectors found no magnetic debris. The AIBN reported that Airbus Helicopters has “performed a worldwide survey on the detection technologies (mainly vibration but not limited to) of cracks inside an epicyclic train for relevant industries. Their conclusion is that no solution currently exists on the market for such degradation detection.”