Analysis concluded after the fatal crash of an Airbus Helicopters H225 last year revealed that gearbox components made by one of two suppliers were more susceptible to surface degradation, Norwegian investigators have disclosed.
The latest interim report from the SHT agency into the fatal loss of the CHC Helikopter Service-operated rotorcraft (LN-OJF) has been released to coincide with the one-year anniversary of the event.
Norwegian investigators have yet to determine the root cause for the 29 April 2016 accident, but reiterate their belief that the loss of the main rotor was triggered by the failure of a second-stage planet gear in the epicyclic module of the main gearbox.
The same fault was pinpointed as the cause of a 2009 crash off the coast of Scotland in which an AS332 L2 (G-REDL) – which shares the same epicyclic module as the H225 – came down 50min into its flight.
That component was dual-sourced by Airbus Helicopters from bearing manufacturers FAG and SNR. Investigations launched by Airbus Helicopters in the wake of LN-OJF noted that parts made by FAG were more than three times more susceptible to spalling – or surface degradation of a bearing – than those made by SNR.
In the period 2001-2016, analysis by the airframer revealed “more spalling events on FAG planet gears than SNR”, says SHT. A total of 22 instances were found on helicopters with the FAG component, versus seven with the SNR.
In both fatal crashes, the failed second-stage planet gear was supplied by FAG. The German bearing manufacturer points out, however, that its “production process has been approved by Airbus Helicopters and has been frozen and unchanged since the beginning of production”.
The report says: "FAG informed that together with Airbus Helicopters, they had reviewed the bearing [life] calculation after the LN-OJF accident. They found no discrepancies from the initial calculations and the approvals from Airbus Helicopters. Further, an internal quality review confirmed that there were no deviations in the manufacturing process."
As part of the safety barriers initiated to enable the H225’s return to service, Airbus Helicopters has mandated that the FAG-made planet gears be replaced.
SHT has yet to determine the root cause for the initiation and propagation of the crack. It believes it began in a "surface micro-pit" in the gear material but spread below the surface, crucially failing to cause any tell-tale metallic particles to be released.
These might then have been picked up by the magnetic chip plugs fitted to the helicopter, providing an early indication of an issue with the gearbox.
However, as the report notes, tests conducted by Airbus Helicopters following the G-REDL crash showed that just 12% of any particles released would be picked up by the chip plugs. Analysis conducted by investigators showed that just 28mm² of material had been released from the planet gear race, leading them to conclude that only 3.3mm² would be detectable, "far below the limit of 50mm² for [main gearbox] removal at the time".
In the event, the second-stage planet gear failed without warning and caused the H225's main rotor to separate while the helicopter was at 2,000ft and travelling at 140kt (260km/h); none of the 13 occupants survived its subsequent uncontrolled descent.
SHT has also widened its probe to examine the certification standards applied to the gearbox. Because it had been introduced on the AS332 L2, which gained French airworthiness approval in 1991, no additional fatigue testing was required on the component during the H225’s validation process in the early 2000s.
"As the first and second stage planet gears and sun gear in the epicyclic module had not changed from the AS332 L2, the earlier requirements were used," says SHT.
UK investigators, in their final report into G-REDL, concluded that the European regulator and Airbus Helicopters should "re-evaluate the continued airworthiness of the MGB [main gearbox] to ensure that it satisfies the latest certification requirements".
However, the subsequent effort did not lead to any "additional changes in design, operational life limits or inspection process of the planet gears", SHT notes.
It also questions the assertion that the removal of a ring of magnets from the gearbox – part of an effort to improve chip detection in the wake of G-REDL – was sufficient.
"For LN-OJF, the fact that the MGB degradation went on undetected by the chip detection system cannot be explained by the ring of magnets or by human factors/maintenance failure, as was the case with G-REDL," it says.
"A more detailed assessment of the actions in response to the safety recommendations, and how they were documented and closed by EASA and Airbus Helicopters will follow in the final report."
However, SHT has ruled out one area of inquiry. Earlier preliminary reports had suggested that the gearbox could have been damaged in a road traffic accident in March 2015, but the agency says it could find "no physical evidence" to link the two events.
The AS332 L2 and H225 remain grounded by Norway and the UK; the agencies have indicated that their positions have not changed in the light of the latest reports.
Both civil aviation regulators are due to meet again with Airbus Helicopters on 4 May.
Guillaume Faury, Airbus Helicopters chief executive, says: "We wish to express our deep regret at this tragedy, and again, we offer our sincere and profound sympathies to the bereaved families."
He points out that the manufacturer was unaware of any "related issues" from G-REDL at the time of the Norwegian accident, but has now put additional safety measures in place.
"We will continue to work with the European Aviation Safety Agency (EASA) and to comply with its airworthiness requirements," he says.
"In the course of the investigation into the 2016 accident, we have implemented a set of protective measures which have been requested and validated by EASA. Nothing in this preliminary report alters this."
Source: FlightGlobal.com
