It seems a long time ago. On 29th April 2016, the Main Rotor Head and mast suddenly detached from an Airbus Helicopters H225 flying from Gullfaks B to Bergen Airport Flesland. The helicopter impacted on a small island east of Turøy. As the final accident report is published our thoughts have to be with the relatives, friends and work colleagues of the 13 passengers and crew who perished on that day. In April 2017 the Accident Investigation Board Norway (‘AIBN’) issued a Preliminary Report concluding that
“…the accident was a result of failure of the main rotor gearbox (MGB) due to a fatigue fracture in one of the eight second stage planet gears in the epicyclic module.” (para 2.1.1)
The crew, engineers and operator couldn’t have done anything to avoid this accident. There were no indications that the Main Gear Box (‘MGB’) was moving towards a catastrophic event. The main question that remained unanswered until this Final Report was issued on 5th July 2018 was why, in such a heavily regulated aviation industry, two similar catastrophic accidents could happen to helicopters with near identical MGBs only seven years apart? There were clear similarities to an AS 332L2 accident off the coast of Scotland in 2009 (G-REDL). The AIBN’s Preliminary Report was also critical of the European Aviation Safety Agency (‘EASA’) regulatory oversight that enabled a return to service of the H225 without clear assurances that the root causes of the accident had been identified and remedied (see HUMS: Safety Alert, February 3rd 2017 Helihub.com, and LN-OJF Preliminary Report: H225 Concerns Remain Open, May 3rd 2017 Helihub.com).
The AIBN’s Final Report (reference 2018-04) can be found on the AIBN reports page. It is comprehensive at 171 pages with more detailed investigations confirming the findings in the Interim Report and making 12 safety recommendations.
The Final Report is also worrying. The following are the key features that cause most concern:
- There is no way of assuring detection of possible failure of the MGB sufficiently in advance of total failure. There was no spalling detected during the growth in cracks in the planet gears and thus little or no probability of detecting cracks growing before a complete fracture (section 184.108.40.206), the investigation discovering that the oil cooler acted as a particle trap preventing 44% of the debris from reaching the oil filter including the largest particles (section 2.5.7). Airbus have improved the capture of particles in the oil by introducing a new full flow magnetic plug increasing the capture rate from 12% to 50% and strengthened inspection criteria and equipment allowing a more detailed analysis of these particles.
- Airbus Helicopters and the European Aviation Safety Agency (EASA) did not realise the safety potential from the G-REDL accident report (section 220.127.116.11). The G-REDL report was described as a “turning point with respect to the continued airworthiness of the AS 332 L2 and the EC 225 LP helicopters”, the UK’s Air Accident Investigation Branch (‘AAIB’) making safety recommendations clearly relevant to the LN-OJF accident which were not followed through by EASA or Airbus (section 18.104.22.168). In particular, the Final Report of the AIBN (section 3.2.9) found that:
- “Less than 10 % of the second stage planet gears ever reached their intended operational time before being rejected during overhaul inspections or non-scheduled MGB removals due to signs of degradation.
- Airbus Helicopters did not perform systematic examination and analyses of unserviceable and rejected second stage planet gears in order to understand the full nature of any damage and its effect on continued airworthiness.
- Airbus Helicopters did not section and inspect any of the second stage planet gears that were scrapped during overhaul. Therefore, it remains uncertain whether any of these gears had subsurface cracks similar to observations made on LN-OJF.
- The differences between the two planet gear bearing designs had not been previously assessed through in-service statistics and calculations.”
- EASA and Airbus acted precipitately when EASA removed the flight prohibition on the H225 on 7 October 2016, with the investigation ongoing and important aspects still open: “The AIBN understands EASA’s role and Airbus Helicopters’ position, but would have expected a more precautionary approach at the time, since the accident involved a critical part in which failure has led to two catastrophic events” (section 22.214.171.124).
- The following issues are currently not fully resolved (section 3.2.11):
- Data, analyses and tests do not conclusively prove that the planet gears still in service will not have the potential to develop subsurface and possible undetectable fatigue cracks from a surface damage;
- The capability to detect and interpret metal particles of few mm2 in the MGB oil system to prevent critical failure;
- Why the cracks in the outer race grew subsurface into the gear bulk material and finally resulted in a fatigue fracture while creating limited spalling.
Airbus do state that they have reduced these risks by replacing planet gears equipped with the type present on LN-OJF to the alternative manufacturer, while at the same time reducing the life limit.
- “Two catastrophic events and the service experience of few second stage planet gears reaching their operating time limit, may suggest that the operational loading environment, on both AS 332 L2 and EC 225 LP, is close to the limit of endurance for the design” (section 126.96.36.199).
In response to the Final Report, Gilles Bruniaux, Head of Aviation Safety issued a statement on the Airbus website which includes the following quote:
“the available degree of scientific and technical knowledge meant it was neither foreseeable nor foreseen that a crack in a plant gear could propagate in a sub-layer, and as a result generate very low levels of detectable particles…With knowledge gained from this investigation, Airbus Helicopters has introduced a series of safety measures on the H225. Some of the technology that has been developed is ground-breaking for the helicopter industry. Airbus Helicopters will continue to pursue innovations and improve safety standards through a proactive approach that sees us challenge internally everything we do. Work on a number of potential improvements to the H225 are in progress, and I remain optimistic that this concerted and complex work will yield new advances.”
HeliHub.com has estimated that 126 H225s and AS332L2s are currently in storage around the world. Until the issues raised in this accident report are fully resolved, Accountable Managers will no doubt be thinking that that is where they should stay.
*Copyright and full responsibility for the content of this article remains with Allan Blake, an independent Aviation Consultant & Journalist. He is the author of “Dynamic Directors: Aligning Board Structure for Business Success” (Macmillan).
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