EC225 crash near Bergen, Norway April 2016

[Mars]

The following article from 'Helihub' appears to capture reservations held by a number on PPRune.

"The Final Report is also worrying. The following are the key features that cause most concern:

1. 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 2.4.2.6), 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.
2. Airbus Helicopters and the European Aviation Safety Agency (EASA) did not realise the safety potential from the G-REDL accident report (section 2.8.6.2). 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 2.1.1.3). 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.”
3. 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 2.14.2.4).
4. 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.

1. “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 2.14.4.1).

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.”"