Report on the air accident near Turøy, Øygarden municipality, Hordaland county, Norway 29 April 2016 with Airbus Helicopters EC 225 LP, LN-OJF, operated by CHC Helikopter Service AS

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Description

The main rotor suddenly detached from the helicopter without warnings. Shortly after, the helicopter impacted a small island near Turøy, northwest of Bergen. All 13 persons on board perished.

The accident was a result of a fatigue fracture in a second stage planet gear in the epicyclic module of the main rotor gearbox. Cracks initiated from a micro-pit at the surface and developed subsurface to a catastrophic failure without being detected.

There are no connections between the crew handling and the accident. The AIBN has also excluded material unconformity and mechanical failure, as well as maintenance actions by the helicopter operator.

Summary report

The accident with LN-OJF

On 29 April 2016 the main rotor suddenly detached from an Airbus Helicopters EC 225 LP Super Puma, operated by CHC Helikopter Service AS. The helicopter transported oil workers for Statoil ASA and was en route from the Gullfaks B platform in the North Sea to Bergen Airport Flesland.

The helicopter had just descended from 3,000 ft and had been established in cruise at 140 kt at 2,000 ft for about one minute. The flight was normal and the crew received no warnings before the main rotor separated from the helicopter.

The helicopter impacted a small island near Turøy, northwest of Bergen. Wreckage parts were spread over a large area of about 180,000 m2 both at land and in the sea. The main rotor fell about 550 meters north of the crash site. The impact forces destroyed the helicopter, before most of the wreckage continued into the sea. Fuel from the helicopter ignited and caused a fire onshore. All 13 persons on board perished.

Investigation findings

An extensive and complex investigation revealed that 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 (MGB). The fatigue fracture initiated from a surface micro-pit in the upper outer race of the bearing, propagating subsurface while producing a limited quantity of particles from spalling, before turning towards the gear teeth and fracturing the rim of the gear without being detected.

The investigation has shown that the combination of material properties, surface treatment, design, operational loading environment and debris gave rise to a failure mode which was not previously anticipated or assessed.

There are no connections between the crew handling and the accident. Nor is there any evidence indicating that maintenance actions by the helicopter operator have contributed to this accident. The failure developed in a manner which was unlikely to be detected by the maintenance procedures and the monitoring systems fitted to LN-OJF at the time of the accident.

Certification and continued airworthiness

The design of the EC 225 LP satisfied the requirements in place at the time of certification in 2004. However, the AIBN has found weaknesses in the current European Aviation Safety Agency (EASA) Certification Specifications for Large Rotorcraft (CS-29).

The accident has clear similarities to an Airbus Helicopters AS 332 L2 Super Puma accident off the coast of Scotland in 2009 (G-REDL). This accident was also identified to be the result of fatigue fracture in a second stage planet gear, however the post-investigation actions were not sufficient to prevent another main rotor loss.

The investigation has found that only a few second stage planet gears ever reached their intended operational time before being rejected during overhaul inspections or non-scheduled MGB removals. The parts rejected against predefined maintenance criteria were not routinely examined and analysed by Airbus Helicopters in order to understand the full nature of any damage and its effect on continued airworthiness.

Lessons learned

From this investigation there are significant lessons to be learned related to gearbox design, safety assessment, fatigue evaluation, condition monitoring, certification requirements and continued airworthiness of the AS 332 L2 and the EC 225 LP helicopters, which also could be valid for other helicopter types.

Based on this investigation, the AIBN issues 12 safety recommendations.

 

Appendices, summary, pictures and video for download

Safety recommendation

Safety recommendation SL No. 2018/01T

The failure mode, i.e. crack formation subsurface with limited spalling initiated from a surface damage, observed in the LN-OJF accident is currently not fully understood. The investigation has shown that the combination of material properties, surface treatment, design, operational loading environment and debris gave rise to a failure mode that was not previously anticipated or assessed.

The Accident Investigation Board Norway recommends that the European Aviation Safety Agency (EASA) commission research into crack development in high-loaded case-hardened bearings in aircraft applications. An aim of the research should be the prediction of the reduction in service-life and fatigue strength as a consequence of small surface damage such as micro-pits, wear marks and roughness.

Safety recommendation SL No. 2018/02T

The MGB, which was later installed in LN-OJF, fell off a truck during transport. It was inspected, repaired and released for flight by Airbus Helicopters without detailed analysis of the potential effects on the critical characteristics of the MGB. The current regulatory framework for large rotorcraft does not make connections between the Instructions for Continued Airworthiness (ICA) and requirements for critical parts subject to an unusual event.

The Accident Investigation Board Norway recommends that the European Aviation Safety Agency (EASA) assess the need to amend the regulatory requirements with regard to procedures or Instructions for Continued Airworthiness (ICA) for critical parts on helicopters to maintain the design integrity after being subjected to any unusual event.

Safety recommendation SL No. 2018/03T

Rolling contact fatigue as observed in the LN-OJF accident was not considered during type certification, neither is it directly addressed in the current certification specifications.

The Accident Investigation Board Norway recommends that European Aviation Safety Agency (EASA) amend the Acceptable Means of Compliance (AMC) to the Certification Specifications for Large Rotorcraft (CS-29) in order to highlight the importance of different modes of component structural degradation and how these can affect crack initiation and propagation and hence fatigue life.

Safety recommendation SL No. 2018/04T

The chip detection system fitted to LN-OJF did not produce any warnings of the impending planet gear catastrophic failure, and the potential of detection was limited. The Certification Specifications for Large Rotorcraft (CS-29) do not specify the chip detection system’s functionality and performance.

The Accident Investigation Board Norway recommends that the European Aviation Safety Agency (EASA) revise the Certification Specifications for Large Rotorcraft (CS-29) to introduce requirements for MGB chip detection system performance.

Safety recommendation SL No. 2018/05T

The LN-OJF accident was a result of a fatigue fracture in one of the eight second stage planet gears in the epicyclic module of the MGB, a critical part in which cracks developed subsurface to a catastrophic failure without being detected. It might not be possible to assess the fatigue reliability of internal MGB components, or design a warning system that works with sufficient efficiency and warning time, thus the MGB should be designed fail-safe.

The Accident Investigation Board Norway recommends that the European Aviation Safety Agency (EASA) develop MGB certification specifications for large rotorcraft to introduce a design requirement that no failure of internal MGB components should lead to a catastrophic failure.

Safety recommendation SL No. 2018/06T

The investigation into the accident to LN-OJF has revealed that the tests performed during the design and certification of the Airbus Helicopters EC 225 LP were in accordance with applicable regulations. However, with regard to the risks associated with offshore operations, there is a less stringent continued operational reliability test requirement for large rotorcraft compared with the Extended Operations and All Weather Operations regime for fixed wing aircraft.

The Accident Investigation Board Norway recommends that the European Aviation Safety Agency (EASA) develop regulations for engine and helicopter operational reliability systems, which could be applied to helicopters which carry out offshore and similar operations to improve safety outcomes.

Safety recommendation SL No. 2018/07T

The investigation into the accident to LN-OJF has found that only a few second stage planet gears in Airbus Helicopters EC 225 LP and AS 332 L2 ever reached their intended operational time before being rejected during overhaul inspections or non-scheduled MGB removals. The parts rejected against predefined maintenance criteria were not routinely examined and analysed by Airbus Helicopters.

The Accident Investigation Board Norway recommends that European Aviation Safety Agency (EASA) make sure that helicopter manufacturers review their Continuing Airworthiness Programme to ensure that critical components, which are found to be beyond serviceable limits, are examined so that the full nature of any damage and its effect on continued airworthiness is understood, either resulting in changes to the maintenance programme, or design as necessary, or driving a mitigation plan to prevent or minimise such damage in the future.

Safety recommendation SL No. 2018/08T

The investigation into the accident to LN-OJF has found that only a few second stage planet gears in Airbus Helicopters EC 225 LP and AS 332 L2 ever reached their intended operational time limit before being rejected during overhaul inspections or non-scheduled MGB removals.

The Accident Investigation Board Norway recommends that the European Aviation Safety Agency (EASA) review and improve the existing provisions and procedures applicable to critical parts on helicopters in order to ensure design assumptions are correct throughout its service life.

Safety recommendation SL No. 2018/09T

The investigation into the accident to LN-OJF has demonstrated that a critical structural component could fail totally without any pre-detection by the existing monitoring means.

The Accident Investigation Board Norway recommends that the European Aviation Safety Agency (EASA) research methods for improving the detection of component degradation in helicopter epicyclic planet gear bearings.

Safety recommendation SL No. 2018/10T

During the investigation into the accident to LN-OJF, considerable time and resources by the AIBN has been drawn to request, wait for release acceptance and review of design and certification documents.

The Accident Investigation Board Norway recommends that the European Commission (DG MOVE) in collaboration with European Aviation Safety Agency (EASA) evaluates the means for ensuring that investigation authorities have effectively free access to any relevant information or records held by the owner, the certificate holder of the type design, the responsible maintenance organisation, the training organisation, the operator or the manufacturer of the aircraft, the authorities responsible for civil aviation, EASA, ANSPs and airport operators.

Safety recommendation SL No. 2018/11T

During the investigation into the accident to LN-OJF, considerable time and resources by the AIBN has been drawn to request, wait for release acceptance and review of design and certification documents. ICAO Annex 13 Chapter 5.12 does not refer explicitly to the protection of sensitive proprietary information regarding design and certification.

The Accident Investigation Board Norway recommends that the International Civil Aviation Organisation (ICAO) evaluates the means for ensuring that investigation authorities have effectively free access to any relevant information or records held by the owner, the certificate holder of the type design, the responsible maintenance organisation, the training organisation, the operator or the manufacturer of the aircraft, the authorities responsible for civil aviation, certification authorities, ANSPs and airport operators.

Safety recommendation SL No. 2018/12T

The LN-OJF accident was a result of a fatigue fracture in one of the eight second stage planet gears in the epicyclic module of the MGB, a critical part in which cracks developed subsurface to a catastrophic failure without being detected. With the knowledge from this investigation, all effort should lead to a robust design in which a single load path should demonstrate compliance to CS 29.601(a), 29.602 and 29.571 without compromising its structural integrity and not only by depending on detection systems or maintenance checks.

The Accident Investigation Board Norway recommends that Airbus Helicopters revise the type design to improve the robustness, reliability and safety of the main gearbox in AS 332 L2 and EC 225 LP.

Location
Turøy, Hordaland, Norway
Occurrence date
29.04.2016
ICAO Location indicator
ENBR
Aircraft
Øvrige helikoptre
Registration
LN-OJF
Meteorological conditions
VMC
County
Hordaland
Type of occurrence
Accident
Type of operation
Offshore
Category of operation
Heavy, helicopter (> 2 250kg)
Aircraft category
Helicopter, Multi-engine, Turboprop/turboshaft
FIR/AOR
ENSV (Stavanger ATCC)
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