Rapport om luftfartsulykke nær Turøy, Øygarden kommune i Hordaland 29. april 2016 med Airbus Helicopters EC 225 LP, LN-OJF, operert av CHC Helikopter Service AS

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Beskrivelse

Helikopteret mistet plutselig og uten forvarsel hovedrotoren. Kort tid etter styrtet det på en holme utenfor Turøy, nordvest for Bergen. Alle de 13 personene om bord omkom i ulykken.

Den direkte årsaken til ulykken var utmattingsbrudd i et andretrinns planetgir i hovedgirboksen. Sprekkene startet i en liten skade på overflaten og utviklet seg uoppdaget under overflaten til en katastrofal feil.

Det er ingen sammenheng mellom ulykken og flygernes handlinger. Havarikommisjonen har også utelukket materialfeil og mekanisk svikt, samt vedlikeholdsfeil hos helikopteroperatøren.

Norsk sammendrag

Ulykken med LN-OJF

Fredag 29. april 2016 kl.1155 løsnet plutselig hovedrotoren fra et helikopter av typen Airbus Helicopters EC 225 LP Super Puma, registrert LN-OJF. Helikopteret var operert av CHC Helikopter Service AS og transporterte oljearbeidere for Statoil ASA (Equinor ASA fra 15. mai 2018). Da ulykken skjedde var helikopteret på vei fra plattformen Gullfaks B i Nordsjøen til Bergen lufthavn Flesland. Flygingen var helt normal og besetningen fikk ingen varsler før hovedrotoren løsnet fra helikopteret.

Fra en høyde på 2 000 fot styrtet helikopteret ned på en holme nær Turøy, nordvest av Bergen. Vrakdeler ble spredd over et stort område på ca. 180 000 m2 både på land og i sjøen. Hovedrotoren landet på en naboøy ca. 550 meter nord for helikoptervraket. Helikopteret ble knust i sammenstøtet, før mesteparten av vraket fortsatte ut i sjøen. Drivstoff fra helikopteret tok fyr og det begynte å brenne på holmen. Alle de 13 personene om bord omkom.

Undersøkelsesfunn

En omfattende undersøkelse har konkludert med at den direkte årsaken til ulykken var utmattingsbrudd i et av de åtte andretrinns planetgirene i hovedgirboksen. Utmattingssprekkene startet i en liten skade på overflaten, en såkalt micro-pit, på øvre ytre lagerbane (på innsiden av tannhjulet). Sprekkene vokste deretter usynlig under overflaten og videre til hele tannhjulet delte seg. Dette skjedde uten vesentlig avskalling (spalling), og sprekken ble ikke detektert.

Undersøkelsen har vist at kombinasjonen av materialegenskaper, overflatebehandling, design, belastning og driftsmiljø bidro til en type feil som ikke tidligere var forventet eller vurdert.

Havarikommisjonen har utelukket enhver sammenheng mellom ulykken og flygernes handlinger. Det er heller ingen funn som tyder på at vedlikeholdsfeil hos helikopteroperatøren har bidratt til ulykken. Feilen utviklet seg på en måte som vanskelig lot seg fange opp, verken av vedlikeholdsregimet eller overvåkingssystemene som var på LN-OJF på ulykkestidspunktet.

Sertifisering og luftdyktighet

EC 225 LP oppfylte konstruksjonskravene på sertifiseringstidspunktet i 2004. SHT har imidlertid funnet svakheter i de nåværende sertifiseringskravene for store helikopter, gitt av det europeiske flysikkerhetsbyrået (EASA).

Ulykken med LN-OJF har klare likheter med en ulykke med en Airbus Helicopters AS 332 L2 Super Puma utenfor kysten av Skottland i 2009 (G-REDL). Denne ulykken var også et resultat av et utmattingsbrudd i et andretrinns planetgir, men tiltakene som ble igangsatt var ikke tilstrekkelige for å forhindre et nytt tap av hovedrotor.

Planetgirene i EC 225 LP og AS 332 L2 helikoptrene hadde i praksis kortere levetid enn forventet før de ble kassert under vedlikehold. Kasserte planetgir ble ikke rutinemessig undersøkt og analysert av Airbus Helicopters slik at skadene og innvirkning på luftdyktighet ble grundig forstått.

Læringspunkter

Undersøkelsen gir viktig læring når det gjelder girbokskonstruksjon, sikkerhetsanalyser, vurdering av materialutmatting, tilstandsovervåking, sertifisering og oppfølging av luftdyktighet for AS 332 L2 og EC 225 LP helikopter, noe som også kan være gjeldende for andre helikoptertyper.

Basert på denne undersøkelsen, fremmer Statens havarikommisjon for transport 12 sikkerhetstilrådinger.

 

Sammendrag, vedlegg, bilder og video til nedlasting

Sikkerhetstilråding

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sikkerhetstilråding SL nr. 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.

Sted
Turøy, Hordaland
Hendelsesdato
29.04.2016
ICAO flyplassindikator
ENBR
Luftfartøy
Øvrige helikoptre
Operatør
CHC Helikopter Service
Reg. merke
LN-OJF
Flygeforhold
VMC
Fylke
Hordaland
Type hendelse
Luftfartsulykke
Type flyging
Kontinentalsokkel
Kategori luftfart
Tung, helikopter (> 2 250kg)
Flykategori
Helikopter, Multi-engine, Turboprop/turboshaft
FIR/AOR
ENSV (Stavanger ATCC)
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