Follow-up of safety recommendations
Here are the replies to the 12 AIBN Safety recommendations that appear to have been received so far:
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, wearmarks and roughness.
Reply from EASA
The Agency intends to commission a research project, the scope of which will include identification of rotor drive system critical parts and associated damage mechanisms, identification of significant design, operational and environmental parameters, identification and characterization of significant threats and recommendation of design standards to ensure flaw tolerant structural integrity. The research project is listed as RES.008 (Rotorcraft main gear box (MGB) design to guarantee integrity of critical parts and system architecture to prevent separation of the main rotor following any MGB failure) in the draft European Plan for Aviation Safety 2019-2023, which is currently undergoing consultation with the Agency's advisory bodies.
EASA status – Open
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.
Reply from EASA
EASA will conduct a Preliminary Impact Assessment (PIA) in order to assess the need to amend the certification specifications for large rotorcraft (CS-29) with regard ·to procedures or instructions for continued airworthiness for critical parts on helicopters to maintain the design integrity after being subjected to any unusual event. Once the PIA is mature, stakeholders will be consulted. Consultation is expected to take place In 02Q2019.
Depending on the outcome of the PIA, EASA will include an appropriate task in the European Plan for Aviation Safety.
EASA status – Open
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.
Reply from EASA
EASA will conduct a preliminary impact assessment (PIA) to assess the need to 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 init iation and propagation and hence fatigue life. The aim of such AMC could be to add specific reference to modes of component structural degradation related to rolling contact fatigue and how these can affect crack initiation and propagation and, hence, fatigue life. Once the PIA is mature, stakeholders will be consulted. Consultation is expected to take place in 02Q2019.
Depending on the outcome of the PIA, EASA will include an appropriate task in the European Plan for Aviation Safety.
In the meantime, EASA is already raising the issue during certification projects via a dedicated Certification Review Item (CRI) providing Interpretative Material to better assess the effect of rolling contact fatigue.
EASA status – Open
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.
Reply from EASA
EASA has recognised the need to improve certification specifications in CS-27 (small rotorcraft) and CS-29 (large rotorcraft) relating to Main Gear Box (MGB) chip detectors.
The current CS 27 /29.130S(a)(23) and CS 27 /29.1337(e) require chip detectors to provide a warning to the flight crew when particles of a sufficient size (or accumulation) are detected and are intended to allow the flight crew to check the correct operation of the relevant elements ' of the drive system.
EASA has conducted a Preliminary Impact Assessment (PIA) on the possible actions to improve the likelihood of detecting chips or particles in gearbox oil. The outcome of the PIA was the inclusion of a dedicated Rulemaking Task (RMT) 0725 in the draft European Plan for Aviation Safety (EPAS) 2019-2023 which is currently undergoing consultation with the Agency's advisory bodies.
The planned RMT.0725 will consider an amendment of the current certification specifications and their associated acceptable means of compliance for demonstrating that the chip detectors perform their intended function.
EASA status – Open
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.
Reply from EASA
The Agency understands that the objective of this Safety Recommendation is that future rotor drive system design requirements will ensure that "no failure of internal MGB components should lead to a catastrophic failure." However, such designs would be so radically different from existing transmission systems that their feasibility needs to be assessed.
EASA considers that the number of potentially catastrophic failure modes should be minimised. Accordingly, any component, the failure of which has a potentially catastrophic failure effect, should not be acceptable if the failure hazard severity can be mitigated to a reduced level and where such measures are considered to be technically feasible and economically justifiable.
It is clear that design choices regarding rotor drive system architecture and individual gearbox design will influence the number of potentially critical parts.
In order to better understand the significance of these design choices, research is planned within the scope of project RES.008 (Rotorcraft main gear box (MGB) design to guarantee integrity of critical parts and system architecture to prevent separation of the main rotor following any MGB failure) in the draft European Plan for Aviation Safety (EPAS) 2019-2023, which is currently under consultation with stakeholders.
EASA status – Open
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.
Reply from EASA
EASA previously evaluated the suitability of the concept proposed in this safety recommendation, i.e. applying the ETOPS principles to helicopters conducting offshore operations, following the receipt of a safety recommendation from the CAA UK. At the end of 2015, EASA concluded that rulemaking was not deemed justified, owing to the differences in term of designs and operating conditions between helicopters and aeroplanes flying ETOPS.
EASA will re-evaluate its conclusion in the light of this safety recommendation. Depending on the results of the review of the concept, a preliminary impact assessment (PIA) may be prepared if deemed appropriate; the PIA would then review possible actions, including rulemaking, and a consultation of stakeholders would be performed.
EASA status – Open
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.
Reply from EASA
EASA will consider amending the Acceptable Means of Compliance (AMC) and Guidance Material (GM) to point 21.A.3A of Annex I (Part-21) to Commission Regulation (EU) No 748/2012, in order to clarify the obligations of Type Certificate Holders to ensure compliance with the requirement of "collecting, investigating and analysing reports of and information related to failures, malfunctions, defects or other occurrences which cause or might cause adverse effects on the continuing airworthiness of the product( ... )".
This will be performed within the frame of rulemaking task RMT.0031 dealing with the regular update of AMC/GM to Part-21. The next NPA is planned to be published 02Q2019.
EASA status – Open
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.
Reply from EASA
EASA issued Certification Memorandum (CM) CM-S-007 in 2015. The purpose of this CM was to supplement the existing guidance for compliance with CS 27 /29.602 (Critical Parts), detailing the need for post certification actions to verify the continued integrity of Critical Parts.
These actions should ensure that critical parts are controlled throughout their service life in order to maintain the critical characteristics on which certification is based. In addition, the effectiveness of any associated design, maintenance and monitoring provisions, which either help ensure the continued integrity or provide advance indication of impending failure of critical parts, should be assessed.
EASA will conduct a Preliminary Impact Assessment (PIA) in order to assess the potential safety benefit and economic: impact of a number of changes to improve the Guidance Material applicable to CS 29.602.
Consideration will also be made to include the provisions of CM-S-007 within the Acceptable Means of Compliance of CS-29 'Book 2'.
Consultation is expected to take place in 02Q2020.
EASA status – Open
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.
Reply from EASA
The Agency intends to commission a research project into rotorcraft gearbox health monitoring. The purpose of this research will be to investigate the use of new technologies, including both hardware and methods of analysis, to improve prognostic health monitoring capability for tilt rotor, helicopter and hybrid aircraft gearbox failures.
The scope of this research will include health monitoring of epicyclic gearbox components. This project is listed as RES.011 (Helicopter, tilt rotor and hybrid aircraft Gearbox health monitoring - ln-situ failure detection ) in the draft European Plan for Aviation Safety 2019-2023 which is currently undergoing consultation with the Agency's advisory bodies.
EASA status – Open
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.
No reply appears to have been received
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.
Reply from ICAO:
Amendment 16 to Annex 13 — Aircraft Accident and Incident Investigation, upgrades Recommendation 5.4.3 to a Standard and will become applicable on 8 November 2018. Standard 5.4.3 reads that “A State shall ensure that any investigations conducted under the provisions of this Annex have unrestricted access to all evidential material without delay.” And Standard 5.6 states that “The investigator-in—charge shall have unhampered access to the wreckage and all relevant material, including flight recorders and ATS records, and shall have unrestricted control over it to ensure that a detailed examination can be made without delay by authorized personnel participating in the investigation.”
Annex 13, 5.18 provides for the participation of accredited representatives appointed by the various States taking part in an investigation. Accredited representatives are entitled to appoint advisers who, in turn, would be able to obtain relevant information stemming from the aircraft manufacturer, type certification holder, operator, maintenance organizations and other information necessary for the investigation.
It is therefore believed that Annex 13 provisions, together With associated guidance material, provide the means to ensure that accident investigation authorities have timely access to relevant information and pertinent records necessary for the investigation.
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.
Reply from Airbus Helicopters:
Airbus is currently defining, together with authorities, the perimeter of a future set of evolutions in order to improve the robustness, the reliability and safety of the main gearbox in AS332 L2 and EC225 LP.
The associated roadmap will be presented beginning of 2019.