**4. Conclusions**

In the present study, the robustness of a hybrid MCDM-based tool proposed by the authors for the aviation sector has been investigated. The latter is performed by accounting for a use case in which the sustainability of composite aircraft components is compared. The proposed tool combines lifecycle metrics linked to environmental, economic and circular economy aspects. Circular economy performance has been associated with a quality feature of the considered components. The tool is able to account for the type of fuel utilized during the use phase of the components. Although liquid hydrogen is not currently certified as an aviation fuel (SAF) by ASTM, its application is being actively researched and developed by various stakeholders in the aviation industry and is considered the most promising fuel option for future aircraft [1]. In this context, it is mandatory to consider and evaluate the sustainability of hydrogen as a potential fuel option for future aircraft.

The environmental impact and cost assessment of the examined components highlighted that a recycled component of near-to-virgin quality can potentially compete with a virgin component, accounting for its whole lifecycle. To this end, the utilization of liquid hydrogen from a renewable source appears necessary. Yet, to achieve a near-to-virgin quality, upgrade techniques and effective remanufacturing methods are required. Furthermore, it has been remarked that the use phase in the aviation sector dominates the overall impact; the latter signifies that the environmental emissions and costs linked to the production and manufacturing phases appear almost negligible compared to these of the use phase. However, when liquid hydrogen from a renewable source, especially from geothermy, has been accounted for, the impact of production and manufacturing comprises a considerable amount of the overall impact. The latter indicates that the decarbonization of the aviation sector may shift the environmental, at least, burden to the production and manufacturing phases. Moreover, although the environmental benefits of using liquid hydrogen are undeniable, the currently high costs of hydrogen compared to kerosene may act as a prohibiting factor for its extensive use in aviation. In addition, the impact of other aspects relating to the production, transportation and storage of liquid hydrogen must also be accounted for, although the latter assessments were outside the scope of this study.

The sensitivity of the MCDM tool to the normalization method applied, as well as to the weights and data variation, has been examined. The sensitivity analysis on the applied normalization method suggested the same ranking for the min-max and z-score methods, while the proportionate normalization method suggested a different ranking. Nonetheless, the first and the last ordered components are identical for all normalization methods. Moreover, the tool was found not to be sensitive to small variations of the weights; on the other hand, larger weights variations suggested a different ranking for the scenarios considered. Finally, the sensitivity analysis on the initial data values did not show a significant change in the final components' rankings compared to the initially obtained ones with respect to the different levels of noise variation for the three studied normalization methods. The latter remarks are quite encouraging and demonstrate the efficiency of the proposed tool as a reliable and robust decision-support tool for the aviation sector.

The goal of this work has been to enhance the reliability of the tool and bolster its credibility for making critical decisions in the aviation sector. Such decisions entail choosing suitable technologies, production and manufacturing procedures for components and materials, as well as determining the appropriate fuel for new aircraft. Future studies could focus on further validating the tool, including its sensitivity to different weights and criteria towards its practical use in the aviation industry, with input from a broader range of experts and stakeholders, ultimately contributing to more sustainable and informed decision-making.

**Author Contributions:** Conceptualization, D.N.M. and S.G.P.; methodology, D.N.M., S.M. and S.G.P.; software, D.N.M., S.M. and S.G.P.; validation, D.N.M., S.M. and S.G.P.; formal analysis, D.N.M., S.M. and S.G.P.; investigation, D.N.M., S.M. and S.G.P.; resources, S.G.P.; data curation, D.N.M., S.M. and S.G.P.; writing—original draft preparation, D.N.M., S.M. and S.G.P.; writing—review and editing, D.N.M., S.M. and S.G.P.; visualization, D.N.M., S.M. and S.G.P.; supervision, S.G.P.; project administration, S.G.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research conducted in this paper has been funded by European Union's Horizon 2020, its research and innovation program, under grant agreement No 101058089, project EuReComp (European recycling and circularity in large composite components).

**Data Availability Statement:** No more data is available due to pricacy restrictions.

**Acknowledgments:** The research conducted in this paper has been funded by European Union's.

**Conflicts of Interest:** The authors declare no conflict of interest.
