**6. Conclusions**

In this study, we reviewed the current situation around the development and application of MEL as well as literature referring to respective challenges, and we performed an analysis of MEL-related events. Overall, the results of our analysis as presented in Table 5 agree with previous work and sugges<sup>t</sup> the need to revisit the processes related to MEL as demonstrated by the several issues mentioned in the safety investigation reports reviewed and related to unauthorised MEL dispatch, ambiguous MELs and airlines operating without a MEL. The lack of a systematic and uniform approach to MEL, apart from depriving the aviation industry of a standardised and reliable application of MEL, might have led to an underestimation of its importance and criticality for safe operations. The ambiguity detected in MEL-related standards along with the diversity of approaches to MEL in various regions might have contributed to building perceptions which suggest, on the hand, that the MEL is a quick-to-achieve compliance requirement by solely replicating or slightly changing MMELs, and, on the other hand, that individual defects emerging in day-to-day operations can be dealt only subjectively.

However, MEL is not just about the aircraft; it is about the aircraft in service operated by humans within a specific environment. While manufacturers try to anticipate varying environment conditions when compiling MMELs, the latter alone are usually inadequate for the development of MELs. As stressed by Leveson [6], system failures do not occur only as a result of random and individual component malfunctions; instead, we must consider interactions between socio-technical system agents (i.e., technology, end-users, organisations, regulators, society and environment) within and across system levels, which are often neglected during current MEL practice. Therefore, in line with the views of Leveson [6], the MEL development process should be viewed as a dynamic process involving the NAAs, air operators, pilots, engineers and flight dispatchers. The work of Karanikas and Chatzimichailidou [49] who suggested a combined qualitative and quantitative approach to compare system configurations encapsulates the newly introduced Systems-Theoretic Process Analysis (STPA) technique [6] and is an example of how system analysis could consider (1) non-binary behaviours of system agents, (2) the degree of influence of each agen<sup>t</sup> on system performance, and (3) the criticality of each agent. The particular approach, as well as any other approach that encompasses systems engineering and socio-technical principles, could comprise the basis for a holistic and systematic methodology for MEL development and application and address the weaknesses of currently used techniques as presented in Section 2.1 of this paper.

**Funding:** This research received no external funding.

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