**1. Introduction**

Airworthiness relates to the ability of an aircraft to conduct safe operations, and aircraft maintenance activities comprise its backbone [1]. Aircraft must be maintained and certified under regulatory standards published by regional authorities to ensure airworthiness in every private or commercial aircraft operation and achieve acceptable flight and ground safety levels while ensuring dispatch reliability. Under this mandate, in collaboration with aircraft manufacturers and aviation regulatory bodies, the Master Minimum Equipment List (MMEL) and Minimum Equipment List (MEL) were introduced in the late 1960s [2]. The MMEL and MEL are documents with a list of aircraft components or systems that may be inoperative for aircraft dispatch [3,4]. The former is developed by the aircraft manufacturer, and the latter is based on the MMEL but further customised by each airline depending on its distinct operational characteristics and needs. MMEL and MEL are reviewed, rejected or approved by the corresponding National Aviation Authority (NAA).

However, although several parties and professionals are involved in the development and review of the MMEL, the MEL still lacks a similarly standardised process. Typically, the MEL per operator and aircraft type is approved by the respective competent authority after it has been compiled according to generic guidelines [3–5] which describe what must be achieved but provide only a little guidance on how to develop a MEL. Although the MMEL can serve as the basis to build MEL, under a systems approach, the existence and performance of each component, as well as the combined e ffects of various malfunctioning components or systems, can resonate and lead to adverse situations that had not been anticipated when examining the performance of the former separately from their environment [6]. Thus, mere reliance on MMEL, which typically refers to behaviours of individual components and subsystems under assumed conditions, might not su ffice to publish a MEL proper for the operational environment of each air carrier.

Considering the above, the overall aim of our study was to examine the current situation around MEL and sugges<sup>t</sup> whether a more standardised framework is necessary. In the next section of the paper, we present an overview of the current MMEL/MEL development process and respective standards, and we refer to the associated topics of reliability, safety/acceptable level of safety, environment and human factors. The paper continues with a review of MEL-related studies and an analysis of MEL-related accidents and incidents to detect the types of relevant problems/issues and identify possible gaps in the MEL development and application process. After a discussion of the overall picture, our paper concludes with recommendations about the application of system engineering and socio-technical system approaches to the development of MELs.

### **2. Master Minimum Equipment List (MMEL)**/**Minimum Equipment List (MEL)**

Aircraft are designed with highly reliable equipment. Nevertheless, failures could occur at any time resulting in an accident/incident or simply flight cancellations and delays. The main objective of the Master Minimum Equipment List (MMEL) is to " ... reconcile an acceptable level of safety with aircraft profitability while operating an aircraft with inoperative equipment" [3]. MMELs/MELs are used to examine the release of an aircraft with inoperative equipment for flight. Their aim is to permit operation for a specific period under certain restrictions pending replacement or repair of the faulty item. However, the repair must be carried out at the earliest opportunity [2–4].

Before the introduction of MEL, the permission or not to operate with inoperative or underperforming systems/components was more a topic of negotiations between the operator and the regulator. Each regulator was forming its judgment and evaluation based on the competence of its sta ff, personal experiences, and information from previous cases depending on the type of aircraft under assessment [2]. This led to operators claiming favouritism when they had discovered that the list of permissively inoperative components and systems of aircraft of the same type belonging to another airline was less restrictive. The regulation and managemen<sup>t</sup> of MELs were institutionalised in the 1960s [2].

According to the International Civil Aviation Organisation (ICAO) [7], the overall goal of a MEL is to describe when an aircraft with inoperative components or systems is still airworthy and authorised for dispatch. Airbus [8] describes MEL as a document based on the MMEL and developed by the air operator to optimise flight planning and dispatch as well as the operator's profitability while maintaining an acceptable level of safety. Nowadays, the general principles governing the compilation, approval, maintenance, and monitoring of MEL are the following [3–5]:


Various professionals are involved in the process of MMEL [3,5]. Interactions within and across (sub)systems are extensively analysed, ensuring that multiple failures would not lead to an unsatisfactory level of safety by considering the impacts of critical failures and/or unserviceable items on flight safety, crew workload and operations. Table 1 illustrates the differences in the MMEL approval process between the Federal Aviation Authority (FAA) and the European Aviation Safety Agency (EASA).

**Table 1.** Differences between the Federal Aviation Authority (FAA) and the European Aviation Safety Agency (EASA) MMEL development and approval [5,9–11].

