*2.1. Methods of MMEL Justification*

Before creating an MMEL, a thorough analysis must be conducted quantitatively and qualitatively to justify whether a component/system should be included in the list. The main tools used for these analyses are the Failure Mode Effect Analysis (FMEA) and Fault Tree Analysis (FTA) [12,13]. While these tools are meticulous and versatile for performing safety assessments, there are also limitations associated with their use. Tables 2 and 3 highlight the advantages and limitations of FMEA and FTA correspondingly [14–16].

The application of FMEA and FTA is based on results from qualitative and quantitative analyses and considers any optional and redundant equipment to inform decisions and develop a MMEL. Qualitative analysis is carried out before quantitative analysis [3,4,13]. It includes an evaluation of the effects of inoperative or underperforming components and systems on aircraft operation, flight crew workload and passenger safety to assess the achievement of an acceptable level of safety for dispatch [4]. Additionally, the qualitative analysis must ensure that the combined impact of multiple inoperative pieces of equipment will not lead to a catastrophic/hazardous failure [4]. Quantitative analysis supplements qualitative analysis [3,4] and is performed for items/equipment/components that are characterised as critical to the safe operation of the aircraft [4]. Furthermore, additional analysis may be required to analyse the rectification interval of each component or system [13]. This type of analysis adopts the System Safety Assessment (SSA) process which is based on the quantitative results from the FTA or FMEA techniques [12,13].

If an item is over the minimum required for safe operations in a particular flight route/condition or the aircraft could be operated under restrictions, inoperativeness of the specific item may be accepted and approved for inclusion in the MMEL. For example, the flight data recorder system in a Bell 412 Helicopter may be out of service for a limited time [17]. In addition, although the number of items with identical functionality installed on an aircraft depends on the manufacturer, operating a piece of equipment in the optional category is subject to the satisfaction of the NAAs that an acceptable level of safety would be maintained. If the functions of the system/item under assessment can be substituted by an alternative system/item with similar functions, then, it would be accepted for MMEL inclusion on a redundancy basis. The condition is that the alternative system would provide an acceptable level of safety as long it is confirmed operative. However, redundancy cannot be claimed for the inclusion of an item in the MMEL if all items/equipment are required to be operative based on the aircraft's type certificate. For instance, in Bell 412, two air data interference units are installed on the aircraft where one may be inoperative provided the second unit is fully operative for flight [17].

In cases that MMEL allows inoperative items, the aircraft can be dispatched to prevent aircraft grounding situations subject to maintenance or replacement of the a ffected component or system within the time frame specified in the MMEL. Nonetheless, to maintain the same level of reliability certain restrictions might apply (e.g., transferring functions to another fully operative system, flight limitations, night/day operations restrictions) to ensure that safety is not compromised.


**Table 2.** Advantages and limitations of Failure Mode E ffect Analysis (FMEA).

**Table 3.** Advantages and limitations of Fault Tree Analysis (FTA).


### *2.2. Factors Considered in MMEL*/*MEL Development and Justification*

This section summarizes the principal factors which are collectively mentioned by Airbus [3], EASA [4], and UK CAA [5] and exert a major influence on the development of a MEL and a ffect decisions to include or exclude items from the list.
