Master Minimum Equipment Lists (MMELs) Considerations
Sofema Online (SOL) www.sofemaonline.com considers the MMEL Development Process
Introduction
The MMEL is a document issued by the aircraft's original equipment manufacturer (OEM) that lists equipment and systems that may be inoperative for certain flight conditions. Each item on the MMEL is associated with a specific operational procedure to ensure safety during the flight. The MMEL is a critical part of an aircraft's operational certification and is a reference for creating an airline's Minimum Equipment List (MEL).
Creating a Master Minimum Equipment List (MMEL) for a new aircraft is a complex and lengthy process that requires a deep understanding of the aircraft's systems, operation, and safety principles. The aircraft manufacturer designs and tests the aircraft, taking into account various scenarios including situations where certain components or systems might not be operational. These tests will inform the initial draft of the MMEL.
For EASA regulatory environment we must consider Operational Suitability Data (OSD) which is a group of interconnected data elements that are necessary to demonstrate the operational suitability of an aircraft type.
The OSD is part of the aircraft type certification and includes, among other things, the Master Minimum Equipment List (MMEL).
The process to generate the MMEL for new aircraft seeking a type certificate involves the following steps:
FAA Process (United States Federal Aviation Administration)
Preliminary Type Certificate Evaluation: The process starts with the aircraft manufacturer working on the aircraft type design, with the FAA's involvement during the initial phase. The FAA evaluates the proposed design, equipment, and system configuration.
>> Based on the aircraft's design, the OEM drafts an initial MMEL, which includes details about all installed systems and equipment and their redundancy, allowing specific components to be inoperative under certain conditions.
>> The OEM performs flight tests to validate the MMEL and demonstrate that the aircraft can be operated safely with each item inoperative.
>> The FAA reviews the initial MMEL and offers feedback and suggestions. This review includes an assessment of the OEM's justifications for each piece of equipment and system being allowed to be inoperative.
>> After the flight tests and any necessary revisions, the FAA approves the final MMEL as part of the Type Certificate for the new aircraft.
EASA Process (European Union Aviation Safety Agency)
>> Just like with the FAA, the process begins with the OEM designing the aircraft and drafting the initial MMEL. The OEM then conducts flight tests to demonstrate the validity of the MMEL.
>> After the flight tests, the OEM submits the MMEL and supporting data to the EASA for evaluation.
>> EASA assesses the MMEL and provides feedback. If the EASA has any objections or suggestions, the OEM must address them.
>> After satisfying EASA's requirements and resolving any issues, the MMEL is approved as part of the Type Certificate.
EASA MMEL Regulatory Introduction (Level of safety and justifications of MMEL items (CS MMEL.140)
The MMEL items are prepared to ensure that an acceptable level of safety as intended by the applicable requirements is maintained taking into account the following factors:
>> Reduction of aircraft functional capabilities and/or safety margins;
>> Change in crew workload and/or degradation in crew efficiency;
>> Consequence(s) to the aircraft and its occupants of the next failure(s) having the worst safety-related impact on the aircraft’s take-off, continued flight and landing when dispatching in a known degraded configuration;
>> Consequence(s) to the aircraft and its occupants of the next external event(s) for which the item was designed to protect against, if applicable.
GM1 MMEL.140 Level of safety (as intended by the applicable requirements)
>> The applicable requirements to be considered for MMEL development include the Type Certification Basis requirements and any operational requirement (including airspace requirements) applicable to the considered item.
Note ‘As intended’ means that strict compliance with the applicable requirement(s) may not be ensured provided appropriate mitigation means are proposed ensuring that an acceptable level of safety is maintained in line with the overall intent of the requirement(s).
GM2 MMEL.140 Level of safety (means to maintain the level of safety)
An acceptable level of safety can be maintained for an MMEL item through one or a combination of the following means:
>> Adjustment of operational limitations;
Transfer of the function/information to an operating system/component performing the required function or providing the required information, provided the change in crew workload and/or crew training remains acceptable;
Development of operational procedures (e.g. such as alternate procedures; additional pre-flight checks), provided the change in crew workload and/or crew training remains acceptable;
Development of maintenance procedures (such as deactivating and securing the system/component of concern, additional verification tasks).
CS MMEL.145 Justification of MMEL items
>> The justifications are provided by the applicant along with each MMEL item.
>> The inclusion of each item in the MMEL is justified following one or more methods, also referred to as MMEL safety methodologies, as agreed with EASA.
>> The justifications include at least a qualitative safety assessment which:
>> Evaluate the consequences of the proposed MMEL dispatch configuration on the aircraft functional capabilities, crew workload and discomfort to occupants and show compliance with CS MMEL.140;
>> Evaluate the consequences of the next worst safety-related failure and, if applicable for the item, separately evaluate the consequences of the external event for which the item was designed to protect against, and ensure the combination of the MMEL dispatch configuration with the next worst safety-related failure or event do not correspond to an hazardous or catastrophic failure condition; and
Quantitive Safety Assessment
Specific cases may be accepted when supported by quantitative safety assessment when both of the following considerations are met:
>> Relief is proposed for items, functions and/or systems involved in catastrophic or hazardous failure conditions, and the severity of the failure condition under MMEL configuration is not mitigated by special operating conditions, limitations or procedures; and
>> When the operation with the inoperative item leaves the aircraft one failure away from a hazardous failure condition, or one or two failures away from a catastrophic failure condition.
Note - When an operational or maintenance procedure is associated to an MMEL item, corresponding symbol is included in the MMEL, and the intent of the procedure is specified in the associated item justification.
Note A qualitative analysis may only be used for conventional and simple systems when the aircraft is certified against requirements other than CS 25/29.1309.
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