How an MMEL Becomes an Operator’s Approved MEL

Aircraft are not always required to have every item of equipment serviceable before they can operate. Their certification includes approved levels of redundancy, allowing certain inoperative items to be accepted temporarily under defined conditions.

The document that establishes these permissions is the Master Minimum Equipment List (MMEL). However, an MMEL is not the document an airline or other operator uses directly. It is the regulatory and technical basis from which the operator develops its own Minimum Equipment List (MEL).

The operator’s MEL must then be approved by the relevant national aviation authority. In the United Kingdom, this is normally the UK Civil Aviation Authority (CAA).

The MMEL: the approved baseline

The MMEL is produced for a specific aircraft type by the type-certificate holder, supplemental type-certificate holder or another approved design organisation. It identifies equipment and systems that may be inoperative at the start of a flight, subject to stated conditions and limitations.

Applicable operational requirements sit within the wider air operations framework. Operators should therefore consult the current EASA Easy Access Rules for Air Operations or the equivalent requirements published by their competent authority.

A typical MMEL entry specifies:

• The relevant aircraft system or item
• The number of items installed and the number required to be operative
• Any operational procedures required by the flight crew
• Any maintenance procedures required before dispatch
• Applicability to particular aircraft configurations
• Repair or rectification intervals
• Associated conditions, limitations and placarding requirements

The MMEL is issued at a defined revision status and approved as the basis for preparing individual operators’ MELs. For aircraft operated under EASA Member State jurisdictions, the MMEL approval statement traditionally confirms that it is approved by EASA as the basis for preparing and approving the individual operator’s MEL. In the United Kingdom, the relevant aircraft and operational approval framework is overseen by the UK CAA aircraft operations division.

The MMEL is therefore a type-level document. It does not automatically authorise every operator to dispatch every aircraft with the listed equipment inoperative.

Why an operator cannot simply adopt the MMEL

The MMEL is based on the aircraft type and its approved design assumptions. An operator’s fleet, routes and procedures may differ from those assumed by the manufacturer.

The operator must therefore account for its particular:

• Aircraft equipment configuration
• Installed options and modifications
• Avionics and communications fit
• Operating environment and route structure
• Navigation and approach capabilities
• Crew procedures and training
• Maintenance capability
• Dispatch and operational-control arrangements

This principle is reflected in standard MMEL wording:

“The MMEL is the basis for the development of the individual operator’s MEL which takes into consideration the operator’s particular aeroplane equipment configuration and operational conditions.”

An operator may use a different format from the MMEL, but the resulting MEL must not be less restrictive than the MMEL. It may impose additional restrictions where the operator’s aircraft, operation or procedures require them. It cannot create wider dispatch privileges than those established in the approved MMEL.

For a practical explanation of how defects are managed before dispatch, see our guide to how aircraft engineers defer defects using the MEL.

Step 1: Establish the aircraft configuration

The MEL development process begins with an accurate understanding of the aircraft to which it will apply.

The operator reviews the aircraft’s approved configuration, including:

• Standard and optional equipment
• Aircraft registration and fleet variation
• Approved modifications and repairs
• Required equipment for the intended operation
• Applicable airworthiness directives and operational requirements
• Equipment installed under supplemental type certificates
• Differences between individual aircraft in the fleet

This is important because an MMEL entry may apply only where particular systems, interfaces or redundancies are installed. An operator cannot assume that an entry applies uniformly across a mixed fleet.

The operator may consequently produce a fleet-wide MEL with aircraft-specific control procedures, or separate MELs for different configurations. Configuration details should be checked against approved technical records and current aircraft documentation, including the relevant aircraft maintenance manuals, IPC, SRM and wiring diagrams.

Step 2: Convert MMEL permissions into operator procedures

The operator then transfers the applicable MMEL entries into its MEL while retaining every relevant condition and limitation.

The operator’s document must provide practical guidance and definitions for the people who use it, including:

• Flight crew
• Maintenance personnel
• Dispatch or operational-control staff
• Engineering and continuing-airworthiness personnel

The MEL must make clear what is required when an item is found to be inoperative. This normally includes the actions needed to:

• Identify and record the defect
• Determine whether the MEL permits dispatch
• Apply any required maintenance procedure
• Apply any required operational procedure
• Placard or identify the affected control, indication or equipment
• Record the rectification interval
• Confirm that all dispatch conditions have been met

An MEL entry is therefore more than a list of equipment that may be unserviceable. It is an operational control document linking aircraft status, maintenance action and flight crew responsibilities.

Clear records and effective communication are essential. Operators can also strengthen this process by using a structured aircraft maintenance shift handover checklist when open defects or deferred items pass between teams.

Step 3: Apply the operator’s operating conditions

The operator must assess whether the MMEL permission remains suitable for its actual operation.

For example, an item that may be inoperative under the MMEL may require additional restrictions within the operator’s MEL because of:

• Particular routes or airspace
• Extended diversion operations
• Low-visibility operations
• Performance or payload limitations
• The operator’s navigation approvals
• Special airports or terrain environments
• The absence of particular maintenance support
• Differences in crew procedures or training

The operator may choose not to include every MMEL item. Excluding an entry is permitted and can be appropriate where the operator does not want to support the associated maintenance or operational procedures.

What the operator cannot do is weaken the MMEL’s conditions, remove a required limitation or extend a permitted rectification period without an appropriate approved basis.

Step 4: Consider multiple inoperative items

A central responsibility remains with the operator: maintaining an acceptable level of safety when more than one item is inoperative.

The MMEL’s standard preamble states that operators are responsible for exercising the necessary operational control to ensure that an acceptable level of safety is maintained. It also requires consideration of exposure to additional failures during continued operation with inoperative items.

This means that two individually permissible defects cannot automatically be treated as acceptable when combined. The operator must consider whether the failures:

• Affect the same safety function
• Remove independent redundancy
• Increase crew workload
• Interact with aircraft performance or procedures
• Create a new failure condition
• Reduce the margin available for another failure

The MEL should contain suitable guidance for assessing combinations of inoperative items, supported where necessary by engineering analysis, manufacturer data or approved operator procedures.

This is also an area where a strong reporting culture matters. Our article on just culture in aircraft maintenance explains how engineers can raise compliance concerns safely when a dispatch decision appears unclear or unsupported.

Step 5: Define rectification intervals

An MEL normally assigns a rectification interval to each permitted inoperative item. These intervals are commonly expressed by categories such as A, B, C or D, although the exact structure and applicable requirements depend on the governing regulatory framework and the approved MEL.

The interval does not mean that the aircraft may operate indefinitely with the item inoperative. It defines the permitted period before rectification, subject to the MEL’s conditions.

The operator must control:

• When the defect was entered into the aircraft records
• When the rectification period begins
• Any calendar, flight-hour or flight-cycle limit
• Extensions, where legally permitted
• The action required when the interval expires

An expired MEL interval is not simply an administrative issue. Once the permitted period has ended, the aircraft must normally be rectified or otherwise placed under an approved and lawful disposition before further dispatch.

MEL planning should also be coordinated with the operator’s wider maintenance programme. Our guide to how aircraft maintenance check intervals are determined explains how maintenance planning uses approved programmes, reliability data, airworthiness directives and aircraft utilisation.

Step 6: Address removal, isolation and placarding

An MEL authorises operation with an item inoperative only within the terms of the applicable entry. It does not, by itself, authorise unrestricted removal of equipment from the aircraft.

Where an item is physically removed, additional approved data and configuration-control requirements may apply. The MMEL and MEL conditions must be followed, and any applicable configuration deviation process, such as an approved Configuration Deviation List (CDL) for external components, must be used where relevant.

The equipment or control affected by an MEL item must also be identified as required. The MMEL preamble and the operator’s MEL will specify conditions concerning isolation, deactivation, securing or placarding. Those conditions must not be omitted when the MMEL entry is incorporated into the operator’s document.

Step 7: Submit the MEL for approval

Once the operator has completed its technical and operational assessment, the MEL is submitted to the competent authority.

For a UK operator requiring CAA approval, the application normally needs to demonstrate that:

• The MEL is based on the applicable approved MMEL
• The aircraft configuration has been correctly assessed
• MMEL conditions and limitations have been retained
• Operational and maintenance procedures are adequate
• Rectification intervals are correctly controlled
• Multiple-inoperative-item risks have been addressed
• Responsibilities for flight crew, maintenance and operational control are clear
• The document is subject to effective revision control

The authority may review the MEL in detail and request changes before approval. Approval is not granted merely because the document reproduces the manufacturer’s MMEL. The CAA must be satisfied that the operator has translated the MMEL into a controlled system suitable for its specific fleet and operation.

Under the applicable UK and European frameworks, the core principle is explicit:

“The MEL and any amendment thereto shall be approved by the CAA.”

Operators should verify the current approval process and applicable guidance directly through the UK CAA air operator certificate guidance.

For an operator under an EASA Member State’s oversight, approval is provided by the relevant national competent authority under the EASA regulatory framework. The authority may apply its own procedures and terminology, but the fundamental relationship remains the same: the MMEL provides the approved type-level basis, while the operator’s MEL requires competent-authority approval.

Step 8: Control MEL amendments

The MEL remains a controlled approval document after its original issue.

Amendments may be required when:

• The MMEL is revised
• The aircraft configuration changes
• New equipment or modifications are installed
• Operational approvals change
• Regulatory requirements are amended
• Manufacturer procedures are updated
• The operator changes its maintenance or dispatch arrangements
• Experience identifies a need for clearer procedures or additional restrictions

An amendment must be reviewed against the current MMEL and the operator’s approved procedures. It must then be submitted through the applicable approval process before it is used, unless the authority has established a specific alternative mechanism.

The statement that “the MEL and any amendment thereto shall be approved by the CAA” is therefore significant. Approval applies not only to the first issue of the MEL, but also to subsequent changes.

The relationship between the MMEL and MEL

• MMEL: Prepared by the type-certificate holder, supplemental type-certificate holder or approved design organisation. It applies to an aircraft type or defined configuration and establishes the maximum permitted relief from fully serviceable equipment.
• MEL: Prepared by the individual aircraft operator. It applies to the operator’s aircraft and operation and applies the MMEL to the operator’s configuration, procedures and operating conditions.
• CDL: Prepared from approved design or aircraft data. It controls certain permitted deviations involving external components.

The MMEL is the starting point, not the final operating document. The MEL is the document used by the operator to decide whether an aircraft may be dispatched with a known inoperative item.

Common mistakes when developing an MEL

Operators can create avoidable compliance problems when they:

• Copy MMEL entries without checking the actual aircraft configuration
• Omit operational or maintenance procedures
• Fail to assess the interaction between multiple inoperative items
• Use an outdated MMEL revision
• Lose control of rectification intervals
• Treat an MEL as permission to remove equipment without approved data
• Use an amended MEL before the required approval or acceptance
• Fail to communicate deferred defects during shift handovers

These issues can affect both dispatch decisions and the quality of the operator’s technical records. For broader guidance on maintenance documentation and compliance, explore the AeroTech Careers Blog.

The essential rule

An operator’s approved MEL is created by taking the approved MMEL and applying the operator’s actual aircraft configuration, operational environment, procedures and safety controls.

The process can be summarised as follows:

1. Identify the applicable approved MMEL.
2. Confirm the aircraft’s actual configuration.
3. Select the relevant MMEL entries.
4. Incorporate all conditions, limitations and rectification intervals.
5. Add operator-specific maintenance and operational procedures.
6. Assess interactions between multiple inoperative items.
7. Establish document-control and defect-tracking arrangements.
8. Submit the MEL and later amendments to the competent authority.
9. Use the MEL only after the required approval or applicable acceptance has been obtained.

The final MEL may look different from the MMEL, but it must never be less restrictive. Its purpose is to provide a controlled, approved method for operating an individual aircraft with specified equipment inoperative, without compromising the level of safety established by the aircraft’s certification and the applicable operational rules.

Managing maintenance records alongside compliance requirements? Analyse your Part 66 logbook with AeroTech Careers to identify missing evidence and strengthen your professional records.