An airliner does not automatically become unfit to fly because one item of equipment is inoperative. Commercial aircraft contain thousands of components, and some failures can be accepted temporarily when the aircraft is operated under defined limitations, procedures and repair deadlines. The document that allows an airline to make that decision is the Minimum Equipment List, or MEL. Under EASA Air Operations rules, an operator’s MEL must be established for the aircraft, based on the applicable Master Minimum Equipment List, and approved by the competent authority. [1]
The MEL is not a list of equipment that an airline is free to ignore. EASA guidance describes it as a controlled means of permitting operation for a limited period with particular equipment inoperative, subject to specified conditions. Its purpose is to maintain an acceptable level of safety while allowing time for rectification; the guidance explicitly says it should not be used to encourage or legitimise operation with inoperative equipment beyond what is necessary. [1]
The MMEL and MEL are different documents
The Master Minimum Equipment List is developed for an aircraft type and approved by the relevant authority. It identifies equipment that may be inoperative under stated conditions without making the aircraft automatically unsafe for the intended operation. EASA publishes MMEL information for aircraft types under its responsibility. [2]
An airline then develops its own MEL from that master document. The operator-specific MEL cannot simply be more permissive than the MMEL. It has to reflect the airline’s aircraft configuration, operational environment, routes, maintenance arrangements and authority requirements. EASA guidance specifically identifies factors such as aircraft configuration, environmental conditions, route structure, locations where maintenance is available and access to spare parts. [1]
An inoperative item must be covered explicitly
If an item required by the type design or operating rules is inoperative, dispatch relief normally exists only where the MEL contains an applicable entry and every associated condition can be met. A failure cannot be accepted simply because maintenance personnel believe it seems unimportant. The operator works from approved technical and operational data. [1]
Some installed items may be outside the scope of the MEL if they are not required for safe operation under the circumstances, but their treatment still follows the operator’s approved procedures. The key principle is controlled configuration: everyone involved must know what is inoperative, why operation is permitted and what restrictions apply. [1]
The entry can require operational procedures
MEL relief can include an operational procedure, commonly identified in the document as an action the flight crew or operational personnel must complete. This might involve changing how a system is used, applying a performance restriction, checking a condition before flight or avoiding a particular type of operation. [1]
The procedure is part of the relief. If it cannot be completed, the MEL permission does not exist simply because the equipment failure itself is listed. Dispatch therefore requires checking the complete entry, including remarks, conditions and any linked procedures. [1]
Maintenance procedures can also be mandatory
Other MEL entries require a maintenance action before the aircraft can be released. Maintenance personnel may need to deactivate a component, isolate a circuit, secure a valve, pull and collar a circuit breaker, inspect a related system or confirm a particular configuration. The exact action is defined in approved maintenance or operator data. [1]
These actions prevent the failed item from creating secondary hazards. A component that is allowed to remain inoperative may still need to be physically secured so it cannot move, leak, energise unexpectedly or confuse the crew. MEL operation therefore depends on maintenance control as well as flight-crew awareness. [1]
Placards make the condition visible
EASA guidance requires appropriate identification of inoperative equipment. A cockpit switch, display or other control can be placarded so crews know the associated function is unavailable or restricted. This is especially important when the failed component remains physically installed and might otherwise appear normal. [1]
The placard is not the legal basis for dispatch; it is one visible part of the control system. The aircraft technical log, maintenance records, MEL reference and crew briefing together establish the documented status. [1]
Rectification intervals limit how long relief can be used
MEL entries can contain rectification categories that set the period within which the fault must be repaired. The categories and counting conventions are defined by the applicable MMEL and operator procedures. This prevents temporary dispatch relief from turning into indefinite operation with degraded equipment. [2]
Maintenance control tracks these deadlines as part of fleet planning. An aircraft may remain technically dispatchable today but become unavailable later if the repair cannot be completed before the rectification interval expires. The commercial schedule therefore has to account for deferred defects as future maintenance commitments. [1]
Different faults can interact
An aircraft with one deferred defect may be acceptable, and another independent defect may also have its own MEL relief. That does not automatically mean both can coexist. Operators have to consider whether combinations create interactions that remove redundancy or violate assumptions behind either entry. [1]
This is one reason engineering and maintenance control review the complete aircraft defect status rather than processing each item in isolation. A second failure can change the safety significance of an earlier deferral even when both items appear in the MEL separately. [1]
Route structure can determine whether relief is usable
EASA specifically notes route structure among the factors an operator considers when developing and using its MEL. An inoperative system may be acceptable for one route but not another if the intended flight depends on equipment required for navigation, weather, airport capability or extended operations. [1]
For example, a particular navigation capability may be unnecessary on a short route served by conventional ground aids but required on a route whose airspace approval depends on that equipment. The MEL therefore answers “may this aircraft operate under these conditions?” rather than simply “may this component be broken?” [1]
Weather can change MEL legality
Some dispatch relief is conditional on weather or visibility. Equipment that is not needed in good visual conditions may become essential when the destination requires a particular low-visibility approach or when icing, thunderstorms or other conditions demand specific systems. EASA guidance lists environmental conditions as an operator-specific MEL consideration. [1]
This means an aircraft can be dispatchable when the flight is planned and later require reassessment if the operating conditions change before departure. Dispatch, engineering and flight crew therefore use current weather and route information alongside the technical defect. [1]
The MEL can affect passenger capacity
Not every deferred defect concerns avionics. Cabin equipment can also be controlled through the MEL where it affects regulatory compliance or passenger safety. An inoperative seat, emergency-lighting component, lavatory feature or other cabin item may require a seat block, passenger restriction or maintenance action depending on the approved entry. [1]
The aircraft can therefore remain airworthy while losing some commercial capacity. Operations control has to understand these effects because the technical solution may change how many passengers can be carried or which seats may be sold. [1]
ETOPS and specialised operations add conditions
Extended-range and other specialised operations depend on specific levels of system availability and redundancy. An item that can be deferred for ordinary operation may not be acceptable for a flight conducted under an approval that relies on that system. The MEL therefore includes conditions or references that preserve the requirements associated with those operations. [2]
Operators check the intended operation, not just the aircraft type. The same physical aircraft can be legal for one route and unsuitable for another until the deferred defect is repaired. [1]
The technical log creates traceability
When a defect is found, it is recorded in the operator’s technical-log or maintenance-record system. If MEL relief is applied, the record identifies the relevant item, actions completed and the continuing defect status. This allows flight crews, maintenance personnel and operations control to see the aircraft’s condition before each subsequent flight. [1]
Traceability also lets the airline track the rectification deadline and parts requirement. A deferred defect is therefore a controlled maintenance task with an audit trail, not an informal agreement that everyone remembers from the previous shift. [1]
Airlines tailor the MEL to their own fleet
Two airlines operating the same aircraft type can have MEL documents that differ in layout and operator-specific procedures because their configurations and operational environments differ. One operator may have optional equipment another does not, different route approvals, different maintenance stations or different authority-approved procedures. [1]
The MMEL provides the master technical basis, while the approved MEL converts that basis into an operational document for one operator. This is why airline personnel use their current company MEL rather than downloading another carrier’s list from the internet and assuming it applies. [2]
MMEL revisions trigger operator action
EASA requires operators to amend their MEL after relevant MMEL changes within the applicable regulatory process. A new MMEL revision can change relief, conditions, procedures or rectification categories based on updated type-design and operational information. [1]
Configuration control therefore applies to the MEL itself. Maintenance and flight crews need the current approved revision because an obsolete copy can contain conditions that no longer match the authority-approved standard. Electronic document systems help airlines control that distribution. [2]
The MEL does not override an Airworthiness Directive
Mandatory airworthiness instructions can impose requirements that take precedence over ordinary MEL relief. If an Airworthiness Directive or other mandatory limitation requires a system to be operative or a specific action completed, the operator must comply with that requirement. An MEL entry cannot be used to bypass a separate legal obligation. [1]
This reinforces the hierarchy of technical data. The MEL is one controlled document within a larger continuing-airworthiness and operations framework that also includes the aircraft maintenance programme, flight manual limitations, operational approvals and mandatory authority instructions. [2]
Maintenance control balances reliability and schedule
Deferred defects give airlines flexibility to avoid cancelling a flight for every non-critical failure, but each deferral creates future workload. Maintenance control decides when and where to repair the item before its deadline, considering spares, labour, hangar access and the aircraft schedule. [1]
Using the MEL effectively therefore does not mean maximising the number of deferred defects. A heavily deferred aircraft can become operationally fragile because route options narrow, defect interactions increase and several rectification deadlines can converge. Airlines generally aim to restore full serviceability as practical while using approved relief to protect the operation when needed. [1]
The captain still needs an acceptable aircraft
Applying MEL relief does not remove the flight crew’s responsibility to operate within the aircraft’s limitations and company procedures. The crew reviews the technical status, understands operational restrictions and confirms the planned flight is compatible with them. The MEL provides the approved framework for that decision; it does not force a crew to ignore changing operational circumstances. [1]
Likewise, engineers cannot create dispatch relief by declaring that a component is “probably fine.” The aircraft either satisfies the approved configuration, or the operator has a valid MEL or other approved means of managing the defect. [2]
The simplest accurate explanation
The Minimum Equipment List is a controlled rulebook that tells an airline when an aircraft can continue operating temporarily with a particular item inoperative. It is based on the aircraft type’s MMEL but customised and approved for the individual operator. [1]
Every relief comes with boundaries: operational or maintenance procedures, placards, route and weather restrictions, equipment interactions and a deadline for repair. Used correctly, the MEL prevents a minor non-critical defect from automatically grounding an otherwise safe aircraft while preserving a documented, authority-approved level of safety. It is not permission to fly broken aircraft indefinitely; it is a tightly controlled way to manage temporary loss of equipment. [2]
Verified Sources / References
- EASA — Easy Access Rules for Air Operations, ORO.MLR.105 Minimum Equipment List and Associated Guidance
- EASA — Master Minimum Equipment Lists
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