A commercial airliner does not automatically become unairworthy every time one item of equipment stops working. Transport aircraft are designed with redundancy, and aviation regulations allow an operator to dispatch an aircraft with certain instruments, systems or components inoperative when an approved Minimum Equipment List—MEL—specifically permits it and every stated condition is satisfied. FAA Advisory Circular 120-125 is the current U.S. guidance covering development and use of an operator’s MEL, Nonessential Equipment and Furnishings programme and Configuration Deviation List for operations under Parts 91K, 121, 125 and 135. [1]
The concept is often misunderstood as permission to “fly with broken things.” In reality, the MEL is a controlled document derived from an approved master framework and tailored to an operator’s aircraft configuration, procedures and regulatory authorisation. It identifies exactly which items may be inoperative, for how long, under what operational conditions and with which maintenance or crew actions. Anything not permitted by the applicable rules and approved MEL cannot simply be ignored because the aircraft appears capable of flying. [2]
The MEL is an approved dispatch tool, not a maintenance wish list
The purpose of an MEL is to define a safe operating configuration when selected equipment is temporarily inoperative. FAA AC 120-125 explains that authorised operators use an MEL under the appropriate Operations Specification or Management Specification. It is therefore part of the operator’s regulatory system rather than an informal engineering document created after a defect occurs. [2]
For each listed item, the MEL can specify how many units are installed, how many must remain operative and which provisos apply. A dispatch condition may require a particular flight phase to be avoided, a redundant system to remain serviceable, a maintenance action to isolate the failed item or an operational procedure to be carried out by the crew. The permission exists only while every applicable condition remains true. [2]
The Master Minimum Equipment List is the starting point
Aircraft types have a Master Minimum Equipment List, commonly called the MMEL, developed through the regulatory approval process. The MMEL provides the high-level basis for which equipment can be considered for dispatch relief. An airline then develops its own MEL within the limitations of that framework and its specific fleet configuration and operation. [2]
An operator’s MEL cannot simply be more permissive than the authorised master basis. It may be more restrictive where the airline’s equipment, routes, procedures or authority require it. This distinction matters because two airlines operating nominally the same aircraft type can have differences in their approved MEL language and procedures. [1]
Redundancy is what makes much MEL relief possible
Airliners often have multiple pumps, sensors, computers, generators or communication systems because the aircraft has to tolerate defined failures. If one redundant unit is inoperative but the remaining architecture still satisfies the conditions established for dispatch, the MEL may allow operation until repair can be completed. [2]
That does not mean every redundant component can be deferred. Some redundancy is required specifically to protect against another failure in flight, and losing one unit can change the safety margin, route capability or dispatch criteria. The approved MEL reflects the safety assessment and operating restrictions associated with the degraded configuration. [2]
Maintenance first has to identify the defect accurately
Before an MEL can be applied, the operator needs to understand what has failed. A cockpit message can be the starting point, but technicians may need built-in test, fault history, wiring checks or physical inspection to identify the actual inoperative item. Applying relief to the wrong component would leave the aircraft in a configuration not covered by the approved document. [2]
The maintenance record must also describe the discrepancy and the action taken. Deferral does not erase the defect from the technical log. It creates a controlled open item with a category, repair interval and any continuing conditions that need to be monitored until rectification. [1]
The MEL can require maintenance procedures before dispatch
Many MEL items include a maintenance procedure, often identified in the operator’s documentation as an M procedure. Technicians may need to deactivate a component, pull and secure a circuit breaker when approved, close a valve, install a placard, perform a test or configure another system before the aircraft can be released. [2]
The purpose is to make the failed item predictable. A component that is known to be inoperative but electrically isolated can present a different risk from one that intermittently activates. The procedure places the aircraft into the exact condition assumed by the MEL relief. [2]
Operational procedures tell the crew how the flight changes
An MEL can also contain an operational procedure, often identified as an O procedure. The crew may have to use a different calculation, avoid a particular type of approach, make an additional check, use an alternate system or observe a restriction throughout the flight. [2]
This makes MEL dispatch a joint engineering and operations process. Maintenance establishes the aircraft configuration and certifies the required technical actions, while the flight crew confirms that the remaining conditions are compatible with the planned route, weather, airports and procedures. A defect that is acceptable for one flight can be unsuitable for another if the operating conditions differ. [2]
Weather can determine whether an MEL item is usable
Some equipment is only necessary in particular weather. A system associated with ice protection, low-visibility approach capability, weather detection or windscreen clearing may have restrictions linked directly to forecast or actual conditions. If the flight cannot remain within those limitations, the MEL relief is not available for that operation. [2]
This is why dispatch control cannot evaluate an inoperative item in isolation from the flight plan. The same aircraft with the same defect might be legal for a clear-weather daytime sector to one airport but not for a route requiring icing capability or a lower-category instrument approach. [2]
Route and airspace requirements can also remove MEL relief
An aircraft may need specific communication, navigation, surveillance or performance capability for the airspace being used. If a deferred item reduces that capability, the flight can require a different route, altitude or procedure. In some cases the planned flight cannot be conducted until the equipment is repaired. [2]
Operational control therefore looks beyond the simple question “does the MEL say yes?” The provisos, route requirements and aircraft authorisations must all align. The MEL allows dispatch only inside the boundaries of the remaining approved aircraft capability. [1]
Repair categories prevent indefinite deferral
MEL programmes use repair categories or intervals so defects do not remain open indefinitely merely because the aircraft can operate safely for a limited period. The exact categories and counting conventions are defined by the approved programme and regulatory guidance. [2]
Planning teams track each deferred item against its expiry and arrange parts, labour and maintenance opportunity before the limit is reached. A deferral that was valid yesterday can become invalid when its repair interval expires, even if the aircraft behaviour has not visibly changed. Time control is therefore a core element of MEL management. [1]
Placarding tells crews and technicians what is unavailable
Where required, an inoperative control, indicator or item is placarded so people interacting with the aircraft do not assume it is available. A clear placard is especially important if the component still looks normal physically but has been deactivated electrically or mechanically. [2]
Placarding is not a substitute for documentation. The technical record and MEL procedure remain authoritative, while the placard provides an immediate human-factors cue at the point of use. It helps prevent somebody from trying to operate a system that maintenance has deliberately made unavailable. [2]
The NEF programme covers genuinely nonessential items
Not every broken cabin or furnishing item belongs in the MEL. FAA AC 120-125 also covers Nonessential Equipment and Furnishings—NEF. These are items that are not required for safe operation under the applicable conditions and are managed through an approved NEF process rather than being treated as normal MEL systems. [1]
The classification is controlled. An operator cannot label a safety-relevant item “nonessential” merely to avoid an MEL restriction. The NEF programme defines how items are assessed, recorded, placarded or secured and repaired within the operator’s system. [2]
The CDL handles missing external parts differently
A Configuration Deviation List—CDL—addresses certain externally exposed aircraft parts that may be missing while the aircraft remains capable of safe flight under specified limitations. A missing fairing, panel or other approved item can increase drag and fuel burn even when it does not prevent flight. [1]
The CDL provides the approved configuration and any performance penalty or operating restriction. This is different from an MEL item, which normally addresses inoperative installed equipment. The two documents can interact on the same aircraft, but they solve different configuration-control problems. [2]
Fuel planning can change when an item is deferred
An inoperative system or CDL deviation can increase drag, require a lower altitude or change the way the aircraft is operated. The operator may therefore apply additional fuel or performance corrections where the approved data require them. Dispatch legality and fuel planning are connected because the degraded configuration has to be reflected in the operational calculation. [2]
Again, there is no universal “MEL fuel penalty.” Some deferred items have no measurable aerodynamic effect, while others can change performance significantly. The applicable MEL, CDL and performance documentation define the exact consequence. [2]
Dispatch has to consider combinations of defects
An aircraft may have more than one deferred item. Individually, each can be permitted by the MEL, but the operator must also consider whether the combination creates an interaction not covered by the individual relief. Two systems can share a power source, backup path or operational purpose, making their simultaneous loss more significant than either defect alone. [2]
Good MEL management therefore includes technical and operational judgement within the approved programme. The existence of two separate permissive entries is not automatically proof that every possible combination is acceptable. Operator procedures govern how multiple deferred defects are reviewed. [2]
The flight crew reviews the deferred configuration before departure
Before accepting the aircraft, the crew reviews the technical status and relevant MEL items. The pilots need to know which systems are unavailable, which operational procedures apply and whether those restrictions are compatible with the flight. This is particularly important when the defect changes automation, navigation, environmental control or abnormal procedures. [2]
Flight crew acceptance does not replace maintenance release, and maintenance release does not remove the crew’s operational responsibility. Both functions contribute to dispatch because one establishes technical conformity and the other confirms that the planned operation can be conducted within the remaining capability. [1]
Maintenance control coordinates repair rather than forgetting the defect
Once an item is deferred, airline maintenance control tracks the defect through the repair interval. Engineers may order components, schedule labour, identify the next suitable maintenance station and monitor whether operating conditions are changing. The aim is controlled continued operation followed by rectification, not permanent acceptance of degraded equipment. [2]
Repeated defects can also trigger reliability or troubleshooting attention even when each individual occurrence could technically receive MEL relief. Airlines use continuing-airworthiness and reliability systems to identify patterns rather than treating every deferral as an isolated administrative event. [1]
An MEL does not override mandatory airworthiness action
Airworthiness Directives, mandatory limitations and other applicable regulatory requirements remain controlling. An operator cannot use an MEL entry to bypass a condition that requires specific equipment to be operative or a mandatory corrective action to be completed. [2]
The MEL operates inside the wider legal and technical framework of the aircraft. Its authority comes from regulatory approval and defined conditions, so it cannot logically be used to set aside the very requirements under which it is authorised. [1]
Why airlines use the system instead of grounding aircraft for every defect
Modern aircraft contain thousands of components, many of which are duplicated or not required in every operating condition. Grounding an otherwise safely dispatchable aircraft for every individual failure would create large operational disruption without necessarily improving safety. The MEL provides a structured way to use the redundancy and design tolerance already built into the aircraft. [2]
The important phrase is structured. The operator does not make the decision from intuition at the gate. It uses approved relief, performs specified procedures, records the defect, respects the repair interval and plans the flight around the remaining capability. That control is what distinguishes legitimate MEL dispatch from simply accepting broken equipment. [1]
The simplest accurate explanation
The Minimum Equipment List is the operator’s approved rulebook for dispatching an aircraft with specifically identified equipment inoperative. It states the minimum quantity that must remain available, any maintenance and crew procedures, operating restrictions and the time allowed before repair. [2]
An MEL does not declare that a failed component is unimportant. It establishes that the aircraft can remain within an approved safe configuration without that component for a controlled period and under defined conditions. NEF programmes manage genuinely nonessential furnishings, while the CDL manages certain permitted external configuration deviations. Together they allow airlines to use aircraft redundancy intelligently while keeping every defect visible, limited, documented and scheduled for rectification. [1]
Verified Sources / References
- Federal Aviation Administration — AC 120-125, Development and Use of an Operator’s MEL, NEF Program, and CDL, Active, issued 1 November 2023
- Federal Aviation Administration — AC 120-125 Full Advisory Circular
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