Almost every commercial flight departs with at least one thing not working. That is not a scandal; it is a designed-for condition, governed by a document most people outside operations have never read. This is what the minimum equipment list actually is, where it comes from, and what the regulation requires it to do.

What a minimum equipment list is

GM1 ORO.MLR.105(a) of the Easy Access Rules for Air Operations puts it plainly:

The Minimum Equipment List (MEL) is a document that lists the equipment that may be temporarily inoperative, subject to certain conditions, at the commencement of flight.

Two phrases carry the weight. Subject to certain conditions — nothing on the list is simply permitted; each entry comes with constraints. And at the commencement of flight — the MEL governs the decision to depart, not what happens once you are airborne.

The same GM explains that the MEL is built by the operator for its own aircraft, taking account of configuration and the variables a manufacturer cannot know: operating environment, route structure, geographic location, and which aerodromes have spare parts and maintenance capability.

It is an alleviating document, not a permission slip

This is the part most summaries leave out, and GM2 ORO.MLR.105(d)(3) is unusually direct about it:

The MEL is an alleviating document having the purpose to identify the minimum equipment and conditions to operate safely an aircraft having inoperative equipment. Its purpose is not, however, to encourage the operation of aircraft with inoperative equipment. It is undesirable for aircraft to be dispatched with inoperative equipment and such operations are permitted only as a result of careful analysis of each item… The continued operation of an aircraft in this condition should be minimised.

An organisation whose MEL usage is climbing is not using a tool more efficiently. It is accumulating deferred defects, which is a maintenance-capacity signal before it is anything else.

Where it comes from: MMEL to MEL

Under ORO.MLR.105(a), the MEL is established based on the relevant master minimum equipment list (MMEL) as defined in the operational suitability data established under Regulation (EU) No 748/2012. The MMEL itself is developed in compliance with CS-MMEL or CS-GEN-MMEL.

The direction of travel only goes one way. ORO.MLR.105(e)(1) requires that the rectification interval in the MEL shall not be less restrictive than the corresponding interval in the MMEL. An operator can tighten; it cannot loosen.

And per ORO.MLR.105(b), the MEL and any amendment to it must be approved by the competent authority. When the MMEL changes in a way that matters — AMC1 ORO.MLR.105(c) defines that as a reduction of a rectification interval, or a change to an item that is more restrictive and applicable to the operation — the operator has an acceptable timescale of 90 days to submit the amended MEL.

What has to be in it, beyond the list

ORO.MLR.105(d) requires three things in addition to the items themselves:

1. a preamble, including guidance and definitions for flight crews and maintenance personnel using the MEL;
2. the revision status of the MMEL upon which the MEL is based, and the revision status of the MEL;
3. the scope, extent and purpose of the MEL.

The preamble does more work than people expect

AMC1 ORO.MLR.105(d)(1) sets out what the preamble should do, and it is a substantial list. Beyond reflecting the MMEL preamble and carrying terms and definitions, it should provide guidance on how to identify the origin of a failure or malfunction to the extent necessary for appropriate application of the MEL, contain guidance on the management of multiple unserviceabilities, and cover placarding of inoperative items — with controls and indicators clearly placarded where crew can reach them in flight.

Multiple unserviceabilities is the one that catches organisations out. Two items that are each individually acceptable may not be acceptable together, and the preamble is where the MEL is supposed to tell you how to think about that.

Rectification intervals, and the limits on extending them

ORO.MLR.105(e) requires the operator to establish a rectification interval for each listed item and to run an effective rectification programme. GM1 ORO.MLR.105(e);(f) is explicit that the definitions and categories of rectification intervals are provided in CS-MMEL — so the category definitions live in the certification specification, not in the air operations rules.

Those categories, how the clock is counted and the rules on extending an interval are covered in detail in MEL Rectification Intervals: How the Clock Actually Works.

What Air Operations does govern is what happens when the interval runs out. Under ORO.MLR.105(f), and subject to competent authority approval, an operator may use a procedure for a one-time extension of category B, C and D rectification intervals. Four constraints come with it:

• the extension must be within the scope of the MMEL for the aircraft type;
• it can be, at most, of the same duration as the interval specified in the MEL;
• it must not be used as a normal means of rectification, and only when events beyond the operator’s control have precluded rectification;
• a plan to accomplish the rectification at the earliest opportunity must be established.

Category A is absent from that list. The extension mechanism does not reach it.

GM1 ORO.MLR.105(f) illustrates what “beyond the operator’s control” is meant to cover: a shortage of parts from manufacturers, or an inability to obtain equipment needed for proper troubleshooting and repair. AMC1 ORO.MLR.105(f) adds the administrative edge — the competent authority should be notified within one month, and the notification should specify the original defect, the reason for the extension, and why rectification was not carried out in time.

(O) and (M): the conditions attached to the relief

Most MEL entries are not bare permissions. They come with compensating procedures, and AMC1 ORO.MLR.105(g) sets out how they are flagged: an item requiring a procedure to ensure an acceptable level of safety should be identified in the remarks or exceptions column — normally (O) for an operational procedure, (M) for a maintenance procedure, and (O)(M) where both are required.

ORO.MLR.105(i) sets the timing: operational procedures are completed when planning for or operating with the item inoperative; maintenance procedures must be completed prior to operating with it inoperative.

Two details from the guidance are worth holding onto. First, AMC1 ORO.MLR.105(g) states that satisfactory accomplishment of all procedures, regardless of who performs them, is the operator’s responsibility. Second, GM1 ORO.MLR.105(g) notes that unless specifically permitted by a maintenance procedure, an inoperative item may not be removed from the aircraft.

What the MEL does not need to cover

GM1 ORO.MLR.105(a) states the default clearly: all items related to the airworthiness, or required for the safe operation, of the aircraft and not included in the list are automatically required to be operative. Silence is not permission.

Genuinely non-safety-related equipment — entertainment systems, galley equipment — need not be listed if it does not affect airworthiness or operation. But the guidance carves out two exceptions that matter in practice: where the equipment serves a second function (cabin safety briefings played through the entertainment system), and where it forms part of another aircraft system such as the electrical system. In both cases procedures belong in the MEL.

Operating outside the MEL

There is one route, and it is deliberately narrow. ORO.MLR.105(j) permits an operator, subject to a specific case-by-case approval by the competent authority, to operate with inoperative items outside the constraints of the MEL but within the constraints of the MMEL.

The same conditions recur: not a normal means of conducting operations, only where events beyond the operator’s control have precluded MEL compliance, named personnel with defined duties controlling its use, and a plan to rectify or return to MEL constraints at the earliest opportunity.

Where the MEL touches the rest of your approvals

AMC1 ORO.MLR.105(d)(3) requires the MEL to include dispatch conditions for flights conducted under specific approvals held under Part-SPA. GM1 ORO.MLR.105(d)(3) gives the examples: RVSM, ETOPS and LVO.

It also has to cover different types of operation the operator conducts — crew training, positioning flights, demonstration flights. A MEL written only around revenue passenger operations has gaps the moment the aircraft is repositioned empty.

The short version

The MEL is the operator’s own, authority-approved document, derived from the manufacturer’s MMEL and never less restrictive than it. It says what may be inoperative at departure, for how long, and on what conditions. Everything not listed is required to work. And the regulation is unusually candid that the document exists to alleviate a situation nobody wants, not to normalise it.

Going further

Building and maintaining an operator MEL — tailoring it from the MMEL, getting it approved and keeping it current — is a discipline in its own right. Our MEL Customisation course covers that process end to end, including the EASA regulatory review behind it.

Further reading

• CS-MMEL: what it is and why it matters — the certification specification behind the MMEL
• MEL Customisation: 6 important steps — turning an MMEL into an operator document
• Why do we need MEL training? — who needs to understand the document, and why

Frequently asked questions