What is the difference between an (M) procedure and an (O) procedure on an MEL item?

An (M) procedure is a maintenance action that must be performed by a certificated maintenance technician before the aircraft can be dispatched with that item deferred — the flightcrew cannot do it themselves. An (O) procedure is an operational requirement that the flightcrew must follow during any flight while the item remains deferred, such as using an alternate checklist or adjusting speeds. Some MEL items require both, and both must be satisfied before or during the flight.

How many days does a dispatcher have before a Category C MEL item must be repaired?

A Category C MEL item must be repaired within 10 consecutive calendar days, not counting the day the discrepancy was recorded in the maintenance log. For example, if the item is entered on the 1st of the month, the repair deadline is the end of the 11th. Dispatching the aircraft after that deadline without a completed repair would be a regulatory violation under 14 CFR 121.628.

Do placards have to be installed for every deferred MEL item, and who is responsible for verifying them?

Yes, a placard must be installed in a location clearly visible to the flightcrew for every item deferred under the MEL, identifying the component as inoperative. This requirement is found in 14 CFR 121.628 and reinforced by AC 120-77. The dispatcher shares responsibility for confirming that the deferral — including the placard — has been properly completed before releasing the flight, since an incomplete deferral means the aircraft has not been placed in the MEL-approved configuration.

Inoperative Equipment, Placards, and (M) and (O) Procedures for Release

When an aircraft system or component is found inoperative before or during flight, the dispatch process does not automatically come to a halt. A structured regulatory framework exists to allow continued safe operations with certain items deferred, provided specific conditions are met. For the aircraft dispatcher, mastering this framework is not optional — it is central to the job. Releasing a flight with deferred maintenance items requires a clear understanding of the Minimum Equipment List (MEL), the Configuration Deviation List (CDL), placard requirements, and the distinction between Maintenance (M) and Operations (O) procedures.

This article walks through the complete picture: what the regulations require, how the MEL and CDL derive their authority, what placards accomplish, and exactly how (M) and (O) procedures affect the dispatcher's release responsibility. Every element connects back to the dispatcher's legal and ethical obligation to ensure that every flight released is airworthy in the eyes of 14 CFR Part 121 and the operator's FAA-approved documents.

The Regulatory Foundation: 14 CFR 121.628 and the MEL

The core rule for inoperative equipment in Part 121 operations is found at 14 CFR 121.628. This section establishes that no person may take off in an aircraft with inoperative instruments or equipment unless one of three conditions is satisfied: (1) the item is identified in the aircraft's FAA-approved MEL and the conditions and limitations of that MEL are followed; (2) the item is not required by the type certificate, applicable airworthiness directives, Part 91 day-VFR operation rules, or any other regulation, and it is deactivated, placarded, and the aircraft is found airworthy by an authorized person; or (3) the aircraft is being ferried to a maintenance base under a special flight permit. For scheduled airline dispatch, condition (1) — the MEL — is the workhorse provision.

The MEL is an FAA-approved, operator-specific document derived from the master minimum equipment list (MMEL) created by the aircraft manufacturer and validated by the FAA. Advisory Circular 120-77 (and its subsequent revisions) provides the FAA's guidance on how operators develop, receive approval for, and manage their MELs. An operator's MEL can never be less restrictive than the MMEL — it can only be equal to or more restrictive. Once an operator receives FAA approval for its MEL, that document becomes a part of the operations specifications, giving it the force of regulation for that certificate holder.

How the MEL Works: Categories, Items, and Deferral Authority

Each MEL item is assigned a repair interval category:

Category A: Items with a specific, manufacturer-defined or FAA-defined interval stated in the MEL remarks column. This could be a number of flight cycles, calendar days, or flight hours.
Category B: Must be repaired within 3 consecutive calendar days, not counting the day the defect was recorded.
Category C: Must be repaired within 10 consecutive calendar days, not counting the day the defect was recorded.
Category D: Must be repaired within 120 consecutive calendar days, not counting the day the defect was recorded.

When a Category C item is deferred, for example, the clock starts the day after the discrepancy is entered in the aircraft maintenance record. The dispatcher must be aware of these intervals when constructing a flight release, because dispatching the aircraft on day 11 of a Category C deferral would mean dispatching an aircraft that is no longer legally operating under its MEL — effectively an unairworthy aircraft.

The Configuration Deviation List (CDL)

The CDL is a separate but related document. Where the MEL covers installed systems and instruments that have failed or been disabled, the CDL addresses missing external components — items that are part of the aircraft's approved type design but whose absence has been analyzed and found acceptable within defined limits. Common CDL items include fairings, access panels, and certain aerodynamic fillets. The CDL is part of the FAA-approved Airplane Flight Manual (AFM) and, like the MEL, may impose performance penalties or operational restrictions when items are missing. The dispatcher must account for those performance penalties when computing takeoff, climb, and landing data for the release.

Placards: Purpose and Requirement

When an item is deferred under the MEL, a placard must be placed in a location clearly visible to the flightcrew — typically on or immediately adjacent to the inoperative component, or in a prominent position on the instrument panel. The placard is not a formality. It serves a direct safety function: it ensures that the crew is immediately aware that a specific item is inoperative and should not be used or relied upon. A placard that simply reads INOPERATIVE meets the basic standard, though many operators use more descriptive placards.

From the dispatcher's standpoint, confirming that a proper placard has been installed is part of verifying that the MEL deferral has been properly executed. If a deferred item has not been placarded per the MEL's instructions and the operator's MEL control program, the deferral is incomplete and the aircraft should not be released. The placard requirement is embedded within 14 CFR 121.628 and reinforced by AC 120-77.

(M) Procedures: The Maintenance Role

Many MEL items carry an (M) procedure — a maintenance procedure that must be performed before the aircraft may be dispatched with that item deferred. The (M) symbol appears in the MEL next to the item number. These procedures typically involve physically deactivating a system, pulling and collaring a circuit breaker, capping a hydraulic line, or otherwise placing the aircraft in a configuration that has been validated as safe for continued flight without that component.

A critical point for dispatchers: an (M) procedure must be accomplished by an appropriately certificated and authorized maintenance technician — it cannot be performed by the flightcrew. Before releasing the aircraft, the dispatcher must have confirmation that the (M) procedure has been completed and documented in the aircraft maintenance record. Releasing a flight on an MEL item that has an (M) procedure without that confirmation is a regulatory violation and a serious safety risk.

(O) Procedures: The Operations Role

An (O) procedure is an operational procedure that the flightcrew must follow when operating with the deferred item. The (O) symbol in the MEL indicates that specific pilot actions — checklists, performance adjustments, technique changes, or crew awareness requirements — are required for each affected phase of flight. Unlike (M) procedures, (O) procedures are the responsibility of the operating crew; however, the dispatcher has an independent duty to ensure the crew is aware of and has access to those procedures.

In practice, the (O) procedure requirements are often communicated via the aircraft release or dispatch paperwork, or through a separate crew notification. The dispatcher reviewing the MEL item must understand what the (O) procedure demands — for example, a restriction on the use of a particular autopilot mode, a change to takeoff speeds, or a requirement for manual fuel balancing — so that any performance data or routing restrictions in the flight release reflect those limitations accurately.

Both (M) and (O) Procedures Together

Some MEL items carry both an (M) and an (O) procedure. In these cases, maintenance must first execute its required action, and then the crew must follow the operational procedure on every departure or flight segment where the item remains deferred. Neither step substitutes for the other. The dispatcher's release is only valid when both conditions are confirmed or provided for in the documentation.

Why It Matters: The Dispatcher's Legal Obligation

Under Part 121, the dispatcher shares legal responsibility for the safety of the flight with the Pilot-in-Command (PIC). A dispatcher who releases a flight without ensuring that all active MEL deferrals are properly documented, that (M) procedures have been accomplished, that placards are in place, and that (O) procedure constraints are reflected in the release is not merely making an administrative error — that dispatcher is potentially violating 14 CFR 121.628, contributing to an unairworthy dispatch, and exposing passengers and crew to risk. The joint responsibility model of Part 121 dispatch means the dispatcher cannot simply rely on the maintenance department or the crew. Independent verification and professional judgment are required.

Key Numbers and Rules

Category B deferral: 3 consecutive calendar days to repair (not counting day of entry).
Category C deferral: 10 consecutive calendar days to repair.
Category D deferral: 120 consecutive calendar days to repair.
MEL vs. MMEL: Operator MEL may be more restrictive, never less restrictive, than the MMEL.
(M) procedures: Must be completed by certificated maintenance personnel before dispatch.
(O) procedures: Must be followed by the flightcrew; dispatcher must ensure they are reflected in the release.
Placard: Required for every deferred MEL item, positioned to be visible to the crew.
CDL items: May impose performance penalties that must be accounted for in dispatch performance calculations.
Authority: 14 CFR 121.628 and AC 120-77 govern MEL construction, approval, and use.

Common Test Traps

Confusing (M) and (O) responsibility: Examiners frequently test whether candidates know that (M) procedures require maintenance personnel — not the PIC or dispatcher — to perform them. A common distractor is a question suggesting the captain can accomplish an (M) procedure.
Miscounting deferral days: The calendar-day count does not include the day the discrepancy was recorded. A Category C item entered on March 1 must be repaired by the end of March 11, not March 10.
MEL versus CDL confusion: The MEL covers inoperative installed equipment; the CDL covers missing structural or aerodynamic components. They are separate documents with separate authority.
Thinking the MEL is optional: Some candidates believe that if an item is not required by any regulation, it can simply be deferred without MEL authority. While 14 CFR 121.628(b) does provide a limited path for such items, this path still requires deactivation and placarding — it is not an open door to operate any defective aircraft.
Forgetting CDL performance penalties: Dispatch performance calculations that ignore a CDL-imposed drag or weight penalty can lead to an incorrect and unsafe release.