What is the difference between an MMEL and an MEL under Part 121?

The Master Minimum Equipment List (MMEL) is the FAA-accepted baseline document for a specific aircraft type, representing the most permissive inoperative-equipment relief legally possible. An individual Part 121 air carrier takes the MMEL and creates its own MEL, which is incorporated into the carrier's Operations Specifications and may be equal to or more restrictive than the MMEL — but never less restrictive. The carrier's approved MEL is the operational document that actually governs dispatch decisions.

How do you count the repair interval days for an MEL deferral category?

Deferral category day counts are measured in consecutive calendar days, and the count begins at midnight on the calendar day immediately after the discrepancy is first recorded in the aircraft maintenance record. For example, a Category C item (10-day limit) discovered on a Monday means the count starts Tuesday as Day 1, and repair must be completed by the end of the Thursday that is ten calendar days later. Hours flown or cycles accumulated during the deferral period have no bearing on the deadline.

Why is an aircraft dispatched under an MEL still considered airworthy?

Under 14 CFR §121.628, an FAA-approved MEL is itself an airworthiness standard — it defines the conditions under which an aircraft with specific inoperative equipment is legally fit for the intended flight, provided all required Maintenance and Operations procedures have been completed. The MEL does not create an exception to airworthiness; rather, compliance with the MEL and its companion Dispatch Deviation Guide establishes airworthiness within the parameters the FAA has already approved for that aircraft type and operator.

Part 121 Minimum Equipment List and Dispatch Deviation Guide

Every revenue flight operated under 14 CFR Part 121 begins with a deceptively simple question: is this aircraft airworthy? In the real world of airline operations, the answer is rarely a simple yes or no. Cabin pressurization controllers, weather radar channels, auto-throttle computers, and hundreds of other systems can develop faults between gate turns. Rather than grounding every aircraft the moment a single component fails, the Federal Aviation Regulations provide a structured, FAA-approved framework that allows continued operations under defined conditions. That framework rests on two interlocking documents: the Minimum Equipment List (MEL) and the Dispatch Deviation Guide (DDG). Mastery of how these documents originate, interrelate, and constrain the crew and dispatcher is essential for the ATP knowledge test and for the judgment calls that real airline operations demand every day.

Legal Foundation

The statutory authority for operating with inoperative equipment traces to 14 CFR §91.213 for general operations and, far more specifically for air carriers, 14 CFR §121.628. Section 121.628 prohibits a certificate holder from dispatching or releasing an airplane with inoperative instruments or equipment unless the airplane has an FAA-approved MEL that covers the item in question, or unless the item meets the narrow conditions allowing deferral without an MEL — meaning it is not required by the type certificate data sheet, not required by an applicable Airworthiness Directive, and not required by the operating rules of Part 121. In practice, that second path is rarely available under Part 121 because the regulatory equipment requirements are so comprehensive. Virtually every Part 121 operator therefore maintains a fully approved MEL.

The MEL is incorporated into the carrier's Operations Specifications (OpSpecs), specifically paragraph D095 or its equivalent. That incorporation gives the MEL the legal standing of an FAA-approved document. Operating in compliance with the MEL means the aircraft is considered airworthy for that flight — a crucial legal and safety distinction that the ATP knowledge test probes repeatedly.

The MMEL: The Permissive Baseline

Before a carrier can write its own MEL, the FAA and the aircraft manufacturer collaborate to produce a Master Minimum Equipment List (MMEL) for each aircraft type. The MMEL represents the most permissive relief legally possible for that type certificate. It is developed jointly, accepted by the FAA, and published in a Flight Standards Information Management System (FSIMS) document specific to each aircraft model.

Individual air carriers then take the MMEL and craft their own MEL. The carrier's MEL may be equal to or more restrictive than the MMEL — it can never grant relief that the MMEL does not already authorize. This is a foundational rule. The FAA has approved the MMEL as the outer boundary; no individual operator can negotiate past that boundary, regardless of the sophistication of its maintenance program or the seniority of its safety personnel. A common test distractor implies that a carrier can obtain special FAA permission to be less restrictive than the MMEL for a specific route or aircraft variant. That is incorrect under Part 121.

Repair Interval Categories

Each MEL item is classified into one of four deferral categories that define how long the aircraft may continue operating with the discrepancy before mandatory repair. The clock for every category starts at midnight on the calendar day immediately following the day the discrepancy is first entered in the aircraft maintenance record — not when the aircraft next departs, not when the crew notices the failure in flight.

Category A: The time limit is specified in the remarks or conditions column of the MEL item itself. There is no standard day count; the item might require repair before the next flight, or within a specific number of flight hours — read the MEL text carefully.
Category B: The inoperative item must be repaired within 3 consecutive calendar days, excluding the day of discovery.
Category C: Repair must occur within 10 consecutive calendar days, excluding the day of discovery.
Category D: The longest standard interval — repair within 120 consecutive calendar days, excluding the day of discovery. Category D items are typically those with minimal operational impact, such as a non-required galley convenience feature.

Understanding these intervals in the context of calendar days — not flight hours or cycles — is critical. A Category C item discovered on a Monday at 14:00 local time must be repaired no later than the end of the day (midnight) ten calendar days later, regardless of how many flights or hours accumulate in the interim.

The Dispatch Deviation Guide

The MEL tells you which items can be deferred and for how long. The Dispatch Deviation Guide (DDG) tells you how to operate safely with each deferral in place. The DDG is a manufacturer-produced companion document that provides specific procedural, placard, and operational limitation guidance for each inoperative item. Where the MEL might simply state that one of two autopilot channels may be inoperative, the DDG specifies exactly what placard must be affixed to which control, what crew briefing must occur, what approach categories or weather minima no longer apply, and what performance adjustments are needed.

Within the MEL and DDG framework, procedures are divided into two categories based on who performs them:

Maintenance (M) Procedures: Steps accomplished by certificated maintenance personnel before the aircraft can be released. These might include deactivating a system, installing a collar or pin, or performing a functional test. A pilot cannot accomplish an M procedure on a Part 121 aircraft.
Operations (O) Procedures: Steps accomplished by the flight crew before or during the flight. These might include a specific checklist action, crew briefing, or performance calculation using degraded-system assumptions.

Both M and O procedures must be fully completed before the aircraft can legally depart. Completing one without the other does not constitute compliance. This is a high-frequency trap on the ATP knowledge test and an error with real safety consequences in line operations.

Dispatcher and Captain Authority

Under Part 121's shared responsibility model, both the aircraft dispatcher and the pilot in command must agree to release the flight. When an MEL item is open, the dispatcher must account for it in the dispatch release — verifying that all M and O procedures are documented as complete, that the deferral category is still within its allowable time, and that any operational limitations imposed by the MEL or DDG are reflected in the flight plan and release. The captain independently reviews the open items and has the authority to refuse the flight if, in their judgment, the condition compromises safety — even if the MEL technically permits dispatch.

Key Rules at a Glance

The carrier MEL is always equal to or more restrictive than the MMEL — never more permissive.
The MEL must be FAA-approved and incorporated into OpSpecs to be legally operative.
Deferral category clocks run in calendar days, beginning at midnight after the day of discovery.
Category A: per item text; Category B: 3 days; Category C: 10 days; Category D: 120 days.
An aircraft dispatched in compliance with its MEL is legally airworthy for that flight.
Both Maintenance and Operations procedures must be completed — neither alone is sufficient.
The DDG provides the specific procedural and limitation detail that makes MEL relief safe in practice.

Common Test Traps

The MMEL is not the carrier's operating document. The carrier's FAA-approved MEL is what governs dispatch. Citing only the MMEL without carrier approval is insufficient.
Category D does not mean unimportant. Even a 120-day deferral item may carry significant O procedures that must be briefed and accomplished every flight during the deferral period.
The day of discovery is excluded from the count. If you discover a Category C item on a Tuesday, you count Wednesday as Day 1; the item must be fixed by the end of Thursday ten days later.
Airworthiness is not suspended by an open MEL item. This is perhaps the most conceptually misunderstood point — MEL compliance restores legal airworthiness; it does not create a condition of non-airworthiness with a waiver attached.
Pilots cannot authorize or sign off M procedures. Under Part 121, only certificated maintenance personnel complete and log Maintenance procedures; the flight crew's role is the Operations side only.