What are the MEL categories A B C and D repair intervals for airline dispatchers?

Category A has no fixed standard interval — the specific deadline is stated in the MEL or an associated procedure for each individual item. Category B allows 3 calendar days, Category C allows 10 calendar days, and Category D allows 120 calendar days. In all cases the count begins the day after the discrepancy is first written up, and the clock runs continuously through weekends and holidays.

Does an MEL deferral clock pause when the aircraft is grounded for unrelated maintenance?

No. MEL repair interval clocks run on calendar days and do not pause for any reason, including unrelated groundings, holidays, or periods when the aircraft is out of service. Only a fully completed and properly signed-off repair stops the clock. An aircraft grounded for three days due to a separate issue does not gain extra time on a concurrently open MEL deferral.

What is the difference between an MEL Category A deferral and the other categories?

Unlike Categories B, C, and D — which have fixed standard intervals of 3, 10, and 120 calendar days respectively — Category A deferrals carry an interval that is specific to that particular item and spelled out in the MEL remarks or a referenced procedure, sometimes expressed in flight cycles or hours rather than calendar days. Category A items are often the most operationally complex deferrals and require dispatchers to locate the specific deadline rather than applying a universal default.

MEL Categories A, B, C, and D Repair Intervals the Dispatcher Tracks

When an aircraft component becomes inoperative, the airline cannot simply ignore the discrepancy and keep flying indefinitely. The Minimum Equipment List (MEL) — a carrier-specific document approved under the FAA's Master Minimum Equipment List (MMEL) program — assigns every deferrable item to one of four lettered categories: A, B, C, or D. Each category carries a maximum repair interval, a clock that starts the moment the discrepancy is first reported or discovered. The aircraft dispatcher, jointly responsible with the pilot-in-command for the legality of every flight under 14 CFR 121.533, must monitor these intervals and ensure no aircraft departs after its deferral window has closed.

Understanding MEL categories is not merely an academic exercise. A dispatcher who misreads a category or loses track of a repair clock can inadvertently release an aircraft that is no longer airworthy under the operator's approved MEL, exposing the carrier to certificate action and — far more importantly — placing passengers at elevated risk. This article explains what each category means, how the interval clock operates, and what practical steps dispatchers use to stay ahead of expiring deferrals.

The Regulatory Foundation

The authority for MEL operations in air carrier service flows primarily from 14 CFR 121.628, which prohibits an aircraft from being dispatched or taken off with inoperative instruments or equipment unless, among other conditions, the inoperative item is covered by an approved MEL and all conditions or placarding requirements specified in that MEL are met. The FAA's Advisory Circular 120-77 (Operators Approval Guidance for RII, MEL, and CDL Programs) provides the operational and administrative framework that translates the regulation into airline practice. Together these documents establish that the MEL must be developed from the MMEL, approved by the certificate-holding district office, and carried on the aircraft — and that deferral categories must be respected without exception.

How the Four Categories Work

The MMEL uses a lettered category system to communicate urgency. Airlines may be more restrictive in their MEL but may never be less restrictive than the MMEL. The categories are as follows:

Category A — No Standard Interval (Case-by-Case): Items in Category A do not have a fixed calendar-day interval. Instead, the MEL or a separate document (often an Operations Specification or a specific maintenance procedure) states the exact interval for that particular item, which may be measured in flight cycles, flight hours, or a specific number of calendar days unique to the discrepancy. Category A items are typically the most complex or safety-sensitive deferrals, requiring an individualized assessment. The dispatcher must locate and apply the interval stated in the applicable MEL remarks or referenced procedure — there is no universal default.
Category B — Three Calendar Days: An inoperative item deferred under Category B must be repaired within three consecutive calendar days, not counting the day the discrepancy is first reported. For example, if a B-category item is written up on Monday, the repair deadline expires at the end of Thursday (day 1 = Tuesday, day 2 = Wednesday, day 3 = Thursday). Dispatchers track B items carefully because the short window can close quickly across multi-day rotations, overnight stations, or irregular operations caused by weather.
Category C — Ten Calendar Days: Category C allows ten calendar days, again not counting the day of discovery. These items are less immediately critical to continued safe operation but still carry an enforceable deadline. Because C items can span a full week-and-a-half, they can quietly approach expiration during busy schedule periods. Dispatch offices often set internal alerts at the 7-day mark to ensure maintenance has adequate time to procure parts or schedule labor before the aircraft must be grounded.
Category D — One Hundred Twenty Calendar Days: Category D represents the longest standard interval — 120 calendar days from the day after discovery. These items have been evaluated as having minimal effect on airworthiness and operational capability under the conditions specified in the MEL. Despite the generous window, dispatchers should not treat D items as indefinite; 120 days arrive faster than expected in high-utilization operations, and the aircraft must be out of service for the repair if the deadline passes.

How the Clock Starts and Stops

The interval clock begins on the calendar day following the day the discrepancy is first reported or entered in the aircraft maintenance record — not the day the MEL deferral paperwork is completed, and not the day the aircraft next flies. This distinction matters enormously. If a crew writes up an item at 11:55 PM on a Tuesday, Wednesday is Day 1 regardless of whether maintenance reviewed the logbook entry immediately or the next morning. Dispatchers should confirm the write-up date stamped in the maintenance log, not assume the deferral was initiated on the day they first see it in the aircraft status system.

The clock does not pause during weekends, holidays, or periods when the aircraft is out of service for unrelated maintenance. Calendar days are literal: every day counts. An aircraft grounded for an unrelated inspection does not receive additional time on a concurrently running MEL deferral. Only a completed repair — properly signed off in the maintenance record — stops the clock. A partial repair that does not restore the item to airworthy condition does not reset the interval.

The Dispatcher's Role in MEL Tracking

Under the joint responsibility framework of 14 CFR 121.533, dispatchers bear an affirmative duty to ensure the aircraft is airworthy before releasing it. In practice this means:

Reviewing the aircraft status at each dispatch cycle. Most carriers use computerized maintenance-tracking systems that flag open deferrals with color-coded urgency indicators. Dispatchers must verify these flags before releasing a flight, particularly for aircraft that have been at out-of-base stations where MEL items may have been deferred by local maintenance without immediate notification to dispatch.
Confirming the deferral category and expiration date. A quick cross-reference between the maintenance log entry date and the MEL category tells the dispatcher exactly when the window closes. Building a simple spreadsheet or using the airline's system to display expiration dates in plain date format (rather than days remaining) reduces arithmetic errors during high-workload periods.
Coordinating with maintenance control. When a B or C category item is approaching its expiration, dispatch must coordinate with maintenance control to ensure a repair station or parts are staged. If the repair cannot be completed before the deadline, the dispatcher cannot legally release that aircraft — the flight must be cancelled, the aircraft ferried empty to a maintenance base (under appropriate conditions), or the repair completed at the current station.
Verifying operational and placarding requirements. Each MEL deferral comes with (M) maintenance procedures and/or (O) operational procedures that must be completed before flight. Dispatch is responsible for confirming the crew has been briefed on applicable (O) items. Releasing a flight without ensuring these conditions are met negates the legal protection the MEL deferral provides.

Why It Matters: Safety and Legal Consequences

Operating with an expired MEL deferral is equivalent to operating with inoperative required equipment — a violation of 14 CFR 121.628. The aircraft is effectively unairworthy for that flight. Beyond certificate and civil penalty exposure for the carrier and the dispatcher personally, the deeper concern is the compounding risk: MEL categories are set by the aircraft manufacturer and the FAA based on analysis of failure modes and redundancy. An expired deferral means the aircraft has been operating at a reduced safety margin for longer than the engineering analysis supports. Dispatchers who internalize this understanding treat MEL expiration dates with the same urgency as fuel calculations — there is no gray area.

Key Numbers and Rules

Category A: Interval specified in the MEL or associated document — no universal default.
Category B: 3 calendar days (day of discovery not counted).
Category C: 10 calendar days (day of discovery not counted).
Category D: 120 calendar days (day of discovery not counted).
Intervals run continuously — weekends, holidays, and unrelated groundings do not pause the clock.
Only a fully completed and signed-off repair stops the clock.
The MEL may be more restrictive than the MMEL but never less restrictive.
Both (M) and (O) procedures must be completed before a deferral is legally operative.

Common Test Traps

Counting the day of discovery. The interval starts the day after the discrepancy is reported. Examinees who include the write-up day miscalculate expiration dates — the most common arithmetic error on dispatcher practical exams.
Assuming Category A means no deadline. Category A has a deadline; it is simply item-specific rather than a standard number of days. Treating A items as indefinitely deferrable is a dangerous misreading.
Pausing the clock for groundings. Calendar days count regardless of whether the aircraft is flying. An aircraft grounded for three days does not get those days back on an open MEL deferral.
Confusing the MEL with the CDL. The Configuration Deviation List covers missing or damaged structural or exterior items (fairings, panels, doors) whose absence is acceptable under defined conditions. It is a separate document from the MEL, though both may be addressed in AC 120-77. Mixing them up on an exam can cost points.
Overlooking (O) procedure verification. Dispatch is not done once the category and date check out. Ensuring the required operational procedures are communicated to and acknowledged by the flight crew is part of the dispatcher's legal obligation before the release is signed.