Definition
The MMEL is the master document — approved by the authority rather than by the operator — that defines the universe of equipment items on a given aircraft type that may be inoperative for dispatch. It is published by the type certificate holder and approved through the FAA's Flight Operations Evaluation Board (FOEB) process for US-certificated types, or through EASA's type certification authority for EASA-certificated types. The MMEL takes precedence over the operator's MEL: the operator's MEL may be more restrictive than the MMEL (requiring earlier rectification, or not allowing dispatch with a given inoperative item at all), but the operator's MEL can never be less restrictive — no operator can dispatch with a configuration that the MMEL does not permit.
MMEL items are assigned rectification interval categories. Category A items must be rectified within a time period specified in the MMEL's Remarks column — often stated in calendar days (e.g., three calendar days, ten calendar days). Category B items must be rectified within three consecutive calendar days, excluding the day of discovery. Category C items must be rectified within ten consecutive calendar days, excluding the day of discovery. Category D items must be rectified within 120 consecutive calendar days, excluding the day of discovery. These categories originate from FAA Order 8900.1 Volume 4 Chapter 4 (the FOEB guidance) and are adopted in EASA AMC M.A.301-3 with the same letter designations. An item that appears in the MMEL without a category designation defaults to Category D in FAA practice. Rectification intervals begin at flight termination on the day the discrepancy is entered in the maintenance release or aircraft record.
Operational provisions (O-items) and maintenance procedures (M-items) in the MMEL define conditions that must be met for dispatch with the inoperative item. An O-item requires the flight crew to perform specific procedures from the Flight Manual Supplement or MEL procedure page before and/or during flight. An M-item requires a maintenance action — typically deactivation, securing, or collaring — before the flight departs. Both are captured in the operator's MEL procedure pages, which are approved as part of the operator's MEL approval by the FAA (under §121.628 through Principal Operations Inspector review) or EASA National Aviation Authority.
The CDL is a fundamentally different document, though it is often managed alongside the MEL. Where the MEL addresses inoperative functional equipment — systems, avionics, instruments — the CDL addresses the physical absence of external aircraft elements: gear doors that can be opened but not closed, fairings that are missing, inspection panel covers, antenna covers, static wicks, and similar non-structural external parts that are absent due to damage or maintenance action. The CDL is part of the AFM (specifically the Airplane Flight Manual Supplement or a separate CDL chapter) and specifies the performance penalties that apply when the item is missing — typically a specific increase in drag, a speed reduction, a fuel-flow penalty, or a combination. The pilot in command must factor the CDL penalties into the applicable performance calculation for that flight.
The legal framework for CDL operation differs by operation type. Under 14 CFR §91.213, non-commercial operators may dispatch with CDL items if listed in the AFM and the performance adjustments are applied. Under §121.628 and §135.179, commercial operators must follow the approved MEL and CDL processes as part of their operations specifications. Under EASA Part-CAT CAT.GEN.MPA.105, commercial air transport operators must operate in conformity with the aircraft AFM, which for CDL items means applying the AFM-specified penalties. EASA AMC M.A.301-3 addresses the continuing-airworthiness aspect: CDL items must be rectified within the time limit specified in the CDL or, where no limit is specified, at the next scheduled maintenance opportunity.
Why It Matters for Flight Schools
For Part 135 operators and ATO/AOC combined organizations, MMEL management is a day-to-day operational compliance function. A training aircraft returning from a student cross-country with an inoperative autopilot servo (perhaps a Category C item) needs immediate MEL deferral documentation — the discrepancy entered in the aircraft logbook with the MEL item reference, the category, the open date, and the rectification due date — before the aircraft is dispatched again. Failure to create this documentation at first grounding, or permitting a flight after the rectification interval has expired, is a direct §121.628 or Part-CAT compliance violation.
The MMEL-to-MEL derivation process creates compliance risk at organizations that use generic or out-of-date MELs. When the MMEL is revised by the manufacturer (reflecting a new system modification, a change in operational experience, or a regulator-initiated revision), operators must review their derived MEL against the revised MMEL and update the MEL through the approval process before the MMEL revision effective date. Operators who miss MMEL revision cycles end up with MELs less permissive than the current MMEL (a recoverable position) or — if the MMEL has restricted a previously permitted item — inadvertently operating with a non-compliant MEL.
How Aviatize Handles This
Aviatize's maintenance control module manages MEL deferrals as structured, time-tracked records. When a squawk is entered and the responding certifying staff member determines the item is MEL-deferrable, the work order interface prompts for: the MEL item reference, the MMEL category (A/B/C/D), the open date, the calculated rectification due date (automatically calculated from the open date and the category interval), any applicable O- or M-procedures confirmed, and the authorized person who approved the deferral. The resulting MEL deferral record is visible to dispatch — the smart planning and booking module checks the open MEL deferred items list against each aircraft before allowing a booking, and prevents scheduling past a rectification due date without a resolved or re-deferred MEL entry.
For CDL items, the same tracking infrastructure records the absent part, the AFM CDL reference, the applicable performance penalty, and the expected rectification date. The CDL deferral is surfaced in the dispatch release for each flight, ensuring the pilot in command acknowledges the performance adjustment. Approaching rectification due dates for both MEL and CDL items generate lead-time alerts to the maintenance team, enabling the work to be planned and parts procured before the aircraft is grounded rather than on the day of expiry.