When an airliner arrives at the gate with a discrepancy—a burned-out cabin light, an inoperative weather radar antenna fairing, or a malfunctioning passenger entertainment system—the crew and dispatcher must immediately answer one question: can this aircraft legally depart? Two separate but complementary FAA-approved documents govern that decision: the Minimum Equipment List (MEL) and the Configuration Deviation List (CDL). Although both authorize flight with something less than a fully airworthy aircraft, they address fundamentally different categories of discrepancies and carry different legal foundations, different performance implications, and different operational constraints.
Understanding the distinction between an MEL and a CDL is not merely an academic exercise. At the ATP level, dispatchers and pilots in command share legal responsibility for the airworthiness determination before every flight. An error in applying one document when the other is called for can expose a carrier to an illegal operation under 14 CFR Part 121 or Part 135—and in the worst case, can compromise safety margins built into the aircraft's original certification.
What the MEL Is and How It Works
The Minimum Equipment List is a carrier-specific document derived from the aircraft manufacturer's Master Minimum Equipment List (MMEL), which the FAA approves as part of the aircraft's type certification data. The MMEL establishes the outer boundary: it lists every system and component that may be inoperative under specified conditions while the aircraft remains airworthy. A carrier then creates its own MEL—which can only be more restrictive than the MMEL, never more permissive—and submits it for FAA approval, typically through its Certificate Management Office. Once approved, the MEL becomes part of the carrier's operations specifications (OpSpecs) and has the force of an FAA-approved document.
Each MEL entry describes a specific piece of equipment, categorizes it by repair interval (Categories A, B, C, or D), and may impose Operations (O) procedures or Maintenance (M) procedures—or both—as a condition of continued operation. Category A items must be repaired within a timeframe specified in the remarks column. Category B items must be repaired within three consecutive calendar days, excluding the day the discrepancy was recorded in the aircraft maintenance record. Category C items allow ten consecutive calendar days, and Category D items allow 120 consecutive calendar days. These intervals begin at 0001 local time on the day following the day the discrepancy is entered in the aircraft maintenance logbook.
The legal basis for operating an aircraft under an MEL rests on 14 CFR §91.213 for general aviation operators, and on 14 CFR §121.628 and §135.179 for Part 121 and Part 135 operators respectively, in conjunction with the carrier's operations specifications. A Part 121 or 135 carrier's MEL is authorized under its OpSpecs (commonly referenced under paragraph D095 in current FAA templates, though this designation has varied historically), which references the approved MMEL. Critically, the MEL addresses installed equipment that is not functioning—a component that is physically present but not operating correctly.
What the CDL Is and How It Works
The Configuration Deviation List is a fundamentally different animal. It addresses missing external components—parts of the airframe or powerplant nacelle that are physically absent rather than merely inoperative. These are typically aerodynamic fairings, access panels, small doors, or similar secondary structures whose absence was evaluated by the aircraft manufacturer during the original type certification process. If the manufacturer's structural and aerodynamic analysis shows that flight is still safe without a particular part—albeit with some performance penalty—that part can be listed in the CDL with associated performance adjustments.
The CDL is part of the FAA-approved Airplane Flight Manual (AFM) for the specific aircraft type, which makes it a type design document rather than an operator-generated document. This is a critical legal distinction: a carrier does not create a CDL the way it creates an MEL. The CDL already exists (or does not exist) as part of the AFM. If a component is not listed in the CDL, the aircraft may not depart with that part missing, regardless of how minor the part appears. The operator's responsibility is simply to consult the AFM CDL section and apply the specified performance penalties and any limitations stated therein.
Typical CDL items include wing-to-body fairings and landing gear door fairings. Static discharge wicks, by contrast, are installed equipment and are typically addressed in the MEL rather than the CDL. Each CDL entry specifies the performance decrement the flight crew must account for—often expressed as a drag increment, a reduction in maximum allowable gross weight, or a speed limitation. These adjustments must be applied to weight-and-balance and takeoff performance calculations before departure.
Why the Distinction Matters Operationally
The MEL/CDL distinction matters in at least three practical dimensions: document authority, performance accountability, and repair obligation.
First, on document authority: an MEL item requires a logbook entry, a placard (if required by the MEL), and often a specific maintenance action (the M procedure) to deactivate or isolate the component. A CDL item typically requires only an AFM performance adjustment and a logbook entry documenting which part is missing. The CDL does not require isolation or deactivation of anything because the component is simply not there.
Second, on performance: MEL items may or may not carry a direct performance penalty. Many MEL items involve avionics, cabin systems, or redundant systems whose absence does not change the aircraft's drag or weight. CDL items, by definition, affect aerodynamics—every CDL item has an associated performance impact that must be incorporated into dispatch calculations.
Third, on repair obligation: both documents carry repair intervals, but the CDL's interval is stated in the AFM rather than in a carrier-generated document. Carriers may not unilaterally extend CDL repair intervals beyond what the AFM states.
Advisory Circular AC 91-67 addresses the interface between the MEL and CDL in the context of minimum equipment requirements, clarifying that the two documents are complementary and that dispatchers and PICs must consult both whenever a discrepancy exists. A single aircraft could simultaneously carry an MEL item (an inoperative weather radar) and a CDL item (a missing fairing), and both documents would apply concurrently. The combined effect on dispatch legality and performance must be evaluated together.
Key Numbers and Rules
- MEL repair categories: Cat A — as specified; Cat B — 3 calendar days; Cat C — 10 calendar days; Cat D — 120 calendar days (all beginning at 0001 local time on the day following the day of discovery entry).
- CDL authority: Derived from the approved AFM; operators cannot create or modify CDL entries—they can only apply them.
- MEL authority: Derived from the MMEL; operators create a carrier MEL that is equal to or more restrictive than the MMEL.
- Stacking discrepancies: An aircraft may carry multiple open MEL items and CDL items simultaneously, provided each individually complies with its applicable document and the combined effect does not create an unsafe condition. Some MMELs explicitly prohibit combinations of certain items.
- Performance adjustment obligation: CDL performance penalties are mandatory and must be applied to takeoff and en route performance computations before departure—they cannot be deferred or estimated informally.
- No CDL entry = no dispatch: If an external component is missing and not listed in the aircraft's CDL, the aircraft is not airworthy for that flight, period. There is no provision to invent a CDL item on the ramp.
- MEL does not cover missing parts: If a component has been removed rather than failed in place, the MEL generally does not apply. The CDL or a ferry permit may be the only options.
Common Test Traps
- Confusing document origin: Examinees often state that the carrier creates the CDL. Wrong—the CDL is part of the manufacturer's AFM and is type-design data. The carrier creates the MEL (from the MMEL).
- Applying the MEL to a missing part: If a fairing has fallen off, reaching for the MEL is incorrect. The MEL covers inoperative installed equipment, not absent hardware. The CDL is the correct reference.
- Ignoring performance penalties: Assuming a CDL item has no performance impact is a classic trap. Every CDL entry carries a stated performance decrement; ignoring it renders the dispatch non-compliant and potentially unsafe.
- Mixing up repair-interval start times: MEL intervals begin at 0001 local time on the day following the day the discrepancy is entered in the logbook, not the day it was noticed verbally or written on a sticky note. Precise logbook entry discipline is legally significant.
- Assuming a CDL always exists: Not every aircraft or every external part has a CDL entry. If the AFM has no CDL section, or the missing component is not listed, there is no authorization to dispatch—not even with a performance penalty applied informally.