14.3 Master Minimum Equipment List (MMEL), MEL & CDL Procedures

MEL, CDL, and the Dispatch Decision

An aircraft discrepancy does not automatically ground a flight, but neither a maintenance log entry nor a pilot's acceptance supplies independent permission to operate with required equipment inoperative. Under § 121.628, dispatch with inoperative instruments or equipment depends on the approved Minimum Equipment List (MEL) and its authorization, applicable conditions, and required procedures. Dispatchers verify the operational implications; qualified maintenance personnel perform the maintenance actions required by the approved system.

Document hierarchy

The Master Minimum Equipment List (MMEL) supplies the type-level basis for relief. The operator's approved MEL applies to its aircraft and operation and may be more restrictive. The AFM, airworthiness requirements, issued OpSpecs, maintenance program, and operating procedures can impose additional restrictions. The existence of an MMEL item alone does not authorize a particular carrier to defer it.

A Configuration Deviation List (CDL) addresses specified missing external parts and associated limitations or penalties. It is not permission to dispatch with any damaged or missing structure. Its approved conditions can affect drag, weight, altitude, speed, performance, or fuel. Some missing equipment can be covered by an MEL provision; the difference should not be reduced to “every internal item is MEL and every external item is CDL.” Identify the actual approved item.

Read the whole entry

A typical MEL entry specifies equipment quantity installed, quantity required, remarks or exceptions, and repair category. Check the aircraft applicability and operational purpose. A generator permitted inoperative for one kind of operation might not be acceptable for ETOPS or for a low-visibility approach. A release to a destination needing that capability can be unacceptable even though the airplane is eligible for some other flight.

The (M) symbol identifies required maintenance procedures; (O) identifies operating procedures. Authorized personnel and documentation requirements come from the approved procedures. Do not invent a rule that every (M) step universally requires one particular license irrespective of approved task permissions, or that only a dispatcher can perform every (O) step. Confirm the required actions have been completed by the proper personnel and that flight-specific restrictions are conveyed.

Repair intervals and the day of discovery

Calendar days are not simply elapsed hours from the discrepancy timestamp. If a Category B item is discovered Monday at 1400, with an approved local-calendar-day system and no different applicable provision, Tuesday, Wednesday, and Thursday are its three counted days. It expires at the end of Thursday, rather than exactly 72 hours after Monday at 1400. An operator's defined time reference can be UTC or another approved basis; establish it before making the calculation.

Category A requires special attention because the entry can use calendar days, flight hours, cycles, or another stated interval. Do not apply the B/C day-of-discovery convention automatically to every A item. For example, an entry requiring repair within two flight cycles is counted using that entry's terms, not a two-day window.

Extension authority

An extension is not created by writing “parts unavailable” in the log. The operator must have the applicable continuing single-extension authority and comply with its approved procedures, oversight, and limits, as addressed in the issued MEL authorization such as D095. Do not attribute this universally to an invented M005 MEL-extension provision or claim Category D can never be extended under any authority.

Identify the eligible category, authorized extension duration, reason, approval, records, and any notification requirement from the actual issued authority. Category A's specific interval cannot simply be converted into a B/C/D extension by the dispatch desk. A practical test can provide a particular authorization; use its exact conditions. Without evidence of an allowed extension, treat the ordinary deadline as controlling and arrange repair or a different aircraft/plan.

Cumulative restrictions

An MEL fuel allowance and a CDL drag allowance can both affect the same flight. Apply the approved combination method, not a guessed additive or multiplicative rule. If a supplied exercise explicitly says independent multipliers of 1.03 and 1.02 apply, a 10,000-pound baseline becomes 10,000 × 1.03 × 1.02 = 10,506 pounds. If the approved procedure prescribes addition or a special combined table, that method controls instead.

Likewise, one acceptable item can interact with another. An inoperative APU plus a generator issue can compromise a redundancy assumption; a pack restriction can limit altitude and alter route fuel; an anti-skid defect can change runway performance; and a navigation defect can make a chosen approach or oceanic approval unavailable. Read interaction restrictions rather than assuming two individually deferable items are always acceptable together.

Worked dispatch review

A hypothetical airplane has an inoperative pressurization component. The MEL permits operation only at or below a supplied altitude and requires an operating procedure. Dispatch first confirms the maintenance action and repair interval. Next, the route is replanned at the permitted altitude using approved performance, terrain, oxygen, weather, and fuel data. Destination and alternate capability are checked at the expected landing weights. If the lower altitude puts the route into unsafe icing or terrain that the aircraft cannot clear, the flight cannot use that original plan.

A signed maintenance deferral establishes the item disposition; it does not prove all those route-specific checks were made. Conversely, a dispatcher cannot authorize a mechanic to ignore an expired interval simply because the route is short. The flight requires both a valid maintenance basis and a valid operational plan.

Flight-specific documentation

Preserve the discrepancy, deferral basis, interval, required procedures, operational restrictions, performance/fuel corrections, and crew briefing under the operator's approved workflow. During flight, a new failure calls for the AFM/QRH and operational reassessment; an MEL is principally a dispatch document, not a replacement for airborne emergency procedures. At arrival, ensure new discrepancies and follow-up actions reach maintenance control and the next dispatch team.

Official References

Source check: 2026-10-12. Transport systems certification; Part 121 oxygen and equipment; FAA aircraft systems handbook. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.