MEL and CDL Recordkeeping Requirements: Part 135 Minimum Equipment List & Configuration Deviation List Records

Last reviewed June 16, 2026 · By Chad Griffith

To operate with inoperative equipment under a Minimum Equipment List (MEL), 14 CFR 91.213(a) requires an approved MEL plus an FAA-issued Letter of Authorization, and the aircraft records available to the pilot must list every inoperative item. Each deferral is tracked to a repair category (A, B, C, or D) with a hard due date.

What is an MEL and what records does it require?

A Minimum Equipment List (MEL) is an FAA-approved document that permits an aircraft to be operated with specific instruments and equipment inoperative, under stated conditions and limitations. Under 14 CFR 91.213(a), an aircraft may be operated with inoperative items only if an approved MEL exists for that aircraft, the operator holds a letter of authorization issued by the responsible Flight Standards office authorizing operation under the MEL, and the aircraft is operated under all conditions and limitations in the MEL and that letter. The rule states that "the Minimum Equipment List and the letter of authorization constitute a supplemental type certificate for the aircraft" — meaning MEL use is not optional self-help; it is a discrete FAA authorization.

The recordkeeping hook is explicit: 91.213(a)(3) requires that "the aircraft records available to the pilot must include an entry describing the inoperative instruments and equipment." Every deferred item must be recorded, dated, placarded, and carried in the records the pilot can access before flight. The MEL itself is derived from the manufacturer/FAA Master Minimum Equipment List (MMEL) and an operator's MEL may be no less restrictive than that MMEL.

MEL repair categories A, B, C, and D (the deferral clock)

When an item is deferred, it is assigned a repair category that sets the maximum time the aircraft may operate before the item must be fixed. The FAA defines four categories in MMEL Policy Letter PL-25 and Advisory Circular guidance. The day of discovery — the calendar day the malfunction was recorded in the maintenance record — is excluded from the count for Categories B, C, and D.

Each deferral therefore carries its own expiration date driven by its category. A Category B item discovered Monday must be repaired by end of the following Thursday; a Category D item gives roughly four months. Missing a repair interval makes the aircraft non-airworthy for that item, even though the deferral was legal when it began — which is why the deferral due-date, not just the original entry, is the record that matters.

The Letter of Authorization (LOA / OpSpec D095) is itself a record

You cannot use an MEL without FAA authorization, and that authorization is a document you must be able to produce. For Part 91 operators the authorization is a Letter of Authorization commonly issued as LOA D195; for Part 135 operators it is issued through operations specifications as OpSpec D095. 14 CFR 91.213(a)(2) requires the letter of authorization, and 14 CFR 135.179 requires that the MEL be authorized by the Flight Standards office through operations specifications and that "the flight crew shall have direct access at all times prior to flight to all of the information contained in the approved Minimum Equipment List."

That makes three things part of your provable record set before any deferral is even written: (1) the current FAA-approved MEL document itself, (2) the LOA or OpSpec (D195/D095) authorizing its use, and (3) the MEL preamble and definitions. An auditor checking a deferral will first confirm you were authorized to defer at all. If the authorizing document is expired, superseded, or simply not locatable, the underlying deferral entries do not save you.

What is a CDL, and how is it different from the MEL?

The Configuration Deviation List (CDL) is a separate authorization that addresses missing external parts, not inoperative systems. A CDL lists secondary airframe and engine parts — fairings, access panels, aerodynamic seals, static wicks, and similar non-structural external items — that may be missing for flight under stated conditions and limitations. The CDL is developed by the aircraft manufacturer, approved by the FAA, and published in the Aircraft Flight Manual (AFM), typically as an appendix or section, rather than in the MEL.

The practical distinction: the MEL governs equipment that is installed but inoperative (for example a failed transponder or one inoperative generator); the CDL governs structure/parts that are physically absent (for example a missing wheel-well fairing). CDL relief usually comes with performance penalties or operating limitations — reduced takeoff weight, fuel-burn corrections, or speed restrictions — that must be applied for the flight. Because the two lists live in different documents and carry different limitations, a complete recordkeeping system has to capture both: the MEL deferral with its repair category and the CDL item with its applicable penalty and any time limit.

Operating without an MEL: 14 CFR 91.213(d)

Some smaller aircraft operate with inoperative equipment without an MEL. 14 CFR 91.213(d) permits this for aircraft such as rotorcraft, non-turbine-powered airplanes, gliders, and lighter-than-air aircraft for which a Master MEL has not been developed (and certain small aircraft for which one has). Even then, the inoperative item must not be required by the VFR-day type-certification equipment list, must not be required by the aircraft's equipment list or Kinds of Operations Equipment List, must not be required by 14 CFR 91.205 (or any other rule) for the specific kind of flight, and must not be required to be operational by an airworthiness directive.

Under this no-MEL path the inoperative instrument or equipment must be removed and the cockpit control placarded with the maintenance recorded per 14 CFR 43.9, or deactivated and placarded "Inoperative," and a pilot or maintenance person must determine it is not a hazard. The recordkeeping obligation does not disappear without an MEL — it shifts to the placard plus the maintenance-record entry. Most turbine Part 135 fleets, however, operate under an approved MEL via OpSpec D095, so the MEL path above is the common one.

How FileFlo keeps MEL and CDL records audit-ready

MEL and CDL compliance is fundamentally a proof-and-expiration problem: every deferral has a category-driven due date, every authorization (D095/D195) has to be current and locatable, and a single fleet can carry dozens of open deferrals on different clocks. FileFlo is the records and proof layer for that. It classifies each uploaded compliance document to its specific CFR citation — tagging an MEL authorization to 14 CFR 91.213/135.179, a deferral entry to its repair category, or a CDL page to the AFM — reads the relevant dates, and tracks expirations and deferral due-dates with 90/60/30/7-day alerts so no Category B, C, or D interval closes silently. It flags missing records (for example, an open deferral with no recorded category, or a fleet operating against an expired LOA) and exports an inspector-ready audit binder on demand.

To be precise about scope: FileFlo is not an MEL-management system, a dispatch system, or a maintenance-tracking system. It does not generate your MEL, decide whether an item is deferrable, compute aerodynamic CDL penalties, or replace your maintenance records of origin. It sits on top of those systems as the document-intelligence and proof layer — ensuring that whatever your MEL/dispatch/maintenance tools produce is classified, date-tracked, and instantly retrievable when the FAA asks you to show it.

Building an MEL/CDL recordkeeping system that survives an audit

A defensible MEL/CDL records set has four properties. First, authorization is provable: the current FAA-approved MEL plus the LOA/OpSpec (D195 or D095) are on file and current, because 91.213(a) and 135.179 make that authorization a precondition to any deferral. Second, every open deferral is dated and categorized: each carries its repair category (A/B/C/D), its day of discovery, and a computed due date, satisfying the 91.213(a)(3) requirement that the records available to the pilot describe the inoperative items. Third, CDL items are tracked separately with their AFM reference and any performance penalty or time limit. Fourth, currency is visible before it lapses, not reconstructed under audit pressure.

The test an inspector applies is simple: for any aircraft on any date, can you show that every deferred item was within its repair interval, every missing CDL part was within its limitations, and the operator was authorized to defer at all? Operators that pass do so by treating the deferral due-date — not just the original logbook entry — as the record of record, and by keeping the authorizing OpSpec/LOA as live a document as the deferrals it permits.

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