Minimum Equipment List (MEL) Explained

An aircraft is certificated with all its installed equipment working. In practice, a reading light, a fuel quantity indicator, or one of two redundant units will fail between flights, and grounding the aircraft for each one would be neither practical nor, for many items, necessary for safety. A minimum equipment list (MEL) is the controlled exception: it names the items that may be inoperative for takeoff, the conditions that apply, and how soon each must be repaired.

The rule behind it is short. Under 14 CFR 91.213, no one may take off an aircraft with inoperative instruments or equipment installed unless an approved MEL exists for the aircraft and the other conditions are met, including that the inoperative item is recorded where the pilot can see it and the aircraft is operated under all the MEL's conditions and limitations. Parts 121 and 135 say the same in sections 121.628 and 135.179, with the authorization given through operations specifications.

One point holds through all of it. The MEL is not a general permission to fly broken. It is a list of specific items, each with its own limits, and equipment not on the list must work.

The three sections share a structure. Each requires an approved MEL, an authorization from the responsible Flight Standards office (a letter of authorization under Part 91, operations specifications under Parts 121 and 135), a record the pilot can see of what is inoperative, and operation within the MEL's conditions. Parts 121 and 135 add that the flight crew must have direct access, before flight, to all of the information in the MEL, in print or another approved form.

Each also lists what an MEL may not contain: equipment required by the type certification basis that is essential for safe operation, equipment required by an airworthiness directive to be operable (unless the directive provides otherwise), and equipment the operating rules require for the specific operation. That middle item is where this subject meets airworthiness directives: a directive can take an item off the table.

Sections 121.628 and 135.179 also say an MEL, as authorized by the operations specifications, constitutes an approved change to the type design without requiring recertification. Section 91.213(d) lets some Part 91 aircraft, for example rotorcraft and non-turbine airplanes with no MMEL, operate with inoperative equipment without an MEL if the conditions listed there are met. Anyone relying on that paragraph should read it in full.

A configuration deviation list (CDL) is separate from the MEL. Where the MEL covers inoperative instruments and equipment, a CDL covers deviations from the aircraft's as-certificated configuration, typically missing external parts. The FAA's Policy Letter 25 keeps the two apart: items already identified in the MEL or the CDL are not treated as nonessential equipment and furnishings. For where each document sits among the flight manuals, see the AFM and QRH and the flight operations library.

For the people who maintain the library, an MEL is a derived document with a parent that keeps changing. The MMEL is revised, and the operator's MEL has to follow. For Part 91 operators and Part 133, 137 and 141 certificate holders, the FAA's standard letter of authorization gives 90 calendar days from the date of a mandatory MMEL revision to obtain it, carry it on board, and incorporate the changes in the operator's procedures. Other operators should check their own authorization. Under ORO.MLR.105, the operator must amend the MEL after an applicable MMEL change within the acceptable timescales, which EASA's acceptable means of compliance puts at 90 days from the change's effective date, and its operational and maintenance procedures with it.

That makes four things worth putting under document control:

• The revision of the MMEL each MEL is based on, stated on the MEL itself, so a reader can tell at a glance whether it is current.
• The (M) and (O) procedures, which point into the maintenance manual and the flight manual. When an AMM task changes, every MEL procedure that cites it needs review.
• Effectivity: which aircraft each MEL applies to, since relief depends on what is installed.
• Access: the crew must be able to reach the current MEL before flight, so a controlled copy that is always at hand matters as much as a correct one.

None of this is exotic, but it is exactly the kind of cross-document dependency that a PDF folder handles badly. The same reasoning applies to the rest of the aircraft manual family and to the instructions for continued airworthiness, and our piece on service bulletin currency covers how revisions travel through a library. Cockpit-side procedures are covered in what an FCOM is.