Aircraft on Ground or AOG is a term in aviation maintenance indicating that a problem is serious enough to prevent an aircraft from flying. Generally there is a rush to acquire the parts to put the aircraft (A/C) back into service, and prevent further delays or cancellations of the planned itinerary. AOG applies to any aviation materials or spare parts that are needed immediately for an aircraft to return to service. AOG suppliers refer qualified personnel and dispatch the parts required to repair the aircraft for an immediate return to service. AOG also is used to describe critical shipments for parts or materials for aircraft "out of service" or OTS at a location.

In aviation, master minimum equipment list, or MMEL, is a categorized list of on-board systems, instruments and equipment that may be inoperative for flight. Specific procedures or conditions may be associated with operation of the relevant item. It is considered by default that any equipment or system related to airworthiness which is not included in the MMEL is required to be operative. The MMEL is defined on a per aircraft model basis.

MEL (Minimum Equipment List): MEL is based upon the MMEL (Master Minimum Equipment List). MMEL is defined on a per aircraft model basis. MEL is prepared by the operator by taking reference of the MMEL keeping in mind the type and number of equipment installed. Initial issue of the MEL and its subsequent revisions will be approved by competent authority.

The philosophy behind MEL is to authorize release of flight with inoperative equipment only when the inoperative equipment does not render the aircraft unairworthy for the particular flight to avoid revenue loss to the operator and discomfort to the passengers.

Limitations, procedures and substitutions may be used to provide conditions under which the inoperative equipment will not make the operation unsafe or the aircraft unairworthy. This is not a philosophy which permits reduced safety in order to fly to a base where repairs can be made, but rather a philosophy which permits safe operations for a take off from a maintenance base or en-route stop.

It may not include items like galley equipment, entertainment systems, passenger convenience equipment, which do not affect the airworthiness of an aircraft. All items which affect the airworthiness of aircraft or safety of those carried on board and are not included in MEL are required to be operative.

Minimum equipment lists are issued to specific aircraft and specific operators. In order to use a minimum equipment list, that specific company must receive a letter of authorization from the national aviation authorities of the countries where the aircraft will operate.

A minimum equipment list is required in the United States by the Federal Aviation Administration:

• When operating any turbine-powered aircraft such as jets or turboprops.
• When operating under part 135 (Commuter and on-demand operations)
• When operating under part 125 (Non-airline large aircraft operations)

The CDL evolved over several years from what was commonly known as a “missing parts list,” which was a list of non-structural external parts of an airplane that were found missing after flight. The missing parts list is known today as the CDL.

The CDL plays an important role in the operator’s ability to safely continue flight operations. It is a list of externally exposed aircraft parts that may be missing for flight while the aircraft remains Airworthy. CDLs are developed by aircraft manufacturers, approved by the FAA, and tailored for each model aircraft.

A CDL is developed for most U.S.-built transport 14 CFR part 25 aircraft and many 14 CFR part 23 aircraft by aircraft manufacturers during the initial certification process. However, they are not a required element for aircraft certification. The manufacturer makes the decision to develop or not to develop a CDL. If deemed necessary, the aircraft manufacturer develops a proposed CDL and submits it to the responsible Aircraft Certification Office (ACO). The ACO reviews, evaluates, conducts the required testing, and coordinates with the appropriate Aircraft Evaluation Group (AEG), if needed, to resolve any problems and/or discrepancies.