In aviation, a master minimum equipment list, or MMEL, is a categorized list of onboard systems, instruments, and equipment that may be inoperative for flight. Specific procedures or conditions may be associated with the 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.
The MMEL contains the conditions, limitations, and procedures required for operating the aircraft with these items inoperative. The MMEL forms the basis for development and review of an individual operator's Minimum Equipment List (MEL).
The Minimum Equipment List (MEL) is a document that lists the equipment that may be temporarily inoperative, subject to certain conditions, at the commencement of flight.
In order to guarantee the correct usage of the MEL all pilots should be trained accordingly.