What is a minimum equipment list for an aircraft? How are MEL and MMEL related? Who creates the list, and how do they decide what is on it?
As mentioned here:
A minimum equipment list is a list of equipment that must be installed and operable for the aircraft to be considered airworthy. It is aircraft-specific and spells out which pieces of equipment may be inoperable while maintaining airworthiness. If something is found to be inoperative, the pilot goes to the MEL, finds the entry for that item, and determines if the airplane must be grounded until that piece of equipment is fixed.
An MEL for a specific aircraft originates from a master minimum equipment list (MMEL). The MMEL is a list of all equipment on an aircraft type; it details which equipment is allowed to be inoperative without grounding the aircraft. (Think of a MMEL as a general, broader MEL.)
FAA publishes MMELs by manufactures.
The MEL is NOT a list of equipment that must be installed and operable for the aircraft to be considered airworthy. The correct definition is:
The MEL is a list of aircraft equipment that can be inoperable PRIOR to dispatch. MEL items are only applicable before your flight is dispatched. Once you are in the air you need to consult the aircraft QRH.
The MMEL (Master Minimum Equipment List) is generic across a type of aircraft. e.g. Cessna Citation CE-500, CE-550, CE-560. A single MMEL is published by Cessna to cover all of these models from the factory (Master).
A MMEL can not, and does not, take into account differences that may occur during upgrades and modifications, nor differences that may be present within specific airframe serial numbers.
A MEL (Minimum Equipment List) is a 'custom' document generated and approved for a specific airframe serial number, or a group of aircraft that are identical (fleet). In a fleet situation of like type airframes with minor differences, a MEL can spell out which serial numbers are applicable for a given item. Once a MEL is approved, it becomes a 'bible' for dispatch or no-dispatch.
A MMEL is not enforceable, as it is not minded by a LOA, but is more guidance in determining whether dispatch is recommended. A MEL can be more restrictive than a MMEL, but not less restrictive. By the definition of 'airworthy'; therefore, the MEL may ground a plane for certain operations, but does not make the aircraft 'unairworthy' for other operations. e.g. An operation operating an aircraft under a Part 135 certificate with a more restrictive MEL approved for the 135 may ground the plane for 135 operations, yet the owner may operate the same airframe under Part 91 using their MMEL if allowed.
The FAA would love all jets to adhere to at least a MMEL, and push operators to get a MEL approved through the LOA (Letter of Authorization) process.