When is the use of a Minimum Equipment List (MEL) considered mandatory?

The use of a Minimum Equipment List (MEL) is considered mandatory after an FAA Letter of Authorization is granted. The MEL is a document that details the equipment which may be inoperative for a specific aircraft type and still allow the aircraft to be deemed airworthy for flight. This list is only valid when the operator has received the Letter of Authorization from the FAA, which officially recognizes that the operator can use the MEL in lieu of the required equipment. Until this authorization is granted, the operator is unable to utilize the MEL framework for operations, therefore ensuring safety and compliance with federal regulations.

In contrast, simply having an MEL issued by the operator does not meet the regulatory requirements; the FAA must be involved to ensure that all necessary scrutiny has been applied. Additionally, an MEL does not apply when no discrepancies are found, as the list is designed for when equipment is inoperative. Operating under part 121 does not itself make the MEL mandatory; it is the FAA authorization that is crucial for its mandate.