Which Minimum Equipment List (MEL) categories would require a commercial airliner to be grounded (Aircraft On Ground or AOG) if they're inoperative? For example, any category A MEL item is considered an AOG item; is it the same for category B items?
FAR §91.213 requires that all equipment installed on the airplane is in an airworthy state.
(a) Except as provided in paragraph (d) of this section, no person may take off an aircraft with inoperative instruments or equipment installed unless the following conditions are met:
(1) An approved Minimum Equipment List exists for that aircraft.
(2) The aircraft has within it a letter of authorization, issued by the FAA Flight Standards district office having jurisdiction over the area in which the operator is located, authorizing operation of the aircraft under the Minimum Equipment List. The letter of authorization may be obtained by written request of the airworthiness certificate holder. The Minimum Equipment List and the letter of authorization constitute a supplemental type certificate for the aircraft.
Any inoperative equipment not listed on the MEL has to be operational before flight.
The purpose of the MEL is to provide relief from the requirement that everything works on the airplane to be able to legally takeoff.
Although all equipment listed on the MEL can be deferred there are specific conditions that have to be followed to ensure the flight can be operated safely.
For example take this excerpt from a commercial aircraft MMEL.
The crew can dispatch provided they mark the lavatory inoperative and lock the door.
The deferred equipment can't be deferred forever. Notice the "C"; that is a repair category and it indicates how many days dispatch can occur without fixing the inoperative equipment.
- A: Time frame listed by the inoperative equipment
- B: 3 calendars days before repair is required
- C: 10 calendar days before repair is required
- D: 120 calendar days before repair is required
If a piece of inop equipment CAN be deferred using the MEL, then the aircraft isn't grounded. When there is no MEL relief & the part has to be changed out is when you have an AOG scenario.