MEL — Minimum Equipment List

Two Documents, Two Authors

Not every piece of equipment on an aircraft has to be working for that aircraft to fly safely and legally — but which items can be inoperative, and under what conditions, isn't left to individual judgment. It's governed by two related but distinct documents. The MMEL (Master Minimum Equipment List) is established by the organisation responsible for the aircraft's type design — the manufacturer — and approved by the aircraft's State of Design (the manufacturer's home regulator, such as the FAA for Boeing or EASA for Airbus). It sets the baseline: the absolute maximum set of items that could ever be allowed to be inoperative on that aircraft type, aircraft-model-wide, under the most permissive interpretation possible.

From MMEL to MEL

Every airline operating that aircraft type must then produce its own MEL (Minimum Equipment List), tailored to its own fleet, routes and operations — but critically, an airline's MEL must always be equal to or more restrictive than the manufacturer's MMEL, never more permissive. An airline cannot decide on its own that an item the MMEL doesn't allow to be deferred can nonetheless fly; it can only choose to be stricter than the baseline the manufacturer and regulator already set. The airline's MEL then has to be separately approved by its own regulator — the State of the Operator — which checks it against both the MMEL ceiling and the airworthiness standards that apply to the aircraft's State of Registry.

What "Deferring" an Item Actually Means

When a piece of equipment fails before a flight, the MEL is what a dispatcher and maintenance crew check to determine whether that specific aircraft can still depart with the item inoperative, and if so, under what conditions — a particular seat's reading light being out is a completely different category of problem from a redundant hydraulic pump being out, and the MEL treats them accordingly. Deferring an item under the MEL isn't the same as ignoring it: deferred items are logged, tracked, and — depending on their category — required to be fixed within a defined time window (commonly ranging from before the next flight up to a matter of days), and some MEL entries carry specific operational limitations attached, such as reduced maximum altitude or a requirement to disable a related system entirely until repair.

Why This System Exists

The MEL framework exists precisely so that a single minor equipment fault doesn't automatically ground an aircraft and cascade into widespread delays and cancellations, while still guaranteeing that nothing safety-critical is ever allowed to fly inoperative — it's a deliberately engineered middle ground between "everything must work perfectly" and "anything can be broken," built on the manufacturer's and regulator's own judgment about what genuinely matters for a given flight.