Minimum Equipment List (MEL) is an FAA or EASA-approved document that lists the aircraft systems and components that may be inoperative while the aircraft remains airworthy and legal to fly.
How It Works#
An MEL is derived from the aircraft manufacturer's Master Minimum Equipment List (MMEL), which the manufacturer develops and submits to the certifying authority. The MMEL sets the outer boundary of what is permissible. An airline or operator then creates their own MEL, which can be equal to or more restrictive than the MMEL, but never more permissive.
Each MEL entry covers a specific piece of equipment. It tells the operator three things: how many units may be inoperative, under what conditions, and what procedures (if any) must be followed. Those procedures fall into two categories:
- (M) items: A maintenance action is required before flight.
- (O) items: An operational procedure is required, performed by the crew.
When a pilot or dispatcher releases a flight with an item deferred under the MEL, the aircraft enters a dispatch deviation status. The inoperative item is logged in the aircraft's maintenance logbook and tagged. A repair interval (Category A, B, C, or D) sets the deadline for fixing it.
Example in Aviation#
A regional airline crew arrives at the gate for a morning departure. The co-pilot's windshield heat system shows inoperative during preflight checks. The crew consults the MEL and finds that windshield heat is listed as a deferrable item under certain conditions, including a restriction to operations below a specific altitude and outside known icing conditions. The dispatcher applies the deferral, the maintenance crew tags the component and logs the entry, and the flight departs legally within those limits.
Why It Matters#
Without an MEL, any inoperative item would ground the aircraft entirely. The MEL exists because aviation regulators and manufacturers have already determined that certain equipment failures do not meaningfully reduce safety when specific conditions and procedures are met. It keeps operations efficient without compromising airworthiness.
For student pilots and enthusiasts, understanding the MEL explains why a commercial aircraft sometimes operates with a known discrepancy and why that is perfectly legal. For working pilots and dispatchers, the MEL is a core tool in day-to-day operations. Misapplying it or ignoring its conditions creates a direct regulatory violation and a potential safety risk.
Key Takeaways#
- An MEL is operator-specific and must be approved by the relevant aviation authority.
- It is always derived from, and never more permissive than, the MMEL.
- Each deferral includes conditions, time limits, and required procedures.
- Inoperative items are logged and tagged before the flight departs.
- The MEL keeps aircraft in service legally while maintaining defined safety standards.