MEL Categories A, B, C and D: Aircraft Defect Rectification Times Explained

Aircraft incorporate redundancy, so certain equipment may be inoperative without compromising the safety of a flight. This does not permit defects to be ignored. When an item is deferred under an operator's Minimum Equipment List (MEL), it must be rectified within an approved time limit.

These limits are grouped into four MEL rectification categories: A, B, C and D. The applicable category, together with the conditions and procedures in the relevant MEL entry, determines whether the aircraft may be dispatched and when the defect must be repaired.

What are MEL deferral categories?

An MEL is an operator-specific document, derived from the aircraft manufacturer's Master Minimum Equipment List (MMEL) and approved by the relevant aviation authority. It identifies equipment that may be inoperative for a limited period while the aircraft remains airworthy, provided all stated conditions are met. For UK operators, the UK Civil Aviation Authority's MEL guidance provides the regulatory context for MEL approval and control.

An MEL entry may specify:

• The number of affected components permitted to be inoperative
• Operational limitations
• Maintenance procedures
• Placarding requirements
• Conditions for dispatch
• A rectification interval

The rectification interval is identified by an MEL category letter: A, B, C or D. An MEL deferral is a controlled and time-limited process, not permission to operate indefinitely with a defect. When the applicable interval expires, the defect must be rectified before further flight unless an authorised extension is available under the operator's approved procedures and the applicable aviation authority requirements.

MEL Category A: Interval stated in the MEL

Category A defects have no standard rectification period. The allowable interval is stated directly in the relevant MEL entry, usually in the remarks or exceptions section.

Depending on the item, the MEL may require rectification:

• Before the next flight
• Within a stated number of flight hours
• Within a stated number of flight cycles
• Within a defined number of calendar days
• Before a specified operating condition occurs

For example, an item may require repair within a specified number of flight hours or cycles, whichever occurs first. Another item may permit operation only until the next suitable maintenance opportunity.

Category A: Rectify within the specific interval stated in the MEL entry.

MEL Category B: Three consecutive calendar days

A Category B defect must be rectified within three consecutive calendar days, excluding the day on which the defect was discovered or the MEL was invoked.

If a defect is entered in the technical log on a Monday, Monday is not counted. The three-day period begins on Tuesday:

• Tuesday: Day 1
• Wednesday: Day 2
• Thursday: Day 3

The defect must therefore be rectified by the end of Thursday, subject to the operator's approved procedures.

Category B: Three consecutive calendar days, excluding the day of discovery.

MEL Category C: Ten consecutive calendar days

A Category C defect must be rectified within 10 consecutive calendar days, excluding the day on which the defect was discovered.

If the defect is identified on 1 June, the counting period begins on 2 June. Day 10 is 11 June. Category C commonly applies to equipment that is not immediately critical to safe operation when the MEL conditions are met, but which still requires timely rectification.

Category C: Ten consecutive calendar days, excluding the day of discovery.

MEL Category D: 120 consecutive calendar days

A Category D defect has the longest standard rectification interval: 120 consecutive calendar days, excluding the day of discovery.

This category is normally reserved for items whose temporary loss does not affect safe operation, provided every MEL condition is satisfied. It may include certain cabin convenience items, such as individual reading lights, although the permitted item and associated limitations always depend on the approved MEL.

A 120-day interval does not make a defect permanent or insignificant. The item must remain controlled and tracked, and it must be rectified before the applicable deadline.

Category D: 120 consecutive calendar days, excluding the day of discovery.

MEL category rectification times

• Category A: As specifically stated in the MEL entry
• Category B: Three consecutive calendar days, excluding the day of discovery
• Category C: Ten consecutive calendar days, excluding the day of discovery
• Category D: 120 consecutive calendar days, excluding the day of discovery

These category intervals form part of the wider MMEL and MEL framework. The FAA's MMEL resources offer further official context on how master lists support operator-specific dispatch procedures.

How is the MEL deferral period counted?

A common error is to count the day on which a defect is discovered as Day 1. For Categories B, C and D, that day is excluded. The following calendar day is Day 1.

For a Category C defect found on 10 March:

• 10 March: Defect discovered, not counted
• 11 March: Day 1
• 20 March: Day 10

The rectification deadline is therefore based on 20 March, in accordance with the operator's approved MEL procedures. Operators may impose more restrictive limits, so the technical log, MEL control system and maintenance procedures must always be used to confirm the actual due date. For practical guidance on maintaining accurate records, read our guide to filling out a CAP 741 logbook.

An MEL deferral is not simply a maintenance delay

Before an aircraft is dispatched with inoperative equipment, the applicable MEL item must be followed in full. This involves more than recording the defect and assigning a category.

The crew or maintenance team may need to complete:

• An operational procedure, often called an O procedure
• A maintenance procedure, often called an M procedure
• Deactivation or isolation of the failed system
• Installation of a cockpit or cabin placard
• A technical log entry
• Performance or route restrictions
• Additional checks before each flight

A system may be permitted to remain inoperative only when another redundant system is serviceable, a particular type of operation is avoided, or a specified maintenance action has been completed. If the conditions in the MEL cannot be met, the aircraft cannot be dispatched under that MEL item. Learn more about the broader regulatory considerations in our guide to aviation compliance challenges.

MEL versus CDL: What is the difference?

The MEL applies to installed equipment that is inoperative. The Configuration Deviation List (CDL) applies when specified external aircraft parts are missing or damaged, such as certain fairings, access panels or aerodynamic components.

A CDL dispatch may include operating limitations or performance penalties because missing external items can affect drag, fuel consumption or handling characteristics. Both documents provide controlled means of operating an aircraft with defined defects or configuration deviations, and both require strict compliance with their stated conditions and limitations.

Why MEL categories matter

MEL categories balance operational reliability with the need to restore full aircraft capability promptly. They allow operators to manage defects in minor or redundant systems without unnecessary cancellations, while ensuring that every deferred defect is monitored and rectified within approved limits.

The essential points are:

• Category A: Follow the exact interval stated in the MEL.
• Category B: Repair within three consecutive calendar days.
• Category C: Repair within 10 consecutive calendar days.
• Category D: Repair within 120 consecutive calendar days.
• Categories B, C and D: Do not count the day the defect was discovered.
• All categories: Follow the specific MEL entry, associated procedures and operator manuals.

An MEL is not a shortcut around airworthiness requirements. It is an approved safety control that permits continued operation only when the remaining systems, procedures and limitations maintain an acceptable level of safety.