Minimum Equipment List (MEL) – Aviation Maintenance

The Minimum Equipment List (MEL) is a core document in aircraft maintenance and flight operations, allowing airlines and operators to dispatch aircraft with certain inoperative equipment—provided strict regulatory requirements, procedures, and limitations are met. The MEL is tailored to each operator’s specific fleet and is based on the manufacturer’s Master Minimum Equipment List (MMEL), but adapted for the installed equipment, modifications, and operational contexts of each aircraft.

What is the Minimum Equipment List (MEL)?

The MEL is an operator-specific legal document, approved by regulatory authorities such as the FAA (Federal Aviation Administration) or EASA (European Union Aviation Safety Agency). It lists, by system and component, which equipment may be inoperative for a flight to proceed and under what circumstances. The MEL is not a blanket permission for equipment to be inoperative; rather, it is a carefully structured list of exceptions to the rule that all installed equipment must be operative before flight.

For each item, the MEL specifies:

• The minimum number required for dispatch
• The maximum time the item may remain unserviceable (rectification interval)
• Required operational or maintenance procedures (called (O) and (M) procedures)
• Any limitations to ensure continued safety and legal compliance

The MEL thus forms the bridge between regulatory requirements for airworthiness and the realities of line operations, where minor, non-critical equipment failures may occur.

Why Do MELs Exist? Purpose and Philosophy

Controlled Flexibility with Safety

Aircraft are complex machines with built-in redundancy. Not every system is essential for every flight. By permitting temporary, controlled operation with some items inoperative (typically non-critical to safety or airworthiness), the MEL helps:

• Maximize dispatch reliability
• Minimize unnecessary delays and cancellations
• Focus maintenance resources efficiently

However, this flexibility is strictly limited by regulatory oversight and the MEL’s detailed procedures. No relief is permitted for critical systems (such as flight controls, fire suppression, or essential navigation) unless specifically analyzed and approved.

Risk Management and Redundancy

The MEL is a product of risk assessment, engineering analysis, and operational experience. Each entry has been evaluated to ensure that operating without the item, within specified conditions, does not compromise safety. The MEL’s conditions (such as daylight-only operation or alternate procedures) are designed to compensate for the loss of function until repairs are made.

Regulatory Compliance

The MEL is legally enforceable. Once approved, it becomes a supplement to the Aircraft Flight Manual (AFM) and is mandatory for the operator. Operations outside its terms are regulatory violations, potentially resulting in fines or grounding.

Regulatory Foundations and Legal Authority

• United States: The MEL is required and regulated by 14 CFR 91.213, 121, and 135, as well as FAA guidance and OpSpecs.
• Europe: EASA and national authorities require MELs for commercial and turbine-powered operations, referencing MMELs and ICAO Annex 6.
• Approval: MELs must be approved by the relevant aviation authority before use, documented with a Letter of Authorization (LOA) or Operations Specification (OpSpec).

MEL vs. MMEL: What’s the Difference?

• MMEL (Master MEL): Created by the aircraft manufacturer and approved by aviation authorities. Covers all possible equipment for a specific aircraft type/model.
• MEL (Minimum Equipment List): Created by each operator, customized for their specific aircraft’s installed equipment and procedures. Must be at least as restrictive as the MMEL. The MEL is the document legally binding for dispatch.

Other related documents:

• KOEL: Lists equipment required for specific types of operations (AFM section).
• CDL: Covers allowable external parts that may be missing.
• NEF: Lists non-essential furnishings that can be inoperative.

Structure of the MEL

Each MEL is organized for rapid reference and compliance:

Categories of Rectification Interval

Note: The clock starts at midnight after defect discovery and log entry. Some categories allow for one extension, subject to procedures.

(O) and (M) Procedures

• (O): Operational procedure required by flight crew (e.g., alternate checklist, confirming alternate navigation).
• (M): Maintenance action required before dispatch (e.g., deactivation, placarding, securing loose items).

Example Entry:

MEL Creation and Approval

1. Obtain MMEL: Start with the latest MMEL for your aircraft type.
2. Customize: Remove items not installed; add STC/modification items; tailor procedures and restrictions to your operations.
3. Develop (O) and (M) Procedures: Reference manuals or create operator-specific steps.
4. Review: Internal quality checks and compliance documentation.
5. Submit for Approval: Authority (FAA/EASA) reviews and approves, issuing LOA/OpSpec.
6. Maintain and Update: Keep MEL current with MMEL revisions, aircraft changes, and regulatory updates.

MEL Use in Daily Operations

Step-by-Step Dispatch with the MEL

1. Log the Defect: Enter the issue in the maintenance logbook.
2. Locate MEL Entry: Identify the affected item by ATA chapter/item.
3. Check Applicability: Confirm MEL entry fits aircraft configuration and planned operation.
4. Review Procedures & Limitations: Complete all (O) and (M) steps, placarding, and documentation.
5. Verify Dispatch Legality: Ensure minimum required equipment and rectification interval are met.
6. Communicate: Inform flight, dispatch, and maintenance personnel.
7. Make Dispatch Decision: The PIC is responsible for final compliance. If any MEL condition is unmet, the aircraft cannot be dispatched.

Example Scenarios

• Landing Light Inoperative for Night Flight: If one of two landing lights is out, and the MEL requires both for night, the flight is not legal for night operation until repaired. For day operation, it may be dispatched with the light placarded and a repair made within 3 days.
• VHF Radio Out: MEL may permit dispatch with one of two radios inop, but may require an alternate means of communication, placarding, and a 10-day repair limit.

MEL Limitations and Violations

• MEL relief only applies to listed items.
• Deferring repairs outside MEL limits or on unlisted items is a regulatory violation.
• All MEL entries must be followed precisely, including placarding, documentation, and time limits.
• MEL must be updated with any MMEL changes, equipment modifications, or regulatory updates.

Best Practices for MEL Management

• Regular Training: Ensure all crews and maintenance staff are fluent in MEL use and compliance.
• Version Control: Only current, approved MELs are in circulation.
• Robust Documentation: Accurate logbook entries and defect tracking.
• Proactive Communication: Immediate notification of MEL changes or defects to all relevant personnel.

Conclusion

The Minimum Equipment List is a cornerstone of safe, efficient, and legally compliant flight operations. By providing controlled flexibility, the MEL allows operators to maintain dispatch reliability without compromising safety or regulatory obligations. Rigorous adherence to MEL procedures and diligent document management are essential to uphold airworthiness and avoid costly regulatory action.

For assistance with MEL development, compliance, or training, contact our aviation maintenance experts.