Aircraft are complex machines with numerous systems working in tandem to ensure safe operation. However, it may come as a surprise that aircraft can still fly even when certain systems or components are not functioning or even missing entirely.
This is possible due to carefully planned redundancies and a crucial safety protocol known as the Master Minimum Equipment List (MMEL) and configuration deviation list (CDL).
Thanks to these systems, even if a part of the aircraft malfunctions, it can remain in service without compromising the safety of the flight or the schedule of the operator.
What is the master minimum equipment list (MMEL)?
The master minimum equipment list (MMEL) is a document created by the manufacturer during the design phase of an aircraft, outlining the minimum equipment that must be serviceable for the aircraft to fly safely. Aircraft are designed with redundancies in place for their systems, such as multiple engines and generators, to ensure the continued safe operation of the aircraft. For example, some aircraft have four engines, each with two generators providing alternating current (AC) power for various systems. In such cases, the manufacturer, in collaboration with the relevant authorities, determines how long the aircraft can safely operate with one generator unserviceable.
Who creates the MMEL list?
The MMEL is developed by the aircraft manufacturer and is approved by the relevant aviation authority. Different authorities may impose slightly different restrictions on the MMEL, meaning an identical aircraft model may have varying limitations depending on its registration. This ensures the aircraft complies with specific regulations in various regions or operational environments.
Purpose of the MMEL
Modern aircraft have hundreds of systems, each with many components working together. With so many parts in play, inevitably, some systems may occasionally become unserviceable. Without the MMEL, aircraft would be grounded whenever a part failed. This list allows aircraft to continue flying, often with certain operational restrictions, until repairs can be made. This way, the MMEL provides flexibility and ensures minor issues don’t unnecessarily disrupt operations.
What is the minimum equipment list (MEL), and how does it differ from the MMEL?
The MEL is an operator’s version of the MMEL. While the MMEL is set by the manufacturer, the operator may create an MEL that is more restrictive but never less so than the original list. Operators often apply more stringent restrictions for various reasons, including safety, operational considerations, and regulatory compliance.
Reasons an operator may impose a more restrictive MEL include:
- Operational safety considerations
Operators may opt for stricter restrictions to ensure greater safety, particularly when flying in challenging environments or high-risk routes. - Regulatory or legal requirements
Some regulatory authorities (such as the UK Civil Aviation Authority or EASA) may impose additional constraints for certain regions or types of operations, necessitating a more restrictive MEL. Airlines may also follow guidelines from their insurers or regulators, which can impact their choice of MEL. - Type of operation
Airlines operating on long-haul flights, remote areas, or mountainous terrain may be more restrictive in their MEL to avoid potential problems. - Fleet maintenance capabilities
Operators with fewer maintenance facilities or limited spare parts may prefer to ground aircraft with unserviceable equipment, even if the MMEL allows continued operations. This reduces the risk of further complications during flights and the need for unexpected repairs. - Customer and brand expectations
Airlines that place high importance on passenger experience or brand reputation may choose to ground aircraft with specific inoperative systems, such as entertainment systems or cabin lighting, especially on premium flights. This ensures the comfort and satisfaction of passengers and preserves the airline’s image. - Operational flexibility vs. risk
Operators must balance the flexibility the MMEL provides with the potential risks of flying with unserviceable equipment. If a system’s failure could compromise safety, the operator may choose to implement a stricter MEL.
How is the MEL applied to a live aircraft for dispatch purposes?
When a component fails, such as an inoperative air conditioning pack, the aircraft may still be cleared for departure if the MEL allows it. For example, an aircraft can typically fly with one air conditioning pack unserviceable, as long as extended-range operations (ETOPS) are not involved. ETOPS refers to flights that take place over routes where emergency diversions to suitable airports are limited.
In this particular circumstance before dispatching the aircraft, the auxiliary pressurisation system must be tested to ensure it operates correctly, as this system is the backup for the air conditioning packs.
Once verified, the unserviceable pack is switched off, and a placard is placed on the flight deck to alert the crew. Additionally, the previous crew should have recorded the defect in the aircraft’s tech log, which needs to be closed before the aircraft can depart.
To close the defect, the engineering team refers to the MEL and cross-references the tech log entry. The operator monitors the situation to rectify the issue before the MEL expires.
How long does a MEL last before rectification is needed?
The MEL is divided into four categories, each with a specific time frame for rectification:
- Category A: Items must be repaired within the time interval specified in the remark column of the operator’s approved MEL, excluding the day the malfunction was recorded.
- Category B: Items must be repaired within three consecutive calendar days (72 hours).
- Category C: Items must be repaired within ten consecutive calendar days (240 hours).
- Category D: Items must be repaired within one hundred and twenty consecutive calendar days (2880 hours).
What is a configuration deviation list (CDL)?
A CDL is distinct from the MEL and MMEL. While those lists allow some equipment or systems to be unserviceable, the CDL covers items that can be completely missing from the aircraft without compromising safety. For example, a service door located behind the wing or a static discharger from each flight control surface may be missing without affecting the aircraft’s operation. These items can be excluded from the aircraft’s configuration, provided they do not impact its aerodynamics or safety.
Conclusion
It might surprise some passengers to learn that the aircraft they are boarding may have several inoperative systems . However, the MMEL and MEL systems ensure that these issues are carefully managed, and safety is always the top priority.
Airlines work closely with authorities to control these situations, ensuring that aircraft continue to operate safely and efficiently. Passengers can rest assured that any decisions made regarding inoperative systems are taken with their safety in mind.
