Recently, I was operating 4 sectors with a change of aircraft after the second sector. The second aircraft had an acceptable minimum equipment list (MEL) and the same was mentioned in dispatch documents. On closer scrutiny, we realized that the MEL number mentioned and the unserviceability were not in conformity. We double-checked before flagging the issue and soon enough the MEL number was amended and fresh dispatch documents were issued.
I completed my task as planned but was still wondering about events and wanted to dig deeper to understand the failure of the safety defense mechanism. I soon uncovered a disturbing statistics which provides a summary of the MEL events.
- Aircraft dispatch against MEL requirements – 50%
- Operating aircraft without an approved MEL – 17%
- Ambiguous and incomplete MEL-related cases – 21%
- Human error/decision making regarding MEL application and management – 12%.
The incident highlights the importance of being vigilant and proper training and continued education of engineers/pilots to strengthen the organization’s safety system.
MEL Purpose
Before the 1960s aircraft dispatch was a topic of discussion between aircraft operators and regulators. However, things changed after the institution of MEL. The MEL is a document that lists aircraft components or systems that may be inoperable for dispatch. Aircraft manufacturers produce the master minimum equipment list (MMEL), and operators are required to develop their MELs based on the master document. The MEL must be more restrictive than the MMEL and must be developed based on the environment the aircraft would be operated. A thorough analysis is conducted before including a system/component in the list using techniques and tools such as Fault Tree Analysis (FTA) and Failure Mode and Effects Analysis (FMEA). These tools have proved useful and yield satisfactory outcomes, however, their efficacy has been doubted with the increasing complexity of systems, diverse environment, and human-machine interface. While some have questioned the acceptable level of degraded safety itself.
Typical all commercial aircraft are designed and certified with a significant amount of equipment redundancy, and as a result, it can be demonstrated that the airworthiness requirements are satisfied by a substantial margin. It may also be the case that some equipment that is fitted to the aircraft is not actually required for safe operation under all operating conditions, e.g. instrument lighting in day VMC. Other equipment, such as entertainment systems or galley equipment, maybe installed only for passenger convenience. If it is the case that this non-safety-related equipment does not affect the airworthiness or operation of the aircraft when inoperative, it is not required to be listed in the MMEL/MEL or for that matter to be given a rectification interval. However, in the case that the “non-safety” related equipment has a safety-related function for example the use of the entertainment system for passenger briefings – then this item must be included in the MMEL/MEL with an appropriate rectification interval.
The MEL is a joint operations and maintenance document prepared by an operator to:
- Identify the minimum equipment and conditions for an aircraft to maintain the Certificate of Airworthiness in force and to meet the operating rules for the type of operation;
- Define operational procedures necessary to maintain an acceptable level of safety and to deal with inoperative equipment; and
- Define maintenance procedures necessary to maintain an acceptable level of safety and procedures necessary to secure any inoperative equipment.
Regulatory Definitions
A minimum equipment list (MEL) is a list that provides for the operation of aircraft, subject to specified conditions, with particular equipment inoperative (which is) prepared by an operator in conformity with, or more restrictive than, the MEL established for the aircraft type. (ICAO Annex 6: Operation of Aircraft).
The MEL is approved by the operator’s national airworthiness authorities.
The master minimum equipment list (MMEL) is a list established for a particular aircraft type by the organisation responsible for the type design with the approval of the State of Design which identifies items which individually may be unserviceable at the commencement of a flight. The MMEL may be associated with special operating conditions, limitations or procedures. (ICAO Annex 6: Operation of Aircraft)
The operator shall include in the operations manual a minimum equipment list (MEL), approved by the State of the Operator which will enable the pilot-in-command to determine whether a flight may be commenced or continued from any intermediate stop should any instrument, equipment or systems become inoperative. Where the State of the Operator is not the State of Registry, the State of the Operator shall ensure that the MEL does not affect the aeroplane’s compliance with the airworthiness requirements applicable in the State of Registry. (ICAO Annex 6 Part I Chapter 6 Para 6.1.1)
An operator may not operate an aircraft that does not comply with the approved MEL, except with the explicit permission of the Appropriate Regulatory Authority. Such permission will not be granted to allow the aircraft to operate outside conditions set by the corresponding MMEL.
In most cases, multiple unserviceabilities of unrelated aircraft systems cannot be addressed by an MMEL nor, therefore, by a MEL. The decision as to whether or not to accept for flight an aircraft which has multiple unserviceabilities which would individually be allowable by MEL provisions ultimately rests with the designated Aircraft Commander, subject to guidance promulgated on a proactive or ad hoc basis by the aircraft operator.
In any case, if multiple unserviceabilities exist, the MEL should be consulted for each individual item to check if there are any incompatibilities for each of the associated dispatch conditions.
European Regulations and supplementary information concerning the MEL for fixed-wing air carrier aircraft are contained in IR-OPS and EU-OPS. The FAA system is similar except that FAA considers an approved MEL to be a Supplementary Type Certificate (STC) issued to a particular aircraft by serial number and registration number as a way of providing authority to fly it in a condition other than that at which it was originally type-certificated.
The MEL is entirely separate from the Configuration Deviation List (CDL), which is a list of secondary airframe and engine components which may be recorded as missing for without prejudicing the acceptance of an aircraft for flight.
Further Reading
- A complete list of FAA current MMELs for all aircraft types (commercial and GA) from the FAA Flight Standards Information Management System.
- “Master Minimum Equipment Lists (MMEL) and Minimum Equipment Lists (MEL)”, UK CAA CAP 549
- ICAO Annex 6 Part I Attachment G
- A Recall on the Correct Use of the MEL, an article in the Safety First magazine, #25, January 2018
EASA
- ARO.OPS.205 Minimum equipment list approval
- ORO.MLR.105 Minimum equipment list
- CAT.IDE.A.105 Minimum equipment for flight
and associated Acceptable Means of Compliance and Guidance Material
- EU-OPS 1.030/JAR-OPS 3.030
Pic courtesy: https://podtail.com
