Most people involved in the operational side of commercial aviation know the Minimum Equipment List. The ‘MEL’ is a regulatory program that allows an aircraft to be dispatched with inoperative equipment or systems and lists any conditions or restrictions required for dispatch under MEL relief. When an item from the MEL is applied, it is often called a ‘deferral.’1
It is assumed the wings are attached, and the engines can produce normal controllable thrust. The MEL contains many of the most critical systems and associated components required to maintain the safety of flight and takes into consideration redundancies built into system designs. Therefore it is understood that equipment and flight-critical system components not listed in the MEL must be operative for flight. Items considered non-airworthiness items/nonessential furnishings or passenger convenience items need not be listed in the MEL but must be defined in the operator’s maintenance program. (The maintenance control manual, for example, may list what qualifies as passenger convenience items).
Master Minimum Equipment List
An operator’s MEL is a flight operations document based on a Master MEL (MMEL) published by their regulatory authority (FAA, EASA, Transport Canada, etc.). The MMEL is specific to an aircraft type/make/model and is designed to cover the most common variations of that aircraft type. (ex. one MMEL for all the Airbus A318, 319, 320, and 321 variants, one MMEL for Boeing 787-8, 787-9, 787-10 variants). The operator’s MEL cannot be less restrictive than the MMEL, but it can be more restrictive and even customized in certain situations to fit their operational needs.2
The systems and components contained in the MMEL are known as MEL items. The Aircraft Evaluation Group (AEG)3 from the state of aircraft design (not necessarily manufacturer) decides which items are eligible for MMEL relief. In the US, the FAA (AEG) decides what items are required in the Boeing MMELs as it is the FAA who has regulatory oversight of Boeing as a US-based OEM.
Each regulatory authority will normally republish the Master MMEL from the state of design to comply with their own regulations which can be in the form of another MMEL or an MMEL supplement.4 The FAA republishes the EASA Airbus MMELs into another FAA MMEL so US based operators comply with both the intent of the EASA Airbus MMEL and FAA federal air regulations that may differ from EASA. For example: FAA MMEL for the Bombardier CL605 business jet allows dispatch with an inoperative nosewheel steering system, but the EASA MMEL does not. However, suppose MEL relief is available for dispatch. In that case, the captain has the final say whether they operate (or not), and many factors will be considered in his/her/their decision-making. (i.e., weather and surface conditions at departure and arrival airports, crew competency, airfield congestion).
What is included in the Operator’s MEL?
- The MEL item/subitem and follows the ATA 100 numbering system
- Aircraft Effectivity, using Fleet Identification Numbers (FIN), Manufacturing Serial Numbers (MSN), or Line Numbers (LN) depending on the operator.
- The Maintenance Repair Interval (How long MEL relief is allowed after which it must be repaired
- The number of applicable systems or components installed
- How many of the applicable items/systems are required for flight with that relief applied
- Provisos – Conditions required or restricted for dispatch. (May be inoperative, provided…). Some critical system MEL items may be listed in the MMEL as ‘NOGO’ or ‘Must be Operative.’5
- Associated Operations and/or Maintenance Procedures that are required to satisfy the associated provisos from the MMEL. Maintenance Procedures are carried out by Qualified Maintenance personnel when the MEL item is first applied (or before each flight if required by provisos). Operations Procedures are carried out by Flight Crew when the MEL item is first applied (or before each flight if required by provisos).
Example OPERATIONS PROCEDURES (to be completed by flight crew before flight)
When can the MEL be applied?
As the MEL is a dispatch document, it must be applied before flight. However, it can be applied right up until the thrust/power levers are advanced to takeoff power, and the aircraft commences takeoff roll. Nevertheless, a takeoff may be rejected before V16, come to a stop, and then MEL relief for the new fault will be considered after consulting with maintenance. However, once the aircraft becomes airborne, any further faults are addressed by other flight operations manuals such as the Quick Reference Handbook (QRH) and Flight Crew Operating Manual (FCOM).
For how long can MEL relief for a specific item be applied?
This is determined by the Repair Interval listed in the MEL: CAT A, B, C, or D.
CAT A – is variable and depends on the MEL provisos.7,9,10
CAT B – 3 Calendar Days7,8
CAT C – 10 Calendar Days7,8
CAT D – 120 Calendar Days7
NOTES1. Called a deferral because something is inoperative, and applying the MEL deviates from the approved maintenance program, which states that the aircraft, its systems, and components must be operative.
2. Although the MEL is indeed a Flight Operations document, Maintenance is the main user of the MEL. That said, everyone involved with the maintenance, dispatch, and operation of an aircraft must review the MEL, all provisos but only the procedures applicable to them. Sometimes, provisos in the MMEL are not written very well, and the Operator may choose to customize the phraseology used. This is fine if the MEL captures the intent of the item in the MMEL. Additionally, the Operator can seek other MEL relief for system functions, extra safety equipment, etc., that are not in the MMEL. For example, excess safety equipment beyond what is required by regulations can be added to the operator MEL.
3. Aircraft Evaluation Group - Assigned to each aircraft certification directorate, addresses Flight Standards considerations during type certification, evaluates operational and maintenance aspects of certification, and evaluates continuing airworthiness requirements of newly certificated or modified products and parts.
4. In Canada, Transport Canada publishes an MMEL Supplement called the Transport Canada Supplement, which essentially ‘Canadianizes’ MMELs to comply with Canadian Air Regulations and Canadian MEL policy.
5. Operative means a system and/or component will accomplish its intended purpose and is consistently functioning normally within its design operating limit(s) and tolerance(s). When a MEL item specifies that an item of equipment must be operative, it does not mean that its operational status must be verified; it is to be considered operative unless reported or known to be malfunctioning. Verified Operative means that it must be checked out and confirmed operative at the interval(s) specified for the MEL item. (i.e., before every flight). When an MMEL item specifies that an item of equipment must be verified, but no interval is specified, verification is only required at the time of deferral.
6. V1 is defined as the speed beyond which the takeoff should no longer be aborted if a fault occurs. Meaning that in case you experience any trouble with your aircraft before reaching V1, you can choose to abort the takeoff and apply all the necessary means to bring the aircraft to a safe stop. (Max braking, careful use of reverse thrust). If pilots experience serious aircraft malfunction after V1, the takeoff continues, and the fault is dealt with in the air. Otherwise, aborting a takeoff after V1 will probably lead to a runway overrun and could severely damage the aircraft and injure the occupants.
7. MEL relief in Calendar Days begins at 0001hrs (UTC) after the day of defect discovery.
8. CAT B/C Repair Intervals may be extended depending on procedures established between the operator and their regulatory authority. This may be done by the regulatory authority or by someone with self-extension privileges (like myself) on behalf of the authorities. This is strictly monitored, and I must have a valid reason for the extension such that it is required because of circumstances beyond the operator’s control. For example, a part is ordered for a CAT C MEL item, but the part doesn’t arrive before the 10-days expires. I may then offer an extension for the associated MEL item, depending on its course.
9. Some operators use A* as a separate MEL Repair category to note the item is self-imposed.
10. CAT A Repair Category may be listed in Calendar Days, Flight Days, Number of Flights, Cycles, Flight Hours. When a time is specified in cycles, number of flights, or flight hours, it shall start at the beginning of the first flight following the fault discovery. Otherwise, Calendar Days (or just days) commences at 0001hrs (UTC) the day following fault discovery.
By Timothy Templeman
MEL Controller, Boeing 787, and Airbus A320
Air Canada Flight Operations