• MMEL

The Airbus MMEL is a dispatch document that is produced by the aircraft manufacturer and approved by the certification authorities.

The MMEL is used as a reference by the operators to create their own MEL, which will permit the dispatch and operation of an aircraft with one or more inoperative equipment or unavailable system function while maintaining an acceptable level of safety.

• MEL

The MEL is a dispatch document developed by the operator based on the aircraft manufacturer’s MMEL. The MEL cannot be less restrictive than the MMEL and must be approved by the operator’s national airworthiness authorities.

The MEL permits the Operator to assess the impact on their operations (flight schedule, route, environmental conditions,...) while operating an aircraft with system, functions or component inoperative, thus to optimize aircraft dispatch reliability and profitability without impairing safety.

The MMEL gives the baseline to maintain an acceptable level of safety when flying with failed equipment.

To demonstrate an acceptable level of safety, Airbus has defined a safety methodology that is applied for the development of each MMEL item. This safety methodology is based on the EASA certification requirements that defines this acceptable level of safety, based on safety classifications. This safety methodology was reviewed and accepted by EASA.

The consequence of the failure itself is assessed based on its safety impact at aircraft level. The safety objective is MINOR. Specific limitations or procedures can be put in place to mitigate initial consequences and meet the safety objective.

The next critical failure, which is the failure that would have the most critical consequences when combined with the initial failure present at dispatch, is assessed. The safety objective is MAJOR. As for step 1, it is a purely qualitative analysis, and as for step 1, additional limitations or procedures can be put in place to meet this safety classification.

All HAZ and CAT failures in which the MMEL item is involved are analyzed. This time, the analysis is quantitative. Compliance with all the safety objectives has to be ensured. This can lead to reduced exposure with a limited time for the dispatch.

For each MMEL item, an acceptable level of safety is ensured by complying to all the safety objectives for all steps. 

Based on the above assessments, a dispatch status is defined for each MMEL item as either:

• “GO” when the dispatch is permitted for a limited period of time without specific dispatch condition, or
• “GO IF” when the dispatch is permitted for a limited period of time with specific dispatch condition, or
• “NO GO” when the dispatch is not permitted and corrective maintenance action must be undertaken before the aircraft can continue operations.

The Operator MEL is a dispatch document which should be tailored according to the operator’s routes and procedures and within the constraints defined by the aircraft manufacturer’s MMEL.

As per regulations, when there are failures or defects that cannot be rectified, and which are covered by an MEL item, the MEL must be applied prior to departure and accepted by the Captain.

The “departure” corresponds to the “commencement of the flight”. “The commencement of the flight” is defined as the moment when the aircraft starts to move under its own power for the purpose of takeoff (i.e. the taxi phase).

EASA and FAA require Operators to define procedures in their MEL for the management of any failure that occurs during the taxi-out phase.

EASA regulations

The EASA regulations require that Operators define an appropriate guidance for flight crew for the management of failures if they occur between the start of taxi and commencement of take-off roll.

The EASA regulations also state that the captain may decide to continue with the flight based on their “good judgment and airmanship”. Additionally, their regulations allow flight crew to consult the MEL if it will help them to make a decision. Communication with dispatch, or the Operator’s maintenance control centre, may assist the flight crew in their assessment of the MEL item and aid the Captain to decide if they will continue with the flight or not.

The final decision to continue with the flight is the responsibility of the Captain. This decision should be based on any operational considerations that could impair the current flight and also consider any impact on the subsequent missions of the aircraft.

FAA regulations

The FAA regulations require that Operators establish a procedure for the Pilot In Command (PIC) to communicate with the aircraft dispatch and maintenance organizations when a failure occurs after an aircraft departs the gate or ramp area, during pushback, taxi or prior to take-off.

This procedure permits the flight crew to review the situation and determine if the aircraft can be either dispatched with the failure under the MEL item, or if the failure must be rectified before take-off. If a dispatch with the failure under the relevant MEL item is advised, the return to the gate to accomplish the appropriate maintenance or operational procedure must be considered. In coordination with the Operator’s dispatch and maintenance organization, certain MEL procedures may be accomplished by the flight crew without returning to the gate, provided these procedures are approved by the FAA’s Principal Operations Inspector (POI).

Other Local regulations

Other National Aviation Authorities (NAA) may have regulations that differ to regulations defined by EASA or FAA. It is the responsibility of each Operator to check the applicable regulations with their relevant NAA.

The MEL is defined as a dispatch document and therefore the MEL is not applicable in flight. However, if a system or equipment defect is detected during flight, the MEL information may be useful to assess the likely dispatch condition for the next flight. A detailed description of the defect detected should be entered in the technical logbook, and operator’s dispatch or Maintenance Control Centre notified so they can consult MEL when the aircraft arrives.

If several aircraft system functions or equipment are inoperative, operators should consult the MEL for each individual item to check if there are any incompatibilities for each of the associated dispatch conditions. It is the flight crew’s responsibility to assess the situation and to decide whether or not to dispatch the aircraft with multiple inoperative items.

From the moment of the failure has occurred until the dispatch of the aircraft, the following steps should be followed to ensure that the aircraft can be dispatched in an airworthy condition.

The identification of the failure is usually based on the ECAM alert’s title and the dispatch assessment is provided in the MEL Entries section under the “CONDITION OF DISPATCH” header (fig.1).

(fig.1)
Example of a MEL entry with an associated MEL item

(fig.2)
Example of a MEL entry with a “NO DISPATCH” condition

In some cases, MEL Entries section may require additional action by the flight crew or the maintenance crew in order to assess the dispatch conditions, particularly when:

• One ECAM alert refers to several MMEL items (fig.3)
  

(fig.3)
Example of a MEL entry with several associated MEL items

• The dispatch condition assessment depends whether the ECAM alert is actual or false (spurious) (fig.4)

(fig.4)
Example of a MEL with dispatch condition depending whether the alert is actual of false

(fig.5)
Example of a MEL entry with the dispatch condition depending on indications on the associated SD page

On A350, the ECAM Dispatch Messages are a straight forward help for dispatch. The flight crew finds the Dispatch Message in the MEL entries section to get the condition of dispatch or identify the applicable MEL item (fig.6)

(fig.6)
Example of a Dispatch Message on A350

The A380 and A350 MEL also show a “Crew Observations” section in the MEL entries (fig.7) covering failures of monitored systems that are indicated with flight deck effects that don’t have an associated ECAM alert or Dispatch Message, for example, an amber indication on system display (SD) page or when the FAULT light of a pushbutton switch illuminates. The “Crew Observations” section also covers malfunctions that can be visually detected by the flight crew or the maintenance personnel, for example during the external walk around.

(fig.7)
Example of a Crew Observation in the MEL Entries section on A380

If the failure is not linked to an ECAM alert or to a failure reflected in the Crew Observation section (A380 & A350), the correct MEL item should be identified directly into the MEL items section.

When the MEL item is correctly identified, the flight crew should carefully review the dispatch condition.

If there are several dispatch conditions, the title of the associated dispatch condition helps to identify which one is applicable (fig.8).

(fig.8)
Example of a MEL item with two dispatch conditions: 36-12-02A and 36-12-02B

Maintenance personnel may propose to dispatch the aircraft under MEL item provided that all of the associated dispatch conditions are fulfilled.

It is the Captain’s responsibility to accept the aircraft dispatch under the MEL item for the flight; taking into account not only the MEL dispatch condition but also the applicable operator’s policy and the operational constraints.

The maintenance personnel must make an entry into the logbook for the MEL item and determine the deadline for rectification based on the MEL repair interval.

Inoperative items should be repaired as soon as possible and at least within the period of time defined by the repair interval (fig.9).

• The allowable intervals for rectification are classified as the following:

(fig.9)
Principle of the repair interval

•  Specificity of Category “A” repair intervals
  

MEL with category “A” repair intervals can use different references, e.g. calendar days, flight cycles…

For the subsequent flights, the flight crew must check that any open MEL item in the logbook is within the window of the repair interval and that this time limit won’t be exceeded during the next flight mission.

When the dispatch conditions are accepted by the captain (as described in step 5), all necessary operational procedures must also be applied.

In the case when a new MEL item is recorded in the technical logbook, the maintenance personnel must also review all of the pre-existing MEL items to ensure that all of the dispatch conditions for each item are fulfilled.