A Recall on the Correct Use of the MEL

• 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 must be as restrictive as or more 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 systems, functions or components inoperative, thus to optimize aircraft dispatch reliability and profitability without impairing safety.

The MMEL provides a list of items with associated conditions for dispatch.

For every item, Airbus must demonstrate that the associated dispatch conditions are compliant with the certification requirements as specified by EASA. The major steps of this demonstration are the following:

Step 1: Assessment of the MMEL item to identify any operational impact or impact on other system functions, and check if there is any influence on the safety level of the aircraft.

Step 2: Identification and assessment of the operational and safety impact of the next critical aircraft system failure which may occur during subsequent flights.

Step 3: Definition of any maintenance actions or operational procedures that may be necessary as a means of mitigation for the assessed impacts of the MMEL item.
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 conditions, or
• “NO GO” when the dispatch is not permitted and corrective maintenance action must be undertaken before the aircraft can continue operations.

The Operator’s MEL is a dispatch document which should be tailored according to the Operator’s routes, procedures and applicable local regulations, 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 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 takeoff 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 from the regulations defined by the EASA or by the 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 tech log, 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. If there is no MEL restriction, 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.

Aircraft system defects detected in the passenger cabin may be reported by the cabin crew in the cabin logbook. Should the defect have an impact on dispatch, these entries must be transferred to the aircraft’s technical logbook before assessing the relevant MEL item applicability.

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).

If a failure is classified as “NO DISPATCH” (fig.2), the aircraft must not be dispatched until the equipment or function is rectified.

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)

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

• The dispatch condition assessment requires additional information such as ECAM indication on the SD page (fig.5)

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)

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.

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.

It is important to identify the MEL item correctly. The application of a MEL item that does not correspond to the inoperative equipment or unavailable system function may have unintended consequences for the safety of the flight.

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).

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).

 Plan the repair as soon as possible to avoid operational disruption should additional failure occur that may make the dispatch impossible.

•  Specificity of Category “A” repair intervals

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

For the first dispatch after applying the MEL item, all dispatch conditions and associated limitations must be accounted for and any relevant maintenance (m) and operational (o) procedures must be applied to maintain an acceptable level of safety for the operation of the aircraft, even with the inoperative equipment or unavailable system function.

For a complete awareness of aircraft dispatch condition, maintenance personnel should also consult the operational procedure (when applicable).

An incorrect or incomplete application of the maintenance or operational procedure may impair the safety of the flight.

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.

In the case of a multiple MEL items logged, flight crew and maintenance personnel must check before each flight that dispatch conditions of all MEL items are fulfilled.