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A Minimum Equipment List (MEL) is one method aircraft operators may use to obtain relief from Federal Aviation Regulations that generally require that all equipment installed on the aircraft be operative at the time of flight. Depending on the size and type of aircraft, the aircraft may still be airworthy with some specific kinds of equipment inoperative.
A MEL is a precise listing of instruments, equipment and procedures that allows an aircraft to be operated under specific conditions with inoperative equipment. For Part 91 operators, a MEL consists of the FAA Letter of Authorization (LOA) which authorizes the operation of the aircraft specific to the aircraft make, model, serial number and registration number, the MEL Procedures Document that is developed by the aircraft operator, the Master Minimum Equipment List (MMEL) preamble, and the MMEL itself that is developed by the aircraft manufacturer and the FAA. Together the four (LOA, MEL Procedures Document, MMEL preamble, and MMEL) constitute a Supplemental Type Certificate (STC) that must be carried onboard the aircraft during its operation. For operators with Operations Specifications (OpSpecs), the MEL consists of the MEL Procedures Document, OpSpec and MMEL.
MELs are not required for a number of part 91 operations, but are required for operations under parts 91 subpart K, 121, 125, and 135.
With this priviledge come some restrictions. The following instruments and equipment may not be included in an MEL:
- Instruments and equipment that are either specifically or otherwise required by the airworthiness requirements under which the aircraft is type certificated and which are essential for safe operations under all operating conditions.
- Instruments and equipment required by an airworthiness directive (AD) to be in operable condition unless the airworthiness directive provides otherwise.
- Instruments and equipment required for specific operations by part 91.
The aircraft records available to the pilot must include an entry describing the inoperable instruments and equipment. The aircraft is operated under all applicable conditions and limitations contained in the MEL and the letter authorizing the use of the list.
An operator authorized to use an approved MEL issued for a specific aircraft under part 121, 125, or 135 shall use that MEL in connection with operations conducted with that aircraft under part 91 without additional approval requirements. This includes situations where the aircraft is be operated by a certificated air carrier as well as operated by a separate part 91 operator. The FAA will not issue more than one MEL per aircraft. The certificated air carrier is responsible for obtaining approval for multiple users of the MEL, training all persons in the MEL's use, and must keep a complete and current list of all persons trained and authorized to use the MEL. The air carrier is also responsible for determining the aircraft's maintenance status on its return from a part 91 operation. This must be done prior to return to service under parts 121, 125, 129, or 135.
Operators conduct aircraft operations with inoperative instruments and equipment within a framework of a controlled program of maintenance inspections, repairs and parts replacement; however, operators must exercise good judgment and have, at each required inspection, any inoperative instrument or equipment repaired or inspected or the maintenance deferred, as appropriate.
For part 91 subpart K, 121, 135 and 129 operators, repairs must be completed in the timeline as required by the repair category designator (“A,” “B,” “C” or “D”). Part 91 operators other than those fractional ownership program operators under part 91 subpart K are not subject to the timelines for repair imposed by the repair category designator.
Obtaining FAA Approval to Use an MEL
An operator who wishes to conduct operations with an MEL must contact the FSDO that has jurisdiction over the geographic area where the aircraft is based and make an appointment. For part 91 operators, the Flight Standards District Office (FSDO) will assign a Flight Standards inspector to advise the applicant operator about regulatory requirements pertinent to using an MEL. During the initial appointment, the applicant will likely be dealing with a team of inspectors from the operations, airworthiness, and avionics units.
The operator must develop the Operations (O) and Maintenance (M) procedures using guidance contained in the manufacturer’s aircraft flight and/or maintenance manuals, the manufacturer’s recommendations, engineering specifications, and other appropriate sources. An operator may consult FSDO inspectors for advice or clarification, but the operator is responsible for preparing the document. The FAA does not approve procedures documents for part 91 operators with an MEL authorization.
In preparing the procedures document, an operator must consider the following:
- The operator’s procedures document may be more restrictive than the MMEL either by the applicant’s choice or because of ADs or operating rules. The operator’s procedures document may not be less restrictive than the MMEL.
- For part 91 operators, the title page of the procedures document must contain the following statement: This MEL is applicable to part 91 operations only and may not be used for operations conducted under parts 121, 125, 129, or 135.
- The operator must use the Air Transport Association (ATA) numbering system for equipment and instruments, as is used in all MMELs. The operator must use the ATA numbering system in sequence when describing O and M procedures, including the numbers for equipment installed in the aircraft. When equipment is not installed in a specific aircraft, the applicant need not develop O and M procedures for those items of equipment.
- Operators must ensure that the procedures document lists the items of equipment that are actually installed on the specific aircraft (except those items over and above the type design or previously approved by the aircraft evaluation group). This provides guidance to a pilot as to which items of equipment may be inoperative for a particular operation.
- Equipment specifically required by the airworthiness rule under which the aircraft is type certificated, equipment required by AD, and equipment required for specific operations under §91.213(b)(1) through (3) must be operative. It is important to note that all items related to the airworthiness of the aircraft that are not included on the MMEL must be operative.
- The repair interval categories (A, B, C, and D) listed in column 1 of the MMEL applies only to operations conducted under parts 121, 125, 129, and 135.
- Where the MMEL states “As required by 14 CFR,” the procedures document should list the particular regulation by part and section or describe the actual 14 CFR requirement applicable to the operator’s particular operation. For example, where the regulation requires a clock for IFR flight, the operator’s procedures document should say “May be inoperative for VFR.”
- The procedures document must specify suitable limitations in the form of placards, maintenance procedures, crew operating procedures, and other restrictions to ensure an acceptable level of safety.
- The procedures document must specify those conditions under which an item may be inoperative. The remarks must also identify required maintenance or operational tasks. The symbol O or M, placed in column 4 of the MMEL, indicates that an O or M procedure is applicable to that item. Indicating the O and M procedures in the procedures document provides flight crews and ground support personnel with a single procedural reference document.
- If the O and M procedures are already stated in the AFM, the maintenance manual, or other available FAA-approved source, the operator needs only to show the reference; e.g., O: AFM, pp. 3-8 through 3-10, paragraph 3-47. If the operator uses this reference format in the procedures document, the referenced source must be readily available to the ground support personnel and a copy of the references source must be carried in the aircraft and be readily available to the flight crew.
- If O and M procedures are not in the AFM, maintenance manual, or other available FAA-approved source, or if the operator wishes to use a different procedure, then the operator must list the procedure in the procedures document.
- The procedures document may not conflict with AFM limitations, emergency procedures, ADs, or the aircraft maintenance manual.
An operator may begin operations before completion of the procedures document. If the operator has not yet developed a procedure for an item, that item must be operative. When an instrument or item of equipment becomes inoperative, the operator must follow the procedure indicated in the procedures document or the operator could be in noncompliance with the regulations.
The MEL applies only to a particular aircraft make, model, serial number, and registration number and only to the operator who received the authorization. When more than one person has operational control of a specific aircraft, all operators must meet with inspectors from the issuing FSDO to discuss MEL operational considerations. The FSDO may find it appropriate to list all operators on the LOA with the exception being a fractional ownership situation. Each operator must sign the Statement of Operator on the LOA.
When an operator adds or removes aircraft of the same type from its fleet, the operator must notify the FSDO having oversight within 10 calendar-days following the change. The FSDO will reissue the LOA containing the new information. Both the operator and the inspector must sign the new LOA and the operator must surrender the previous LOA upon reissuance of the new one.
Conducting Operations with an MEL
In addition to carrying the documents that comprise the MEL on board the aircraft, the operator must have on board any technical manuals needed to accomplish O and M procedures. The following figure illustrates the sequence of events involved in applying the MEL to inoperative equipment.
Correcting MEL Inoperative Items
The MEL permits operations with inoperative equipment for the minimum period of time necessary for equipment repair. It is important that operators have repairs done at the earliest opportunity in order to return the aircraft to its design level of safety and reliability. In all cases, inoperative equipment must be repaired or the maintenance deferred at the aircraft’s next required inspection. Items of inoperative equipment authorized by the MEL to be inoperative must be inspected or repaired by qualified maintenance personnel or maintenance deferred at the next 100-hour, annual, progressive, or unscheduled inspection. However, if §91.213 requires that an item be repaired, the item cannot be deferred.
A record of inoperative equipment must remain in the aircraft so pilots will be aware of all discrepancies. Since operators frequently do not carry aircraft logbooks in the aircraft, a discrepancy record or log is a good alternative. When an operator uses this type of discrepancy log in lieu of the aircraft’s maintenance records, the operator must retain the log as a part of the aircraft’s records as per §91.417(b). Maintenance procedures and corrective actions must be accomplished and recorded according to §§ 43.9, 91.405, and 91.417.