Minimum Equipment Lists

Minimum Equipment Lists

Why they exist and what they cover

Jack HessburgBy Jack Hessburg

October 2000

Not every device installed on an airplane works all the time. Items of installed equipment or complete systems do become inoperative during revenue operations, which can cause flight delays or cancellations
Loss of service is unacceptable in the airline business. According to FAA data available for 1994, the calculated annual total operating delay cost to the airlines was $2.5 billion and $7.0 billion for losses to passengers. The average delay was approximately 14 minutes. The cost to the operator includes:
• Loss of revenue
• Poor customer relationships
• Increased spares inventories
• Increased numbers of maintenance stations, including skills and manpower required to alleviate such loss
• Costs arising from re-routes, equipment substitution, passenger handling (hotels, buses, meal vouchers)
The cost to the customer takes the form of disrupted plans, missed business appointments, lost time, and late shipments. People and cargo expect to go to their destination on time.
The regulations traditionally specified that all installed aircraft equipment required by the airworthiness and operating regulations must be operative. However, experience showed that, with varying levels of redundancy designed into airplanes, operation of every system or installed component was not necessary when the remaining operative equipment provided an acceptable level of safety. For example, it is possible to safely operate a pressurized airplane with an inoperative pressurization system. Yet, a limitation on its operation in this configuration must be imposed. It must be operated at 10,000 feet pressure at altitude or less or have supplemental oxygen available to the occupants.
Beginning in the mid-1950s, the CAA (FAA's predecessor), recognized this and lists of allowable inoperative equipment, called Minimum Equipment Lists (MEL), were prepared for each airplane model. It granted, only to air carriers, permission to operate with certain items of equipment inoperative. The original intent was to permit "revenue operations" to continue until repairs or replacements could be accomplished. This provided economic airplane utilization as well as offering a more reliable flight schedule to the flying public without compromising safety.
The structure and approval of the early Minimum Equipment Lists (MEL) lists were principally in the hands of the individual CAA Air Carrier Inspectors. Technical evaluation of items for inclusion into a list was based upon the individual inspector's knowledge, competence, and subjective analysis of a specific airplane type. The result was that the MEL for operator "A" and that operator "B" (both using the same model airplane) were frequently different; one airline's inspector being very conservative and disallowing all but the simplest of equipment to be inoperative, the other being far more liberal. Individual operators would claim favoritism when one discovered that their competitor had a less restrictive MEL.
This lack of objective analysis and standardization of MELs' resulted in the institutionalization of the process by the mid-1960s. The FAA adopted centralized control and publication of separate Master Minimum Equipment Lists (MMEL) for each large airplane type.
Present regulations continue to recognize the original MEL concept. FARs 125, and 135 operators were included in the concept by the late 1970s. In 1991, single engine operations under FAR 135 were added to the concept. Lastly, FAR Part 91 operations are also now covered.
Nothing in the concept disallows the authority of the pilot-in-command. The pilot may, at his or her discretion, require that any item covered by the MEL be repaired before flight.
Operations with certain items of equipment inoperative are not considered an abrogation of the airplane Type Certificate. When operating under Parts 121, 125 or 135, an approved Minimum Equipment List is recognized as an approved change to the type design. Therefore, the altered status of the airplane under the MEL remains an acceptable certified configuration. Consequently, adoption of an MEL item does not require re-certification of the type design.
Operating with an approved MEL and a letter of authorization under FAR Part 91 constitutes a Supplemental Type Certificate for the airplane. As such, an MEL approved under Part 91 is issued against a specific airplane(s) i.e. airplane serials number(s). The airplane(s) will be listed on the cover sheet of the approved MEL.
Is a mechanic or inspector in violation of the FAR for releasing an aircraft as airworthy when certain items are inoperative under an approved MEL? No! An air carrier airworthiness release requires certification that the work performed is in accordance with the certificate holder's manual. Because an approved MEL is a part of the certificate holder's manual, a mechanic is relieved of responsibility for the inoperative status of MEL items. The actions taken under the requirements of an approved MEL would "clear" the discrepancy from the aircraft maintenance record and would consequently revalidate the maintenance release.
A mechanic is not responsible for any contingent maintenance required by the MEL for any previously deferred items unless additional or repetitive maintenance is required.
An airplane maintained under FAR Part 91 is returned to service under the provisions of FAR 43.5 and is unaffected by this dilemma as an approved MEL under Part 91 is considered a supplemental type certificate.
To be continued....