The following information is generic in nature and, since most civilian jet airplanes require a minimum flight crew of two pilots, assumes a two pilot crew. If any of the following information conflicts with FAA-approved AFM procedures for a particular airplane, the AFM procedures take precedence. Also, if any of the following procedures differ from the FAA-approved procedures developed for use by a specific air operator and/or for use in an FAA-approved training center or pilot school curriculum, the FAA-approved procedures for that operator and/or training center/pilot school take precedence.
All FAA certificated jet airplanes are certificated under Title 14 of the Code of Federal Regulations (14 CFR) part 25, which contains the airworthiness standards for transport category airplanes. The FAA-certificated jet airplane is a highly sophisticated machine with proven levels of performance and guaranteed safety margins. The jet airplane’s performance and safety margins can only be realized, however, if the airplane is operated in strict compliance with the procedures and limitations contained in the FAA-approved AFM for the particular airplane. Furthermore, in accordance with 14 CFR part 91, section 91.213, a turbine powered airplane may not be operated with inoperable instruments or equipment installed unless an approved Minimum Equipment List (MEL) exists for that aircraft, and the aircraft is operated under all applicable conditions and limitations contained in the MEL.
Minimum Equipment List and Configuration Deviation List
The MEL serves as a reference guide for dispatchers and pilots to determine whether takeoff of an aircraft with inoperative instruments or equipment is authorized under the provisions of applicable regulatory requirements.
The operator’s MEL must be modeled after the FAA’s Master MEL for each type of aircraft and must be approved by the Administrator before its implementation. The MEL includes a “General Section,” comprised of definitions, general policies, as well as operational procedures for flight crews and maintenance personnel. Each aircraft component addressed in the MEL is listed in an alphabetical index for quick reference. A table of contents further divides the manual in different chapters, each numbered for its corresponding aircraft system designation (i.e., the electrical system, also designated as system number 24, would be found in chapter 24 of the MEL).
Maintenance may be deferred only on those aircraft systems and components cataloged in the approved MEL. If a malfunctioning or missing item is not specifically listed in the MEL inventory, takeoff is not authorized until the item is adequately repaired or replaced. In cases where repairs may temporarily be deferred, operation or dispatch of an aircraft whose systems have been impaired is often subject to limitations or other conditional requirements explicitly articulated in the MEL. Such conditional requirements may be of an operational nature, a mechanical nature, or both. Operational conditions generally include one or more of the following:
- Limited use of aircraft systems
- Downgraded instrument flight rule (IFR) landing minima
- Fuel increases due to additional burn, required automatic power unit (APU) usage or potential fuel imbalance situations
- Precautionary checks to be performed by the crew prior to departure, or special techniques to be applied while in flight
- Weight penalties affecting takeoff, cruise, or landing performance (runway limit, climb limit, usable landing distance reduction, and VREF, takeoff V-speeds, N1/EPR adjustments)
- Specific flight restrictions involving:
- Authorized areas of operation (clearly defined geographical regions)
- Type of operations (international, extended operations (ETOPS)) • Altitude and airspace (reduced vertical separation minimums (RVSM)
- Minimum navigation performance specifications (MNPS)
- Speed (knots indicated airspeed (KIAS) or Mach)
- Routing options (extended overwater, reduced navigation capability, High Altitude Redesign navigation)
- Environmental conditions (icing, thunderstorms, wind shear, daylight, visual meteorological conditions (VMC), turbulence index, cross-wind component)
- Airport selection (runway surface, length, contamination, and availability of aircraft maintenance, Airport rescue and firefighting (ARFF) and ATC services)
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Listed below are some examples of both operational and mechanical situations that may be encountered:
- A defective Ground Proximity Warning System (GPWS) would require alternate procedures to be developed by the operator to mitigate the loss of the GPWS and would likely only allow continued operation for two days.
- An inoperative air condition (A/C) pack might restrict a Super 80 or a Boeing 737 to a maximum operating altitude of flight level (FL)250, whereas as a Boeing 757 is only restricted to FL350.
- An inoperative Auxiliary Power Unit (APU) will not affect the performance or flying characteristics of an aircraft, but it does prompt the operator to verify that ground air and electrical power is available for that particular type of aircraft at the designated destination and alternate airports.
- A faulty fuel pump in the center tank may lower the Maximum Zero Fuel Weight (MZFW) by the amount of center tank fuel, as that fuel would otherwise be trapped and unusable should the remaining fuel pump fail while in flight. At the same time, the unavailability of center tank fuel unmistakably decreases the aircraft range while perhaps excluding it from operating too far off-shore.
- An inoperable generator (IDG) may require the continuous operation of the APU as an alternate source of electrical power throughout the entire flight (and thus more fuel) as it is tasked with assuming the function of the defunct generator.
- A failure of the Heads-up Display (HUD) or the auto-pilot may restrict the airplane to higher approach minima (taking it out of Category II or Category III authorizations)
Mechanical conditions outlined in the MEL may require precautionary pre-flight checks, partial repairs prior to departure, or the isolation of selected elements of the deficient aircraft system (or related interacting systems), as well as the securing of other system components to avoid further degradation of its operation in flight. The MEL may contain either a step-by-step description of required partial maintenance actions or a list of numerical references to the Maintenance Procedures Manual (MPM) where each corrective procedure is explained in detail. When procedures must be performed to ensure the aircraft can be safely operated, they are categorized as either Operations Procedures or Maintenance Procedures. The MEL will denote which by indicating an “O” or an “M” as appropriate.
If operational and mechanical conditions can be met, a placard is issued and an entry made in the aircraft MEL Deferral Record to authorize the operation for a limited time before more permanent repairs can be accomplished. The placard is affixed by maintenance personnel or the flight crew as appropriate onto the instrument or control mechanism that otherwise governs the operation of the defective device.
In order to use the MEL properly, it is important to clearly understand its purpose and the timing of its applicability. Because it is designed to provide guidance in determining whether a flight can be safely initiated with aircraft equipment that is deficient, inoperative, or missing, the MEL is only relevant while the aircraft is still on the ground awaiting departure or takeoff. It is essentially a dispatching reference tool used in support of all applicable Federal Aviation Regulations. If dispatchers are not required by the Operator’s certificate, flight crews still need to refer to the MEL before dispatching themselves and ensure that the flight is planned and conducted within the operating limits set forth in the MEL. However, once the aircraft is airborne, any mechanical failure should be addressed using the appropriate checklists and approved AFM, not the MEL. Although nothing could technically keep a pilot from referring to the MEL for background information and documentation to support his decisions, his actions must be based strictly on instructions provided by the AFM (i.e., Abnormal or Emergency sections).
A Configuration Deviation List (CDL) is used in the same manner as a MEL but it differs in that it addresses missing external parts of the aircraft rather than failing internal systems and their constituent parts. They typically include elements, such as service doors, power receptacle doors, slat track doors, landing gear doors, APU ram air doors, flaps fairings, nose wheel spray deflectors, position light lens covers, slat segment seals, static dischargers, etc. Each CDL item has a corresponding AFM number that identifies successively the system number, sub-system number, and item number. Flight limitations derived from open CDL items typically involve some kind of weight penalty and/or fuel tax due to increased drag and a net performance decrement, although some environmental restrictions may also be of concern in a few isolated cases. For example, a missing nose wheel spray deflector (Super 80 aircraft) requires dry runways for both takeoff and landing.
Each page of the MEL/CDL is divided into 6 columns. From left to right, these columns normally display the following information:
- Functional description/identification of the inoperative or missing aircraft equipment item
- Normal complement of equipment (number installed)
- Minimum equipment required for departure (number of items)
- Conditions required for flight/dispatch including maintenance action required (M) by mechanics or other authorized maintenance personnel and operational procedures or restrictions (O) to be observed by the flight crew