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 V$_{REF}$, takeoff V-speeds, N$_{1}$/ 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) 

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. 

