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:  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. Listed below are some examples of both operational and  mechanical situations that may be encountered: 15-19 • 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) • 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)