TM 1-1510-225-10 7-8 between two reference lines, use the reference line to the right of the point. In  this  example,  the  point  is  between  ISA  +  20 and ISA + 30, and is closest to ISA + 20.  Therefore, use  Figure  7-39.    Normal  Cruise  Power,  1700 RPM  - ISA +20 ºC for cruise performance planning. Enter  the  chart  at  24,000  feet  pressure  altitude and    read    across    horizontally    to    determine    cruise torque, fuel flow, and airspeeds. (2)  If  a  weather  forecast  does  not  include  a temperature at the cruise altitude, then the temperature   must   be   calculated   using   a   standard lapse   rate   and   the   temperature   at   the   departure airport. In this example, the temperature at the departure airport  is  +30  ºC.    The  pressure  altitude  is  4000  feet and   the   ISA   temperature   at   that   pressure   altitude would be +7 ºC. Temperature  decreases  at  2   ºC  per   thousand feet;  4000  feet    1000  feet  =  4;  4  x  2  ºC  =  8  ºC decrease in temperature.  Therefore, ISA at 4000 feet PA is +15 ºC (SL ISA) – 8 ºC = +7 ºC. The   actual   surface   temperature   of   +30   ºC   at Airport  A,  PA  4000  feet,  is  23º  above  ISA  (+7  ºC). Therefore, it is ISA +23 at the airport.  If it is ISA +23º at the departure surface and  a standard  lapse rate  of 2º per thousand feet of increased altitude is used, then it will be ISA +23º at the planned cruise altitude. On  Figure  7-11,  ISA  +23º  is  closest  to  the  ISA + 20 reference line; therefore, use Figure 7-39, Normal Cruise   Power,   1700   RPM,   -   ISA   +20 ºC   for   cruise performance planning. (3)  The   following   planning   data   is   derived from   the   Figure   7-39,   Normal   Cruise   Power,   1700 RPM - ISA +20 ºC cruise chart for this example: Torque per engine – 75% Fuel flow per engine – 291 pounds/hour Total fuel flow – 582 pounds/hour IAS (12,000 pounds) – 179 KIAS TAS (12,000 pounds) – 271 KTAS The  above  data  is  used  for  the  corresponding entries on the DD Form 175 and for flight planning. As  a  general  rule,  the  total  fuel  flow  from  the Recommended Cruise charts will sufficiently match the results of a more detailed fuel planning process using: time, fuel, and distance to climb; cruise fuel; and, time, fuel, and distance to descend.  Therefore, for mission planning    purposes,    the    total    fuel    flow    from    the appropriate cruise chart will suffice.  Therefore, for this example, the total fuel in hours  and  minutes  for  entry on the DD Form 175 is 3+24 hours. Fuel for the mission 1975 pounds ÷ fuel flow per hour  582 pounds = 3.39 hours (3+24) The   minimum   reserve   fuel   for   the   mission   is 437 pounds. Fuel flow per hour 582 pounds          x 45 minutes, .75                 = 436.5 (437 pounds) Fuel   available   for   the   mission   minus   required reserve is 1538 pounds. Total mission fuel 1975 pounds         - reserve fuel        437 pounds                               =    1538 pounds Mission endurance fuel, minus reserves, is. Mission    fuel    minus    reserve        1538    pounds ÷  fuel  flow  per  hour  582  pounds           =     2.64 hours (2 + 37)