TM 1-1510-218-10
7-8
intersecting the 24,000 feet reference line. Note which
reference line is closest and use the corresponding
chart for planning. If the point is exactly midway
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 7-43. Recommended 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 7-43,
Recommended Cruise Power, 1700 RPM, - ISA
+20 ºC for cruise performance planning.
(3) The following planning data is derived
from the Figure 7-43, Recommended Cruise Power,
1700 RPM - ISA +20 ºC cruise chart for this example:
Torque per engine – 67%
Fuel flow per engine – 275 pounds/hour
Total fuel flow – 550 pounds/hour
IAS (12,000 pounds) – 171 KIAS
TAS (12,000 pounds) – 259 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+59 or 4+00 hours.
Fuel for the mission 2175 pounds ÷ fuel flow per
hour 550 pounds = 3.95 hours (4+00)
The minimum reserve fuel for the mission is
413 pounds.
Fuel flow per hour 550 pounds x 45 minutes, .75
= 412.5 (413 pounds)
Fuel available for the mission minus required
reserve is 1762 pounds.
Total mission fuel 2175 pounds – reserve fuel
413 pounds = 1762 pounds
Mission endurance fuel, minus reserves, is
3 hours and 12 minutes (3.2 hours).
Mission fuel minus reserve 1762 pounds ÷ fuel
flow per hour 550 = 3.2 hours
