Beechcraft C-12C, Operator And Technical Reference Manual

Page: 710 / 949

TM 1-1510-218-10
(3) The following planning data is derived
from the Figure 7A-39, Normal Cruise Power, 1700
RPM - ISA +20 ºC cruise chart for this example:
guideline and use the corresponding chart for
planning.
If the forecast temperature for FL 240 is –10 ºC,
enter the chart at that temperature. Trace up until
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. Torque per engine – 75%
Fuel flow per engine – 291 pounds/hour
Total fuel flow – 582 pounds/hour
IAS (12,000 pounds) – 179 KIAS
In this example, the point is between ISA + 20
and ISA + 30, and is closest to ISA + 20. Therefore,
use Figure 7A-39. Normal Cruise Power, 1700 RPM -
ISA +20 ºC for cruise performance planning.
TAS (12,000 pounds) – 271 KTAS
The above data is used for the corresponding
entries on the DD Form 175 and for flight planning.
Enter the chart at 24,000 feet pressure altitude
and read across horizontally to determine cruise
torque, fuel flow, and airspeeds.
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.
(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.
Fuel for the mission 1975 pounds
Ί
fuel flow per hour 582 pounds
= 3.39 hours (3+24)
The minimum reserve fuel for the mission is
437 pounds.
Temperature decreases at 2 ºC per thousand
feet; 4000 feet
y
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.
Fuel flow per hour 582 pounds
x 45 minutes, .75
= 436.5 (437 pounds)
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.
Fuel available for the mission minus required
reserve is 1538 pounds.
Total mission fuel 1975 pounds
- reserve fuel 437 pounds
= 1538 pounds
On Figure 7A-11 , ISA +23º is closest to the ISA
+ 20 reference line; therefore, use Figure 7A-39,
Normal Cruise Power, 1700 RPM, - ISA
+ 20 ºC for
cruise performance planning.
Mission endurance fuel, minus reserves, is.
Mission fuel minus reserve 1538 pounds
Ί
fuel flow per hour 582 pounds
= 2.64 hours (2 + 37)
7A-8