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FAHRENHEITICELSIUS
Figure 7A-1. Temperature Conversion Chart
Change 3 7A-3
TM 1-1520-238-10
Section II. MAXIMUM TORQUE AVAILABLE
7A.9. DESCRIPTION. 7A.12. TORQUE FACTOR METHOD.
The maximum torque available charts shows the maximum
specification torque available per engine for 30
minute operation (fig 7A-2 sheet 1) and 10 minute operation
(fig 7A-2 sheet 2) at various conditions of pressure
altitude and free air temperature. Both single and
dual engine operation limits are shown.
The maximum torque available for 2.5 minute operation
(fig 7A-2 sheet 3) shows the maximum specification
torque available when one engine is inoperative;
only single engine operation limits are shown.
The torque factor charts (figs 7A-3 and 7A-4) provide an
accurate indication of available power for the engines
installed in each individual aircraft.
7A.10. USE OF CHARTS.
The primary use of the maximum torque available
charts (fig 7A-2 sheets 1, 2, and 3) is illustrated by the
example. To determine the maximum specification
torque available, it is necessary to know pressure altitude
and free air temperature. Enter the left side of either
the 30 minute or the 10 minute chart (fig 7A-2
sheets 1 or 2) at the known temperature and move right
to the known pressure altitude, and then move down
and read the maximum specification torque available.
This is torque per engine. For dual engine operation, if
the torque per engine exceeds the two engine limit, the
maximum torque available must be reduced to the two
engine limit.
For one engine inoperative, enter the left side of the 2.5
minute limit chart (fig 7A-2 sheet 3) at the known temperature
and move right to the known pressure altitude,
and then move down and read the maximum
specification torque available for one engine. If the
torque exceeds the one engine limit, maximum torque
available must be reduced to the one engine limit.
7A.11 . CONDITIONS.
The maximum torque available charts (fig 7A-2 sheets
1, 2, and 3) are based on 100% rotor rpm, zero airspeed,
JP-4 fuel and ENG INLET anti-ice switch OFF. With
ENG INLET anti-ice switch ON, available torque is reduced
by as much as 19.3% for 30 minute operation and
18.6% for 10 minute operation. For example, if value
from the chart is 90%, with anti-ice ON, torque available
would be 90 - 19.3 = 70.7%, 30 minute limit.
The torque factor method provides an accurate indication
of available power by incorporating ambient temperature
effects on degraded engine performance. The
torque factor method provides the procedure to determine
the maximum dual or single engine torque available
for the engines installed in each individual aircraft.
The specification power is defined for a newly
delivered low time engine. The aircraft HIT log form for
each engine provide the engine and aircraft torque factors
which are obtained from the maximum power
check and recorded to be used in calculating maximum
torque available.
7A.12.1 Torque Factor Terms. The following terms
are used when determining the maximum torque available
for an individual aircraft:
a. Torque Ratio (TR). The ratio of torque available
to specification torque at the desired ambient temperature.
b. Engine Torque Factor (ETF). The ratio of an
individual engine torque available to specification
torque at reference temperature of 35 °C. The ETF is
allowed to range from 0.85 to 1.0.
c. Aircraft Torque Factor (ATF). The ratio of an
individual aircrafts power available to specification
power at a reference temperature of 35 °C. The ATF is
the average of the ETFs of both engines and its value is
allowed to range from 0.9 to 1.0.
7A.12.2 Torque Factor Procedure. The use of the
ATF or ETF to obtain the TR from figure 7A-3 for ambient
temperatures between -15 °C and 35 °C is shown
by the example. The ATF and ETF values for an individual
aircraft are found on the engine HIT log. The TR
always equals 1.0 for ambient temperatures of -15 °C
and below, and the TR equals the ATF or ETF for temperatures
of 35 °C and above.
When the TR equals 1.0 the torque available may be
read directly from the specification torque available
scales. When the TR is less than 1.0, the actual torque
available is determined by multiplying the specification
torque available by the TR (example for TR = 0.98:
90% TRQ X 0.98 = 88.2% TRQ). The torque conversion
chart (fig 7A-4) is provided to convert specification data
to actual torque available. The single and dual engine
transmission limits are shown and should not be exceeded.
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