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3.25.2. Alternate TAS Method. There is an alternate method of finding TAS when given CAS. The
instructions for alternate solution are printed on the computer directly below the F factor table (Multiply
F Factor by TAS obtained with computer to obtain TAS corrected for compressibility). Mathematically,
your answer should be the same regardless of the procedure you use, but the ICE-T method is used most
often because the computation can be worked backwards from TAS. If you wish to maintain a constant
TAS, you can determine what IAS to fly by working the ICE-T method in reverse (also known as
Reverse ICE-T), as illustrated in Figure 3.23.
3.26. Machmeters. Machmeters indicate the ratio of aircraft speed to the speed of sound at any
particular altitude and temperature during flight. It is often necessary to convert TAS to a Mach number
or vice versa. Instructions are clearly written on the computer in the center portion of the circular slide
rule. Locate the window marked FOR AIRSPEED AND DENSITY ALTITUDE COMPUTATIONS and
rotate the disk until the window points to the top of the computer (toward the l0 index on the outer
scale). Within the window is an arrow entitled MACH NO. INDEX (Figure 3.24). To obtain TAS from a
given Mach number, set air temperature over the MACH NO. INDEX and, opposite the Mach number
on the MINUTES scale, read the TAS on the outer scale.
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EXAMPLE: If you are planning to maintain Mach 1.2 on a cross-country flight, place the air
temperature at flight altitude over the MACH NO. INDEX. Read the TAS on the outer scale opposite 1.2
on the inner scale. If the temperature is –20 oC, the TAS will be 742 knots.
Figure 3.22. ICE-T Method.
Figure 3.23. ICE-T in Reverse.
110 AFPAM11-216 1 MARCH 2001
3.27. Airspeed Indicators. The combined airspeed-Mach indicator, shown in Figure 3.25, is usually
found in high-performance aircraft or where instrument panel space is limited. It simultaneously displays
IAS, indicated Mach number, and maximum allowable airspeed. It contains a differential pressure
diaphragm and two aneroid cells. The diaphragm drives the airspeed-Mach pointer. One aneroid cell
rotates the Mach scale, permitting IAS and Mach number to be read simultaneously. The second aneroid
cell drives the maximum allowable airspeed pointer. This pointer is preset to the aircraft's maximum
IAS. Unlike the maximum IAS and unlike the maximum allowable airspeed, Mach number increases
with altitude. An airspeed marker set knob positions a movable airspeed marker. This marker serves as a
memory reference for desired airspeed.
Figure 3.24. Finding True Airspeed From Mach Number.
Figure 3.25. Combined Airspeed-Mach Indicator.
3.28. Air Data Computer. The air data computer is an electro-pneumatic unit that uses pitot and static
pressures and total air temperature to compute outputs for various systems. These output parameters of
voltage and resistance represent functions of altitude, Mach number, TAS, computed airspeed, and static
AFPAM11-216 1 MARCH 2001 111
air temperature. Air data computer outputs are used with the flight director computers, automatic flight
controls, cabin pressurization equipment, and normal basic indicators. The air data computer provides
extreme accuracy and increased reliability.
3.29. Doppler. Doppler radar provides the navigator with continuous, instantaneous, and accurate
readings of groundspeed (GS) and drift angle in all weather conditions, both over land and water. It does
this automatically with equipment that is of practical size and weight. Its operation makes use of the
Doppler effect.
3.29.1. Two basic Doppler radar systems exist—the four-beam and the three-beam. Both types use either
continuous wave (CW) or pulse wave (PW) transmission. CW transmission requires one antenna for
transmission and a second antenna for reception. Both use an X-shaped beam configuration.
Groundspeed is computed by comparing Doppler shift between front and rear beams, and drift angle is
computed by comparing the shift between the left and right beams.
3.29.2. Doppler is not the only source of drift angle and GS. The same basic information is available on
virtually all Inertial Navigation Systems, and now Global Positioning System computers can also give
accurate information under a wider range of conditions.
3.30. Summary. While the operation of some of the equipment in this chapter seems crude and
primitive compared to state-of-the-art navigation computers currently in use, these instruments still have
two very important uses—they act as crosschecks to ensure the computers are functioning properly, and
they become your primary means of navigation when the computers aren't.
112 AFPAM11-216 1 MARCH 2001
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