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Airspeed (Part Two)

in Basic Instruments

Computing True Airspeed
ICE-T Method


To compute TAS using the ICE-T method on the DR computer, solve, for each type of airspeed, in the order of I, C, E, and T; that is, change IAS to CAS, change CAS to EAS, and change EAS to TAS. This process is illustrated by the following sample problem. (Refer to definitions as necessary.)

Given:
PA = 30.000′
Temperature = –37 °C
IAS = 253 knots
Flight Manual Correction Factor = 2 knots

Find: CAS, EAS, and TAS

CAS is determined by algebraically adding to IAS the correction factor taken from the chart in the flight manual. (This correction is insignificant at low speeds but can be higher than 10 knots near Mach 1.) To correct CAS to EAS, use the chart on the slide of the computer entitled F-CORRECTION FACTORS FOR TAS. [Figure 3-22] Enter the chart with CAS and PA. The F factor is .96. Multiply CAS by .96 or take 96 percent of 255 knots. To do this, place 255 knots on the inner scale under the 10 index on the outer scale. Locate 96 on the outer scale and read EAS on the inner scale: 245 knots.

Figure 3-22. ICE-T method.

Figure 3-22. ICE-T method. [click image to enlarge]

Now, we need to correct EAS for temperature and altitude to get TAS. As shown in Figure 3-22, in the window marked FOR AIRSPEED AND DENSITY ALTITUDE COMPUTATIONS, place temperature over PA. Locate the EAS of 245 knots on the inner scale and read TAS on the outer scale. The TAS is 408 knots.

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, the answer should be the same regardless of the procedure being used, but the ICE-T method is used most often because the computation can be worked backwards from TAS. If the desire is to maintain a constant TAS, determine what IAS to fly by working the ICE-T method in reverse (also known as reverse ICE-T). [Figure 3-23]

Figure 3-23. ICE-T in reverse.

Figure 3-23. ICE-T in reverse. [click image to enlarge]

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.

Figure 3-24. Finding true airspeed from Mach number.

Figure 3-24. Finding true airspeed from Mach number.

Example: If you are planning to maintain Mach 1.2 on a crosscountry 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 °C, the TAS is 742 knots.

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 3-25. Combined airspeed Mach indicator.

Figure 3-25. Combined airspeed Mach indicator.

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 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.

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.

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 systems 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.

Doppler is not the only source of drift angle and GS. The same basic information is available on virtually all inertial navigation systems (INS), and now global positioning system (GPS) computers can also give accurate information under a wider range of conditions.

 

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