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be within specified tolerances for various airspeeds. It is considered negligible or is accounted for in
technical order tables and graphs.
3.24.3. Calibrated Airspeed (CAS). CAS is basic airspeed corrected for pitot-static error or attitude of
the aircraft. The pitot-static system of a moving aircraft will have some error. Minor errors will be found
in the pitot section of the system. The major difficulty is encountered in the static pressure section. As
the flight attitude of the aircraft changes, the pressure at the static inlets changes. This is caused by the
airstream striking the inlet at an angle. Different types and locations of installations cause different
errors. It is immaterial whether the status source is located in the pitot-static head or at some flush
mounting on the aircraft. This error will be essentially the same for all aircraft of the same model, and a
correction can be computed by referring to tables in the appendix of the flight manual.
3.24.4. Equivalent Airspeed (EAS). EAS is CAS corrected for compressibility. Compressibility
becomes noticeable when the airspeed is great enough to create an impact pressure which causes the air
molecules to be compressed within the impact chamber of the pitot tube. The amount of the compression
is directly proportionate to the impact pressure. As the air is compressed, it causes the dynamic pressure
to be greater than it should be. Therefore, the correction is a negative value. The correction for
compressibility error can be determined by referring to the performance data section of the aircraft flight
manual or by using the F-correction factor on the DR computer.
3.24.5. Density Airspeed (DAS). DAS is calibrated airspeed corrected for pressure altitude (PA) and
true air temperature (TAT). Pitot pressure varies not only with airspeed but also with air density. As the
density of the atmosphere decreases with height, pitot pressure for a given airspeed must also decrease
with height. Thus, an airspeed indicator operating in a less dense medium than that for which it was
calibrated will indicate an airspeed lower than true speed. The higher the aircraft flies, the greater the
discrepancy. The necessary correction can be found on the DR computer. Using the window on the
computer above the area marked FOR AIRSPEED DENSITY ALTITUDE COMPUTATIONS, set the
PA against the TAT. Opposite the CAS on the minutes scale, read the DAS on the miles scale. At lower
airspeeds and altitudes, DAS may be taken as true airspeed with negligible error. However, at high
speeds and altitudes, this is no longer true and compressibility error must be considered.
(Compressibility error is explained in the equivalent airspeed section.) When density altitude (DA) is
multiplied by the compressibility factor, the result is true airspeed.
3.24.6. True Airspeed (TAS). TAS is equivalent airspeed that has been corrected for air density error.
By correcting EAS for TAT and PA, the navigator compensates for air density error and computes an
accurate value of TAS. The TAS increases with altitude when the IAS remains constant. When the TAS
remains constant, the IAS decreases with altitude. CAS and EAS can be determined by referring to the
performance data section of the aircraft flight manual.
3.25. Computing True Airspeed:
108 AFPAM11-216 1 MARCH 2001
3.25.1. 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'
Given: Temperature: -37 oC
Given: IAS: 253 knots
Given: Flight Manual Correction Factor: 2 knots
Find: CAS
Find: EAS
Find: TAS
Answer: CAS is determined by algebraically adding to IAS the correction factor taken from the chart in
your flight manual. (This correction is insignificant at low speeds but can be higher than 10 knots near
Mach 1.)
3.25.1.1. To correct CAS to EAS, use the chart on the slide of the computer entitled F-CORRECTION
FACTORS FOR TAS. See Figure 3.22. Enter the chart with CAS and PA. The F factor is .96. This
means we 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.
3.25.1.2. 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.
　
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