TYPES OF AIRSPEED

International Standard Atmosphere:

Also known as the ICAO Standard Atmosphere, ISA is a standard against which to compare the actual atmosphere at any point and time.
The ISA is based the following values of pressure, density, and temperature at mean sea level each of which decreases with increase in height:
- Pressure of 1013.2 millibar - Pressure is taken to fall at about 1 millibar per 30 feet in the lower atmosphere (up to about 5,000 feet).
- Temperature of +15 °C - Temperature falls at a rate of 2 °C per 1,000 feet until the tropopause is reached at 36,000 feet above which the temperature is assumed to be constant at -57 °C. (The precise numbers are 1.98 °C, -56.5 °C and 36,090 feet)
- Density of 1.225 kg/m3

1- INDICATED AIRSPEED (IAS):

The airspeed shown on the flight-deck instrument.

An aneroid instrument, the airspeed indicator measures the dynamic pressure of the outside air entering a pitot tube. At sea level, and an atmospheric pressure of 1013.2 mb, and with no wind effect, the airspeed indicated is the true speed of the aircraft relative to the surface.

As the aircraft climbs, the air density decreases and the indicated speed will be less than the True Air Speed (TAS). However, when it comes to controlling the aircraft, because the flight characteristics of the aircraft also alter with reduction in atmospheric density, the indicated airspeed is of greater importance than the true airspeed. This is why control speeds (e.g. V1, V2, etc) are given as KIAS (Knots Indicated Airspeed).

2- COMPUTED AIRSPEED:

Basically, this is IAS with corrections for position error (PE) applied. The term “computed” applies specifically to air data computer systems in which PE corrections are automatically applied to an airspeed sensing module via an electrical correction network.

3- CALIBRATED AIRSPEED (CAS):

This is also associated with air data computer systems and is the computed airspeed compensated for the non-linear, or square-law, response of the airspeed sensing module.

4- EQUIVALENT AIRSPEED (EAS):

It is the Calibrated airspeed (CAS) corrected for adiabatic compressible flow at the altitude of flight. 

At sea level under ISA conditions, equivalent airspeed and calibrated airspeed are the same. At any other altitude, equivalent airspeed will be less than calibrated airspeed. Below speeds of about 200 knots CAS and at altitudes below 10,000', the difference between CAS and EAS is negligible. However, at higher speeds and altitudes CAS must be corrected for compressibility error to determine EAS.
The significance of equivalent airspeed is that, at Mach numbers below the onset of wave drag, all of the aerodynamic forces and moments on an aircraft are proportional to the square of the equivalent airspeed.

5- TRUE AIRSPEED (TAS):

This is EAS compensated for changes in air temperature and density at various flight levels.
At sea level in the International Standard Atmosphere (ISA), and at slow speeds where air compressibility is negligible, IAS corresponds to TAS. When the air density or temperature around the aircraft differs from standard sea level conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. The ASI will indicate less than TAS when the air density decreases due to increase in altitude or temperature.