thumb|right|An [[airspeed indicator is a flight instrument that displays airspeed. This airspeed indicator has standardized markings for a multiengine airplane.]]
thumb|Aircraft have [[pitot tubes for measuring airspeed.]]
In aviation, airspeed is the speed of an aircraft relative to the air it is flying through (which itself is usually moving relative to the ground due to wind). In contrast, the ground speed is the speed of an aircraft with respect to the surface of the Earth (whether over land or presumed-stationary water). It is difficult to measure the exact airspeed of the aircraft (true airspeed), but other measures of airspeed, such as indicated airspeed and Mach number give useful information about the capabilities and limitations of airplane performance. The common measures of airspeed are:
Depending on the country of manufacture or which era in aviation history, airspeed indicators on aircraft instrument panels have been configured to read in knots, kilometers per hour, miles per hour. In high altitude flight, the Mach number is sometimes used for reporting airspeed.
Indicated airspeed
Indicated airspeed (IAS) is the airspeed indicator reading (ASIR) uncorrected for instrument, position, and other errors. From current EASA definitions: Indicated airspeed means the speed of an aircraft as shown on its pitot static airspeed indicator calibrated to reflect standard atmosphere adiabatic compressible flow at sea level uncorrected for airspeed system errors.
An airspeed indicator is a differential pressure gauge with the pressure reading expressed in units of speed, rather than pressure. The airspeed is derived from the difference between the ram air pressure from the pitot tube, or stagnation pressure, and the static pressure. The pitot tube is mounted facing forward; the static pressure is frequently detected at static ports on one or both sides of the aircraft. Sometimes both pressure sources are combined in a single probe, a pitot-static tube. The static pressure measurement is subject to error due to inability to place the static ports at positions where the pressure is true static pressure at all airspeeds and attitudes. The correction for this error is the position error correction (PEC) and varies for different aircraft and airspeeds. Further errors of 10% or more are common if the airplane is flown in "uncoordinated" flight.
Uses of indicated airspeed
Indicated airspeed is a better measure of power required and lift available than true airspeed. Therefore, IAS is used for controlling the aircraft during taxiing, takeoff, climb, descent, approach or landing. Target speeds for best rate of climb, best range, and best endurance are given in terms of indicated speed. The airspeed structural limit, beyond which the forces on panels may become too high or wing flutter may occur, is often given in terms of IAS.
Calibrated airspeed
Calibrated airspeed (CAS) is indicated airspeed corrected for instrument errors, position error (due to incorrect pressure at the static port) and installation errors.
Calibrated airspeed values less than the speed of sound at standard sea level (661.4788 knots) are calculated as follows:
<math>V_c=a_0\sqrt{\bigg(\frac{2}{\gamma-1}\bigg)\Bigg[\bigg(\frac{q_c}{p_0}+1\bigg)^\frac{\gamma-1}{\gamma}-1\Bigg]}</math> minus position and installation error correction.
;where
:<math>V_c \, </math> is the calibrated airspeed,
:<math>a_0 \, </math> is speed of sound at standard sea level
:<math>\gamma \, </math> is the ratio of specific heats (1.4 for air)
:<math>q_c \, </math> is the impact pressure, the difference between total pressure and static pressure
:<math>p_0 \, </math> is the static air pressure at standard sea level
This expression is based on the form of Bernoulli's equation applicable to isentropic compressible flow. CAS is the same as true air speed at sea level standard conditions, but becomes smaller relative to true airspeed as we climb into lower pressure and cooler air. Nevertheless, it remains a good measure of the forces acting on the airplane, meaning stall speeds can be called out on the airspeed indicator. The values for <math>p_0</math> and <math>a_0</math> are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated.
True airspeed
The true airspeed (TAS; also KTAS, for knots true airspeed) of an aircraft is the speed of the aircraft relative to the air in which it is flying. The true airspeed and heading of an aircraft constitute its velocity relative to the atmosphere.
Uses of true airspeed
The true airspeed is important information for accurate navigation of an aircraft. To maintain a desired ground track whilst flying in a moving airmass, the pilot of an aircraft must use knowledge of wind speed, wind direction, and true air speed to determine the required heading. See wind triangle.
TAS is the appropriate speed to use when calculating the range of an airplane. It is the speed normally listed on the flight plan, also used in flight planning, before considering the effects of wind.
Measurement of true airspeed
thumb|A mechanical airspeed indicator for an airplane, showing IAS in [[knot (unit)|knots (inner scale on black background) and miles per hour (outer scale on black background). The pilot sets the pressure altitude and air temperature in the top window using the knob; the needle indicates true airspeed in the lower left window (white background).]]
True airspeed is calculated from calibrated airspeed as follows