airspeed

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: Indicated airspeed (IAS), what is read on an airspeed gauge connected to a pitot-static system. Calibrated airspeed (CAS), indicated airspeed adjusted for pitot system position and installation error. True airspeed (TAS) is the actual speed the airplane is moving through the air. When combined with aircraft direction, wind speed and direction, it can be used to calculate ground speed and direction. Equivalent airspeed (EAS) is true airspeed times root density ratio. It is a useful way of calculating aerodynamic loads and airplane performance at low speeds when the flow can be considered incompressible. Mach number is a measure of how fast the airplane is flying relative to the speed of sound.