Airspeed
The pitot-static system comprises one or more pitot probes (or tubes) facing the on-coming air flow to measure pitot pressure (also called stagnation, total or ram pressure) and one or more static ports to measure the static pressure in the air flow.
These two pressures are compared by the ASI to give an IAS reading.
Since 2010, the International Civil Aviation Organization (ICAO) recommends using kilometers per hour (km/h) for airspeed (and meters per second for wind speed on runways), but allows using the de facto standard of knots, and has no set date on when to stop.
[2] 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.
[3] In high altitude flight, the Mach number is sometimes used for reporting airspeed.
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.
Sometimes both pressure sources are combined in a single probe, a pitot-static tube.
Further errors of 10% or more are common if the airplane is flown in "uncoordinated" flight.
Therefore, IAS is used for controlling the aircraft during taxiing, takeoff, climb, descent, approach or landing.
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 values less than the speed of sound at standard sea level (661.4788 knots) are calculated as follows:
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.
are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated.
The true airspeed and heading of an aircraft constitute its velocity relative to the atmosphere.
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.
where Some airspeed indicators include a TAS scale, which is set by entering outside air temperature and pressure altitude.
where Stated differently,[5] where EAS is a measure of airspeed that is a function of incompressible dynamic pressure.
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.
Thus, the handling and 'feel' of an aircraft, and the aerodynamic loads upon it, at a given equivalent airspeed, are very nearly constant and equal to those at standard sea level irrespective of the actual flight conditions.
At standard sea level pressure, CAS and EAS are equal.
Beyond this speed, Mach buffet or stall or tuck may occur.
