Airspeed

Airspeed is commonly given in knots (kn). 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]

The aviation industry in Russia and China as well as aircrew flying Russian/Chinese aircraft currently use km/h for reporting airspeed. Many present-day European glider planes also indicate airspeed in kilometers per hour. Some older planes, like German World War 2 planes, also indicated airspeed in kilometers per hour.

In high altitude flight, however, the Mach number is sometimes used for reporting airspeed. Sometimes other units are also used for airspeed, including miles per hour (mph) or meters per second.

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.[3]

Outside the former Soviet bloc, most airspeed indicators show the speed in knots (nautical miles per hour). Some light aircraft have airspeed indicators showing speed in statute miles per hour or kilometers per hour.

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.

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:

${\displaystyle V_{c}=A_{0}{\sqrt {5{\Bigg [}{\bigg (}{\frac {q_{c}}{P_{0}}}+1{\bigg )}^{\frac {2}{7}}-1{\Bigg ]}}}}$ minus position and installation error correction.

where

${\displaystyle V_{c}\,}$ is the calibrated airspeed,

${\displaystyle q_{c}\,}$ is the impact pressure (inches Hg): the difference between total pressure and static pressure,

${\displaystyle P_{0}\,}$ is 29.92126 inches Hg; static air pressure at standard sea level,

${\displaystyle A_{0}\,}$ is 661.4788 knots;, speed of sound at standard sea level.

Units other than knots and inches of mercury can be used, if used consistently.

This expression is based on the form of Bernoulli's equation applicable to isentropic compressible flow. The values for ${\displaystyle P_{0}}$ and ${\displaystyle A_{0}}$ are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated.

Equivalent airspeed (EAS) is defined as the airspeed at sea level in the International Standard Atmosphere at which the (incompressible) dynamic pressure is the same as the dynamic pressure at the true airspeed (TAS) and altitude at which the aircraft is flying. That is, it is defined by the equation

${\displaystyle {\frac {1}{2}}\rho _{0}(\mathrm {EAS} )^{2}={\frac {1}{2}}\rho (\mathrm {TAS} )^{2}}$

where

${\displaystyle \rho \,}$ is the density of air at the altitude at which the aircraft is currently flying;

${\displaystyle \rho _{0}\,}$ is the density of air at sea level in the International Standard Atmosphere (1.225 kg/m³ or 0.00237 slug/ft³).

EAS is a measure of airspeed that is a function of incompressible dynamic pressure. Structural analysis is often in terms of incompressible dynamic pressure, so equivalent airspeed is a useful speed for structural testing. 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. Up to about 200 knots CAS and 10,000 ft (3,000 m) the difference is negligible, but at higher speeds and altitudes CAS diverges from EAS due to compressibility.

The true airspeed (TAS; also KTAS, for knots true airspeed) of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. The true airspeed and heading of an aircraft constitute its velocity relative to the atmosphere.

Measurement of true airspeed

A mechanical true airspeed indicator for an airplane, showing speed in knots (kn) and miles per hour (mph). 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.

True airspeed is related to the Mach number ${\displaystyle M}$ and speed of sound ${\displaystyle c}$ by

${\displaystyle \mathrm {TAS} =cM}$

Both the Mach number and the speed of sound can be computed using measurements of impact pressure, static pressure and outside air temperature.

At sea level in the International Standard Atmosphere (ISA) and at low speeds where air compressibility is negligible (and so a constant air density may be assumed), TAS equals CAS. Above approximately 100 knots (190 km/h), the compressibility error rises significantly.

In flight, it can be calculated using an E6B flight calculator or equivalent.

Since temperature variations are of a smaller influence, the ASI error can be roughly estimated as indicating about 2% less than TAS per 1,000 feet (305 m) of altitude above sea level. For example, an aircraft flying at 15,000 feet (4,572 m) in the international standard atmosphere with an IAS of 100 knots (190 km/h), is actually flying at 126 knots (233 km/h) TAS.

• ICAO recommendations on use of the International System of Units
• Acronyms and abbreviations in avionics
• Flight instruments
• Ground speed
• Maneuvering speed
• V speeds

• Glauert H. (1947). "2". The Elements of Aerofoil and Airscrew Theory. Cambridge University Press. ISBN 9781139241953.
• William Gracey (May 1980). Measurement of Aircraft Airspeed and Altitude (PDF). NASA.
• Getting to grips with aircraft performance (PDF). Flight Operations Support & Line Assistance. Airbus Customer Services. January 2002.

• Kevin Brown. "True, Equivalent, and Calibrated Airspeeds". MathPages.