In
aviation, airspeed is the
speed of an
aircraft relative to the
air. Among the common conventions for qualifying 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;
*
Equivalent airspeed ("EAS"), calibrated airspeed adjusted for
compressibility effects;
*
True airspeed ("TAS"), equivalent airspeed adjusted for
air density
The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature a ...
, and is also the speed of the aircraft through the air in which it is flying.
Calibrated airspeed is typically within a few
knots of indicated airspeed, while equivalent airspeed decreases slightly from CAS as aircraft altitude increases or at high speeds.
With EAS constant, true airspeed increases as aircraft
altitude increases. This is because air density decreases with higher altitude.
The measurement and indication of airspeed is ordinarily accomplished on board an aircraft by an
airspeed indicator ("ASI") connected to a
pitot-static system. 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
Stagnation may refer to one of the following
* Economic stagnation, slow or no economic growth.
* Era of Stagnation, a period of economic stagnation in Soviet Union
* Lost Decade (Japan), a period of economic stagnation in Japan
*Stagnation in flu ...
, 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.
Units
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.
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
Mach number (M or Ma) (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound.
It is named after the Moravian physicist and philosopher Ernst Mach.
: \mathrm = \frac ...
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
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.
Outside the former Soviet bloc, most airspeed indicators show the speed in
knots (
nautical mile
A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude. Today ...
s per hour). Some light aircraft have airspeed indicators showing speed in
statute miles
The mile, sometimes the international mile or statute mile to distinguish it from other miles, is a British imperial unit and United States customary unit of distance; both are based on the older English unit of length equal to 5,280 English ...
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
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
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as w ...
at standard sea level (661.4788 knots) are calculated as follows:
minus position and installation error correction.
;where
:
is the calibrated airspeed,
:
is the
impact pressure (inches Hg): the difference between total pressure and static pressure,
:
is 29.92126 inches Hg; static air pressure at standard sea level,
:
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
and
are consistent with the
ISA
Isa or ISA may refer to:
Places
* Isa, Amur Oblast, Russia
* Isa, Kagoshima, Japan
* Isa, Nigeria
* Isa District, Kagoshima, former district in Japan
* Isa Town, middle class town located in Bahrain
* Mount Isa, Queensland, Australia
* Mount Is ...
i.e. the conditions under which airspeed indicators are calibrated.
Equivalent airspeed
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
where
:
is the
density of air at the altitude at which the aircraft is currently flying;
:
is the density of air at sea level in the International Standard Atmosphere (1.225 kg/m
3 or 0.00237 slug/ft
3).
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.
True airspeed
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.
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 true measure of aircraft performance in cruise, thus it is the speed listed in aircraft specifications, manuals, performance comparisons, pilot reports, and every situation when cruise or endurance performance needs to be measured. It is the speed normally listed on the flight plan, also used in
flight planning, before considering the effects of wind.
Since indicated airspeed is a better indicator of power used and lift available, true airspeed is not used for controlling the aircraft during taxiing, takeoff, climb, descent, approach or landing; for these purposes the
Indicated airspeed – IAS or KIAS (knots indicated airspeed) – is used.
Measurement of true airspeed
True airspeed is related to the
Mach number
Mach number (M or Ma) (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound.
It is named after the Moravian physicist and philosopher Ernst Mach.
: \mathrm = \frac ...
and
speed of sound
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as w ...
by
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 , 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 of altitude above sea level. For example, an aircraft flying at in the international standard atmosphere with an IAS of , is actually flying at TAS.
See also
*
ICAO recommendations on use of the International System of Units
*
Acronyms and abbreviations in avionics
*
Flight instruments
*
Ground speed
*
Maneuvering speed
*
V speeds
References
Bibliography
*
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External links
*
Calculators
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{{Authority control
Aerodynamics