True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.
Performance
TAS is the true measure of aircraft performance in cruise, thus listed in aircraft specs, manuals, performance comparisons, pilot reports, and every situation when actual 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.
Airspeed sensing errors
The
airspeed indicatorThe airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot.- Use :...
(ASI), driven by a
Pitot tubeA pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...
and a barometric static port, shows what is called
indicated airspeedIndicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....
(IAS). By the static port the IAS is corrected for the surrounding air pressure, but not for air density. The ratio between pressure and density is temperature dependent.
At sea level in the
International Standard AtmosphereThe International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...
(ISA) and at slow speeds where air compressibility is negligible, IAS corresponds to TAS. When the air density or temperature around the aircraft differs from standard sea level conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. The ASI will indicate less than TAS when the air density decreases due to increase in altitude or temperature.
For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using an
E6BThe E6B Flight Computer, or simply the "whiz wheel", is a form of circular slide rule used in aviation. They are mostly used in flight training, but many professional and even airline pilots still carry and use these flight computers...
flight calculator or its equivalent.
For slow speeds, the data required are
static air temperatureIn aviation terminology, the outside air temperature or static air temperature refers to the temperature of the air around an aircraft, but unaffected by the passage of the aircraft through it.-Aviation usage:...
, pressure altitude and IAS (or
CASCalibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
for more precision). Above approximately 100 knots, the compressibility error rises significantly and TAS must be calculated by the Mach speed. Mach incorporates the above data including the compressibility factor.
Modern aircraft instrumentation use an Air Data Computer to perform this calculation in real time and display the TAS reading directly on the EFIS.
Since temperature variations are of a smaller influence, the ASI error can be roughly estimated as indicating about 2% less than TAS per 1,000ft of altitude above sea level. For example, an aircraft flying at 15,000ft in the international standard atmosphere with an IAS of 100kt, is actually flying at 126kt TAS.
Use in Navigation Calculations
To maintain a desired ground track whilst flying in the 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 also
- Wind triangle
The wind triangle is a graphical representation of the relationship between aircraft motion and wind. It is used extensively in dead reckoning navigation.The wind triangle is a vector diagram, with three vectors....
Low-speed flight
At low speeds and altitudes IAS and CAS are close to
equivalent airspeedEquivalent airspeed is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an airspeed...
(EAS). TAS can be calculated as a function of EAS and air density:
where
is true airspeed
is equivalent airspeed
is the air density at standard sea level (1.225 kg/m
3)
is the density of the air in which the aircraft is flying
High-speed flight
TAS can be calculated as a function of
Mach numberMach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...
and static air temperature:
Where
is the speed of sound at standard sea level (661.47 knots)
is Mach number,
is static air temperature in
kelvinThe kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
,
is the temperature at standard sea level (288.15 K)
For manual calculation of TAS in knots where Mach number and static air temperature are known the expression may be simplified to:
(remembering temperature is in kelvin)
Combining the above with the expression for Mach number gives an expression for TAS as a function of
impact pressureIn compressible fluid dynamics, impact pressure is the difference between total pressure and static pressure. In aerodynamics notation, this quantity is denoted as q_c or Q_c....
, static pressure and static air temperature (valid for subsonic flow):
Where :
is impact pressure
is static pressure
Electronic Flight Instrument SystemAn electronic flight instrument system is a flight deck instrument display system in which the display technology used is electronic rather than electromechanical. EFIS normally consists of a primary flight display , multi-function display and engine indicating and crew alerting system display...
s (EFIS) contain an
air data computerAn air data computer is an essential avionics component found in modern glass cockpits. This computer, rather than individual instruments, can determine the calibrated airspeed, Mach number, altitude, and altitude trend from input data from sensors such as an aircraft's pitot-static system,...
with inputs of impact pressure, static pressure and
total air temperatureTotal air temperature is a term used generally in aviation. In other applications it is called stagnation temperature. Total air temperature is measured by a specially designed temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the...
. In order to compute TAS the air data computer must convert total air temperature to static air temperature. This is also a function of Mach number:
Where
total air temperature
In simple aircraft, without an air data computer or
MachmeterA Machmeter is an aircraft pitot-static system flight instrument thatshows the ratio of the true airspeed to the speed of sound,a dimensionless quantity called Mach number...
, true airspeed can be calculated as a function of
calibrated airspeedCalibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
and local air density (or static air temperature and pressure altitude which determine density). Some airspeed indicators incorporate a
slide ruleThe slide rule, also known colloquially as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction.Slide rules come in a...
mechanism to perform this calculation. Otherwise, it can be performed using
this applet or a device such as the
E6BThe E6B Flight Computer, or simply the "whiz wheel", is a form of circular slide rule used in aviation. They are mostly used in flight training, but many professional and even airline pilots still carry and use these flight computers...
(a handheld circular
slide ruleThe slide rule, also known colloquially as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction.Slide rules come in a...
).
See also
- Equivalent airspeed
Equivalent airspeed is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an airspeed...
- Indicated airspeed
Indicated airspeed is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed , which is the IAS corrected for instrument and installation errors....
- Calibrated airspeed
Calibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...
- Flight planning
Flight planning is the process of producing a flight plan to describe a proposed aircraft flight. It involves two safety-critical aspects: fuel calculation, to ensure that the aircraft can safely reach the destination, and compliance with air traffic control requirements, to minimise the risk of...
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