True airspeed: Definition from Answers.com

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True airspeed (TAS) 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.

1 Performance
2 Airspeed sensing errors
3 Aircraft maneuvering
4 Use in Navigation Calculations
5 Calculating true airspeed
- 5.1 Low-speed flight
- 5.2 High-speed flight
6 See also
7 Notes
8 References
9 External links

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 Indicator (ASI), driven by the Pitot tube, shows what is called Indicated airspeed (IAS). The ASI is calibrated so that IAS corresponds to TAS @ sea level, 15 degrees Celsius and 1013.2 HPa (29.92 InHg) of air pressure - called International Standard Atmosphere or ISA conditions.

When the air around the aircraft differs from said ISA conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. In fact, the ASI will indicate less and less as the air density decreases with altitude and temperature increases.

For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using an E6B flight calculator or its equivalent function on many GPSs. The data required are Outside Air Temperature (OAT), Pressure altitude and CAS (IAS corrected for installation and instrument errors). 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. Thus for a given IAS, the True Airspeed is about 2% higher than IAS per 1,000ft of altitude above sea level. An aircraft flying at 15,000ft with an IAS of 100kt, is actually flying at 130kt TAS, or 130kt through the air.

Aircraft maneuvering

Aircraft maneuvering, is mostly a function of dynamic pressures, so all ASI markings such as Stall speed, yellow, green and white arcs continue to be accurate throughout the altitude range that most civilian aircraft fly.

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

Calculating true airspeed

Low-speed flight

True airspeed (TAS) can be calculated as a function of indicated airspeed (or equivalent airspeed) and air density:^[1]
$TAS=V_I\sqrt{\frac{\rho_0}{\rho}}$
where $T A S$ is true airspeed

V I

is indicated (or equivalent) airspeed

ρ 0

is 1.225 kg/m³, the air density at sea level and 15 degrees Celsius

ρ

is the density of the air in which the aircraft is flying

High-speed flight

TAS can be calculated as a function of Mach number and static air temperature:

$TAS ={a_{sl}} M_a \sqrt{T\over T_{sl}}$

Where: $T A S =$ true airspeed; $a s l$ is the standard speed of sound at 15 °C (661.47 knots); $M a$ is Mach number,; $T$ is static air temperature in kelvins,; $T s l$ is standard sea level temperature (288.15 K)

Combining the above with the expression for Mach number under subsonic compresible flow gives an expression for TAS as a function of impact pressure (pitot tube), static pressure and static air temperature:

$TAS={a_{sl}}\sqrt{{5T\over T_{sl}}\left[\left(\frac{q_c}{P}+1\right)^\frac{2}{7}-1\right]}$

Where: $q c$ is impact pressure; $P$ is static pressure

Electronic Flight Instrument Systems (EFIS) contain an air data computer with inputs of impact pressure, static pressure and total air temperature. In order to compute TAS the air data computer must convert total air temperature to static air temperature. This is a function of Mach number:

$T={\frac{T_{t}}{1+0.2M_a^2}}$

Where: $T t =$ total air temperature

For manual calculation of TAS in knots where Mach number and static air temperature are known the expression may be simplified to:

$\mathrm{TAS} = 39M_a\sqrt{T}$

remembering that temperature is in kelvins and TAS in knots.

In simple aircraft, without an air data computer or Machmeter, true airspeed can be calculated as a function of calibrated airspeed and local air density (or static air temperature and pressure altitude which determine density). Some airspeed indicators incorporate a slide rule mechanism to perform this calculation. Otherwise, it can be performed using this java applet or a device such as the E6B (a handheld circular slide rule).

Notes

^ Clancy, L.J., Aerodynamics, Section 3.8

References

Air Navigation. Department of the Air Force. 1 December 1989. AFM 51-40.
Clancy, L.J.(1975), Aerodynamics, Chapter 3. Pitman Publishing Limited, London. ISBN 0 273 01120 0
Kermode, A.C., Mechanics of Flight, Chapter 2. (Eighth edition 1972) Pitman Publishing Limited, London. ISBN 0 273 31623 0

External links

True, Equivalent, and Calibrated Airspeed at MathPages
Newbyte airspeed converter
avc.obsment.com - True airspeed calculator.
Calculate True Airspeed, Mach, Pitot Tube Impact Air Pressure and more at luizmonteiro.com

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	TAS (abbreviation)
	true-airspeed indicator (aerospace engineering)
	air-temperature correction (navigation)

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True airspeed

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