True airspeed

Traditionally it is measured using an analogue TAS indicator, but as GPS has become available for civilian use, the importance of such air-measuring instruments has decreased.

However, since indicated airspeed only shows true speed through the air at standard sea level pressure and temperature, a TAS meter is necessary for navigation purposes at cruising altitude in less dense air.

The airspeed indicator (ASI), driven by ram air into a pitot tube and still air into a barometric static port, shows what is called indicated airspeed (IAS).

At sea level in the International Standard Atmosphere (ISA) and at low speeds where air compressibility is negligible (i.e., assuming a constant air density), 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.

For low speeds, the data required are static air temperature, pressure altitude and IAS (or CAS for more precision).

Above approximately 100 knots (190 km/h), the compressibility error rises significantly and TAS must be calculated by the Mach speed.

Modern aircraft instrumentation use an air data computer to perform this calculation in real time and display the TAS reading directly on the electronic flight instrument system.

Since temperature variations are of a smaller influence, the ASI error can be estimated as indicating about 2% less than TAS per 1,000 feet (300 m) of altitude above sea level.

To maintain a desired ground track while 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.

At low speeds and altitudes, IAS and CAS are close to equivalent airspeed (EAS).

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

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

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

Some airspeed indicators incorporate a slide rule mechanism to perform this calculation.

Otherwise, it can be performed using this applet or a device such as the E6B (a handheld circular slide rule).