![]() 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. Mach incorporates the above data including the compressibility factor. Above approximately 100 knots (190 km/h), the compressibility error rises significantly and TAS must be calculated by the Mach speed. In flight, it can be calculated either by using an E6B flight calculator or its equivalent.įor low speeds, the data required are static air temperature, pressure altitude and IAS (or CAS for more precision). For this reason, TAS cannot be measured directly. The ASI will indicate less than TAS when the air density decreases due to a change in altitude or air temperature. 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 ratio between the two measurements is temperature-dependent and pressure-dependent, according to the ideal gas law.Īt 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. The differential pressure is affected by air density. 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). It is the speed normally listed on the flight plan, also used in flight planning, before considering the effects of wind. 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. Neither provides for accurate speed over the ground, since surface winds or winds aloft are not taken into account. On jet airliners the TAS meter is usually hidden at speeds below 200 knots (370 km/h). The IAS meter reads very nearly the TAS at lower altitude and at lower speed. 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. Since indicated, as opposed to true, airspeed is a better indicator of margin above the stall, true airspeed is not used for controlling the aircraft for these purposes the indicated airspeed – IAS or KIAS (knots indicated airspeed) – is used. Traditionally it is measured using an analogue TAS indicator, but as the Global Positioning System has become available for civilian use, the importance of such air-measuring instruments has decreased. The true airspeed is important information for accurate navigation of an aircraft. The true airspeed ( TAS also KTAS, for knots true airspeed) of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. Here the speed is displayed both 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. An analog true airspeed indicator for an airplane.
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