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True airspeed
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==Airspeed sensing errors== 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). The differential pressure is affected by [[air density]]. The ratio between the two measurements is temperature-dependent and pressure-dependent, according to the [[ideal gas law]]. 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. The ASI will indicate less than TAS when the air density decreases due to a change in altitude or air temperature. For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using an [[E6B]] flight calculator or its equivalent. For low speeds, the data required are [[outside air temperature|static air temperature]], pressure altitude and IAS (or [[Calibrated airspeed|CAS]] for more precision). Above approximately {{convert|100|kn|km/h}}, 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 [[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 {{convert|1000|ft|m|sigfig=1}} of altitude above sea level. For example, an aircraft flying at {{convert|15000|ft|m}} in the international standard atmosphere with an IAS of {{convert|100|kn|km/h}}, is actually flying at {{convert|126|kn|km/h}} TAS.
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