Airspeed Sensing Errors
The airspeed indicator (ASI), driven by a Pitot tube and a barometric static port, shows what is called indicated airspeed (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 Atmosphere (ISA) and at low 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 E6B flight calculator or its equivalent. For low speeds, the data required are static air temperature, pressure altitude and IAS (or 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.
Read more about this topic: True Airspeed
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