Definition

True airspeed is Calibrated Airspeed (CAS) corrected for altitude and non-standard temperature, i.e. the speed of the aircraft relative to the air mass in which it is flying .

Description

At sea level in the International Standard Atmosphere (ISA), and at slow 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.