Since the temperature and density of air decreases with altitude, so does the speed of sound, hence a given true velocity results in a higher MACH number at higher altitudes. | MACH NUMBER and AIRSPEED vs ALTITUDE MACH NUMBER is defined as a speed ratio referenced to the speed of sound . MACH NUMBER Velocity of Interest at the given atmospheric conditions m Velocity of Sound Since the temperature and density of air decreases with altitude so does the speed of sound hence a given true velocity results in a higher MACH number at higher altitudes. AIRSPEED is a term that can be easily confused. The unqualified term airspeed can mean any of the following a. Indicated airspeed IAS - the airspeed shown by an airspeed indicator in an aircraft. Indicated airspeed is expressed in knots and is abbreviated KIAS. b. Calibrated airspeed CAS - indicated airspeed corrected for static source error due to location of pickup sensor on aircraft. Calibrated airspeed is expressed in knots and is abbreviated KCAS. Normally it doesn t differ much from IAS. c. True airspeed TAS - IAS corrected for instrument installation error compressibility error and errors due to variations from standard air density. TAS is expressed in knots and is abbreviated KTAS. TAS is approximately equal to CAS at sea level but increases relative to CAS as altitude increases. At 35 000 ft 250 KIAS or KCAS is approximately 430 KTAS. IAS or CAS is important in that aircraft dynamics such as stall speed responds largely to this quantity. TAS is important for use in navigation True airspeed windspeed groundspeed . Figures 1 and 2 depict relations between CAS and TAS for various altitudes and non-standard temperature conditions. The first graph depicts lower speed conditions the second depicts higher speeds. As an example of use consider the chart on the next page. Assume we are in the cockpit have read our IAS from the airspeed indicator and have applied the aircraft specific airspeed correction to obtain 370 KCAS. We start at point A and go horizontally to our flight altitude at point B 25 000 ft in this case . To find our Mach we go down vertically to point C to obtain Mach. To .
