15 °C For supersonic airspeeds, where a normal shock forms in front of the pitot probe, the Rayleigh formula applies: The supersonic formula must be solved iteratively, by assuming an initial value for equal to . These formulae work in any units provided the appropriate values for and are selected. For example, = 1013.25 hPa, = . The ratio of specific heats for air is assumed to be 1.4. These formulae can then be used to calibrate an airspeed indicator when impact pressure () is measured using a water manometer or accurate pressure gauge. If using a water manometer to measure millimeters of water the reference pressure () may be entered as 10333 mm . At higher altitudes CAS can be corrected for compressibility error to give equivalent airspeed (EAS). In practice compressibility error is negligible below about and . See also Acronyms and abbreviations in avionics ICAO recommendations on use of the International System of Units Equivalent airspeed Flight instruments Indicated airspeed True airspeed References Bibliography Gracey, William (1980), "Measurement of Aircraft Speed and Altitude" (12 MB), p. 15, NASA Reference Publication 1046. External links A free windows calculator which converts between various airspeeds (true / equivalent / calibrated) according to the appropriate atmospheric (standard and not standard!) conditions A free android calculator which converts various airspeeds according to atmospheric characteristics Newbyte airspeed converter JavaScript Calibrated Airspeed calculator from True Airspeed and other variables at luizmonteiro.com Determine your Calibrated AirSpeed with this android app which uses the "GPS calibration method". Airspeed