A system and method are provided for calculating Mach number and true airspeed without reference to data from a pitot static sensor. The true airspeed and Mach number are calculated using the altitude information from GPS, IRS, Radio Altimeter and other onboard sensors other than the air data computer (ADC). The computed true airspeed or Mach number could be used to confirm the ADC information or in lieu of the ADC information when the ADC information is unreliable or unavailable.