Mach #


Can anybody tell me why the Mach number listed sometimes is higher than the maximum Mach number for the particular aircraft. Thank You very much. PS Flightaware Rocks!!!


It’s calculated using the groundspeed (instead of the airspeed) and using the standard atmosphere model for temperature (instead of actual temp aloft).


What’s the equation?


The speed of sound is sqrt(gammaRtemperature), where gamma is the ratio of specific heats, R is the gas constant, and temperature is in absolute units. I calculated it every 100 feet in the 1976 US standard atmosphere and came up with a nice quadratic curve fit (R^2 > 0.9995) up to FL360 (above that it’s a constant).


Not a simple answer:


I think my head just exploded.


Mine too, my equation is to go to OTHR page 9, get the true airspeed from the air data computer, then go to CALC page 3 and put it in there to compute the mach number. Much easier. :wink:


How would I correct for nonstandard temperature? Just find the Mach # for the temperature at the current altitude?

There are some embarrassed History or English majors, who… [screams] [running away]

Sorry, I couldn’t resist.


Two ways come to mind: Either calculate the speed of sound using the previously mentioned formula and divide the airspeed by that figure, or multiply the Mach number at standard day conditions by sqrt(actual_temp/standard_day_temp).