Mach #


#1

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!!!


#2

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


#3

What’s the equation?


#4

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).


#5

Not a simple answer: mathpages.com/home/kmath282/kmath282.htm


#6

I think my head just exploded.


#7

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:


#8

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.


#9

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).