Hmmmm...
 

I would hate to challange the learned AC K, as his 'Mechanics of Flight' is the basic repository for most aviation knowledge. BUT...

As the density of air at FL390 at -56 is NOT the same as the density at FL450 at -56, I think that we all agree. However, we all agree that the LSS is equal at both atltitudes, by the formula.

Has the basic formula been simplified for the 'not so clever guys that drive the buses'?

As I understand it, the local speed of sound is defind by:

a = sqrt(gRT/M)

Where:

g(Gamma on a UK keyboard!) = Ratio of specific heats, also known as the adiabatic constant
R = Universal Gas Constant
T = Absolute Temperature
M = Molecular mass of gas

Certainly R and M are independant of pressure. I thought g (Gamma) was as well. If that's the case, the the local speed of sound, and therefore Mach No, are only affected by temperature.
I guess that if pressure does have an effect, it must be that gamma varies with pressure...


Looks like I've just managed to repeat dexter:rolleyes: