Oh yes. Explanation on Mach numbers. They are created using two variables: temperature and atmospheric composition. They are largely independent of pressure. So the combination of atmosphere and temperature defines resistance.
As a plane rises, the air gets thinner but it also gets colder. The lowering of temperature seems to have a slightly greater effect than the thinning of the atmosphere. So the speed of sound lowers as altitude increases.
The effect of this is that planes reach a particular Mach number at a lower velocity the higher they fly.
"At altitude, for reasons explained, Mach number is a function of temperature. Aircraft flight instruments, however, operate using pressure differential to compute Mach number, not temperature. The assumption is that a particular pressure represents a particular altitude and, therefore, a standard temperature. Aircraft flight instruments need to operate this way because the stagnation pressure sensed by a Pitot tube is dependent on altitude as well as speed.
Assuming air to be an ideal gas, the formula to compute Mach number in a subsonic compressible flow is found from Bernoulli's equation for M < 1 (above):[7]
\mathrm{M} = \sqrt{5\left[\left(\frac{q_c}{p}+1\right)^\frac{2}{7}-1\right]}\,
"
So on the one hand, you have to establish if a plane has the thrust and lift to reach a particular velocity, net of the drag.
And on the other, you have to consider if the frame and the engine can deal with the shock waves at a particular Mach number.
From what I have read, the 767 has the power to reach Mach 0.86. If it was regularly flown at speeds above 0.5 Mach at lower altitudes, there would be wear and tear issues requiring higher maintenance costs. But it would come apart at the seams if flown a bit above Mach 0.86 at any altitude.
I could be wrong. But that is how I read the sources I have read.
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