gums, 1842
Somewhere in the cobwebs of this old brain it seems that temperature is a big player to get true mach, especially up high. An SR-71 buddy of mine reminded me of this awhile back, but what do WE know? And ask a few Concorde pilots about this. So if the Airbus wants to use only dynamic and total pressures to calculate mach, I am wondering a bit.
You are right, air temperature is required, for accuracy. In the old days
of mechanical instruments, machmeters had only pitot and static input ports
and made assumptions about air temperature vs altitude. Non linear
cams and levers then translated that to mach dial reading, with the temp
parameter effectively factored in from the altitude capsule.
When air data computers arrived, it became much easier to plug the
temperature into the equation, as a synchro or pot input. The driver for
this being the need for much greater accuracy of measurement. Even
some of the earliest electromechanical adc's had tas and mach capability,
so not such a new idea....
Edit:
Went back to check and could not verify it. Referring to Collinson,
there's no mention of temperature in Mach calculations. You definately do
need temperature for true air speed calculation,, but can't confirm the
mach case. Perhaps someone else has more info ?...