@ GY:
How is M number computed?
Not sure if I build the formula correct, but something like this is mentioned. Couldn't find the Root sign
MACH = ROOT(5 * ((Qc/Po+1)^(2/7)-1))
from wiki (thx LoneW 50)
Qc/Po = ratio of impact pressure to static pressure.
@ Bearfoil:
Why was it not available at <60 (but airborne?)
Maybe because pitots (Qc) where clogged with Ice?