I think you have some confusion about indicated airspeed - it's used because it provides pilots with predictable information about how the plane will react, it'

I think you have some confusion about indicated airspeed - it's used because it provides pilots with predictable information about how the plane will react, it's not indicative of the actual speed of the plane: https://en.wikipedia.org/wiki/Indicated_airspeed https://en.wikipedia.org/wiki/Indicated_airspeed They were truly at 0.9 mach, had they increased speed by 11% they would have broken the sound barrier, well before the indicated air speed would have read a >mach 1 number.