As an aside, the speed of sound varies depending on pressure and temperature. At the cruising height of an airliner it's typically significantly less than the s

As an aside, the speed of sound varies depending on pressure and temperature. At the cruising height of an airliner it's typically significantly less than the speed at 0m/20 degrees. In fact at 35,000 feet it's usually closer to 660 mph. When you get close to the speed of sound in air, all sorts of weird aerodynamic effects happen. Because it varies, the term "mach" is used to denote 1x the speed of sound (regardless of what the pressure or temperature is) - so an aircraft flying at mach 0.85 is at 85% the speed of sound for that height and temperature. The only thing that matters to an aircraft at altitude is the airspeed.