If the air was at a lower viscosity, (Or as levee would like to think of it, non-viscous) then airplanes WOULDN'T fly, since the lift couldn't be created. Also, there's a reason commercial jets don't fly that fast. They aren't made to go that fast, and I personally doubt they'd hold up well, and without notice of going supersonic speeds. All airline passengers are naturally acclimated to going that fast and they don't pass out?
Adam Savage from mythbusters did when he got to fly with the blue angels.