Disclaimer: not an aeronautics expert. The problem is that airspeed and velocity are not the same thing. Even with an inertial system where you could get the "

Disclaimer: not an aeronautics expert. The problem is that airspeed and velocity are not the same thing. Even with an inertial system where you could get the "forward" velocity of the plane, that's not guaranteed to be the airspeed. It has to do with the angle-of-attack and fluid dynamics around the plane.