And there is a third mean,whose definition may at first seem a bit strange.A common situation encountered by pilots goes like this:an aircraft can fly at 500 mph in still air.But due to the presence of wind,this so-called airspeed will be different from the aircraft's ground speed.On a stretch between two cities,a pilot encounters a headwind of 50 mph,reducing the aircraft's ground speed to 450 mph.On the return flight,the same wind now decomes a tailwind,increasing the ground speed to 550 mph.At what speed would the aircraft have to fly in still air to complete the round trip in the same stretch of time as when the wind was blowing?