The jet stream is a fast-flowing air current found in the atmosphere at around 36,000 ft above the surface of the Earth. During one summer day, the speed of the jet stream is 40 mph. A plane flying with the jet stream can fly 750 mi in the same amount of time that it would take to fly 510 mi against the jet stream. What is the speed of the plane in still air? The plane flies mph in still air.