When the Aeroplane was on the ground it had the same speed as the earth but let's say the aeroplane flew at 300 mph. That means it flew at the speed of Earth ± 300 mph. +300 mph if it flies in the same direction of the earth's rotation i.e, West to East or -300 mph if in opposite direction. Since the speed of the earth is not that important for calculation we tend to ignore it.