An aircraft of mass 3 10^5 ~kg with total wing area 400 ~m ^2 is in a level flight at a speed of 540 ~km ~h ⁻¹ . The density of air at its height is 1.2 ~kg ~m ⁻³ . The fractional increase in the speed of the air on the upper surface of its wings relative to the lower surface is ____ ( g =10 ~ms ⁻² )