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