An enhanced robust altitude control scheme that indicates the improved performance than the typical sliding mode technique for a Quad-rotor aircraft vehicle is proposed in this article by including an integral action in the sliding mode control architecture in order to eliminate the steady-state error induced by the boundary layer and achieving asymptotic convergence to the desired altitude with continuous control input. The proposed integral sliding mode controller is chosen to ensure the stability and robustness of overall dynamics during the altitude control at a desired height reference on the z-axis. Furthermore, we propose a Control Lyapunov Function (CLF) via Lyapunov theory in order to construct the robust stabilizing controller and demonstrate the stability of the z-dynamics of our system. A suitable sliding manifold is designed to achieve the control objective. At last, the simulations and experimental studies are supported by different tests to demonstrate the robustness and effectiveness of the proposed enhanced robust altiutde control scheme subject to bounded external disturbances in outdoor environment.