As we know, sliding mode control methodology is one of the robust control technique to handle systems with model uncertainties, parameter variations and external disturbances. In this paper, a robust altitude control scheme is proposed for a nonlinear quad-rotor aircraft system based on sliding mode controller with an integral action to eliminate the steady-state error effect. The proposed sliding mode controller is chosen to improve the stability and robustness of overall z-dynamics during the altitude control at a desired height. The stability of the system is guaranteed via Lyapunov stability theory. A suitable sliding manifold is designed to achieve the control objective. At last, the theoretical results are supported by different simulation tests to verify the satisfactory performance of proposed robust control scheme under external disturbances applied to autonomous quad-rotor aircraft.