The nonlinear planar dynamic model of an underactuated autonomous airship and its position/orientation control are addressed, for moderate-amplitude sinusoidal perturbation in three directions. The dynamic model, even if it is reduced into a plan, it presents some strong coupling terms due to the non coincidence of buoyancy and gravity centers and the presence of added masses while two control acting on the airship model. A nonlinear control law is proposed that guaranteed the convergence of the airship in the neighborhood of a desired point in presence of perturbations. Numerical results are presented to show the effectiveness of the proposed nonlinear control law.