In this paper we develop a directional control for a small fixed-wing air vehicle in order to perform path following in presence of unknown crosswind. The dynamic of the cross track error with respect to a desired trajectory is obtained from the lateral airplane equations of motion. The control strategy relies on Lyapunov based backstepping approach and adaptation techniques are considered to estimate the unknown wind disturbance. The dynamic order of the adaptive system is guaranteed to be minimal. Lyapunov stability theory is used to analyze the stability properties of the closed-loop system. In addition, several simulations taking into account unknown wind, have been performed to evaluate the behavior and the robustness of the control scheme.