In this paper the problem of icing detection is considered for wind turbines operating in medium speed wind region (Region 2) and subject to a control law tracking the Maximum Delivery Point of the power coefficient characteristic. Based on an observer of the rotor angular acceleration, rotor inertia is estimated in order to detect its eventual increase due to icing. Moreover, the observed value of rotor inertia can be potentially used for updating the controller parameters or to stop the turbine when icing is too severe. The proposed approach has been tested by intensive MatLab^® simulations using the NREL 5 MW wind turbine model.