This paper deals with the concept of input observability in a fixed speed wind turbine. A linear system has been calculated from the nonlinear equations of the squirrel cage induction generator, supposing it connected directly to the grid and assuming a steady state operating point. The observability of the input from the output of the system could be an interesting way to know if its possible to develop some new controls without introduce special sensors in the system. Furthermore, it is interesting to analyse which is the parameter variation margin of the wind turbine from input-observable state to non-input observable, in order to obtain some restrictions to design future controllers, or limit the operating points.