Fault detection by modal analysis is a highly developed field in civil engineering. For wind turbines, a loss of isotropy for the rotor can be the consequence of a defect in the angle of attack (pitch) of the blades, or the accumulation of ice, or else the presence of structural defects resulting in a loss of stiffness on one or more blades. It is highly desirable to apply classical OMA techniques, and this despite the rotation of the blades. It is shown in this paper that it is possible to apply LTI approaches on wind turbines similarly to the methodology usually applied to standard civil engineering structures using an approximate Fourier modeling of the eigenmodes. The monitoring of anisotropy using the mode shapes of the estimated modes is validated with an example where a global loss of stiffness of 5% of a rotor blade is simulated.