A method for determining the speed vector ({right arrow over (Vd)}), with respect to the surrounding air, of a rotary wing aircraft (HL) equipped with a beam scanning Doppler laser anemometry device for measuring (Step1) a set of projections (Vm) of said speed vector ({right arrow over (Vd)}) in at least four non-coplanar directions, comprising the following steps: detecting (Step2) any anomaly of at least one element from the set of measured projections (Vm), from a comparison with respect to a first threshold (S1), of a deviation between the measurements (Vm) and the measurements (Vp) predicted from a predetermined model depending on values of beam scanning parameters of the anemometry device; and reducing (Step3) the effect of a detected anomaly by invalidating measurements corresponding to said anomaly and computing the components of the speed vector from valid measurements.