A method for estimating the surface condition of a blade rotating about a rotational axis in an air or fluid stream, the method comprising the steps of: - measuring blade deformations indicative of blade loads acting on the blade to identify actual blade loads ( P,R,T), - measuring blade-state information (Ψ,Ω,, V) indicative of a blade operational state to identify a given blade operational state ( α,Q), - providing a clean-surface aerodynamic map () relating blade loads to an ensemble of blade operational states when the blade surface is in a clean state, - providing a dirty-surface aerodynamic map () relating blade loads to an ensemble of blade operational states when the blade surface is in a dirty state, - estimating the surface condition of the blade by comparing the distance of the actual blade loads ( P,R,T) to the blade loads given by the clean-surface aerodynamic map () at the given operational state ( α,Q), and the distance of the actual blade loads ( P,R,T) to the blade loads given by the dirty-surface aerodynamic map () at the given operational state ( α, Q), characterized in that the operational state ( α,Q) is defined by a dynamic pressure and a representative blade angle of attack ( α), the dynamic pressure and the representative blade angle of attack ( α) being derived from the blade-state information (Ψ,Ω,, V,β), the method further comprising the steps of: - defining an error function ( ε), the error function being dependent on a dirtiness indicator ( η) and a deviatory angle of attack (Δ α), the deviatory angle of attack (Δ α) being defined as a deviation in representative blade angle of attack ( α) between the actual blade angle of attack, and the representative blade angle of attack, - computing values for the dirtiness indicator ( η) and the deviatory angle of attack (Δ α) that minimize the error function ( ε), and - selecting the minimizing value for the dirtiness indicator ( η), the minimizing value for the dirtiness indicator ( η) being indicative of the surface condition of the blade. - measuring blade deformations indicative of blade loads acting on the blade to identify actual blade loads ( P,R,T), - measuring blade-state information (Ψ,Ω,, V) indicative of a blade operational state to identify a given blade operational state ( α,Q), - providing a clean-surface aerodynamic map () relating blade loads to an ensemble of blade operational states when the blade surface is in a clean state, - providing a dirty-surface aerodynamic map () relating blade loads to an ensemble of blade operational states when the blade surface is in a dirty state, - estimating the surface condition of the blade by comparing the distance of the actual blade loads ( P,R,T) to the blade loads given by the clean-surface aerodynamic map () at the given operational state ( α,Q), and the distance of the actual blade loads ( P,R,T) to the blade loads given by the dirty-surface aerodynamic map () at the given operational state ( α, Q), characterized in that the operational state ( α,Q) is defined by a dynamic pressure and a representative blade angle of attack ( α), the dynamic pressure and the representative blade angle of attack ( α) being derived from the blade-state information (Ψ,Ω,, V,β), the method further comprising the steps of: - defining an error function ( ε), the error function being dependent on a dirtiness indicator ( η) and a deviatory angle of attack (Δ α), the deviatory angle of attack (Δ α) being defined as a deviation in representative blade angle of attack ( α) between the actual blade angle of attack, and the representative blade angle of attack, - computing values for the dirtiness indicator ( η) and the deviatory angle of attack (Δ α) that minimize the error function ( ε), and - selecting the minimizing value for the dirtiness indicator ( η), the minimizing value for the dirtiness indicator ( η) being indicative of the surface condition of the blade. - defining an error function ( ε), the error function being dependent on a dirtiness indicator ( η) and a deviatory angle of attack (Δ α), the deviatory angle of attack (Δ α) being defined as a deviation in representative blade angle of attack ( α) between the actual blade angle of attack, and the representative blade angle of attack, - computing values for the dirtiness indicator ( η) and the deviatory angle of attack (Δ α) that minimize the error function ( ε), and - selecting the minimizing value for the dirtiness indicator ( η), the minimizing value for the dirtiness indicator ( η) being indicative of the surface condition of the blade.