an electro - thermal or hot gas anti - icer of the “ running - wet ” type is mounted at the leading edge of an airfoil . mechanical de - icers are mounted on the surfaces aft of the anti - icer . the mechanical de - icers throw off ice without creating runback water , thereby eliminating runback refreeze ridges caused by a known hybrid ice - protection system . advantageously , the de - icers are of a type in which the airfoil has a semi - rigid skin that forms an aerodynamic surface of the airfoil , and in which there is at least one mechanical actuator for flexing the skin .