A boundary layer ingestion fan system for location aft of the fuselage of an aircraft is shown. It comprises a nacelle defining a duct, and a fan located within the duct. The fan comprises a hub arranged to rotate around a rotational axis (A-A) and a plurality of blades attached to the hub. A blade blockage, which is the ratio of the blade thickness to the product of the circumferential pitch and the cosine of a blade inlet angle (t/s·cosβ1), is 0.25 or greater at the 0 percent span position.