A boundary layer ingestion fan system for location aft of the fuselage of an aircraft, comprising: a nacelle defining a duct; a fan located within the duct, the fan comprising a hub arranged to rotate around a rotational axis (a-A) and a plurality of blades attached to the hub, each of which has: a span (r) from a root at the hub defining a 0 percent span position (r hub) to a tip defining a 100 percent span position (rtip) and a plurality of span positions therebetween (r ∈ [r hub, r tip]), a leading edge and a trailing edge defining, for each span position, a chord therebetween having a chord length (c), a blade thickness (t) defined for each span position thereof which is normal to said chord and which is defined as a maximum distance between a pressure surface and a suction surface of each blade; wherein a ratio of blade thickness at the 0 percent span position (t hub) to chord length at the 0 percent span position (c hub) is 0.1 or greater, and wherein the fan has, for each span position, a circumferential pitch (s) between each one of said plurality of blades; each of the plurality of blades has a blade inlet angle (β1) which is the angle of a camber line at the leading edge relative to the rotational axis; characterised in that 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·cosβ 1), is 0.25 or greater at the 0 percent span position. a nacelle defining a duct; a fan located within the duct, the fan comprising a hub arranged to rotate around a rotational axis (a-A) and a plurality of blades attached to the hub, each of which has: a span (r) from a root at the hub defining a 0 percent span position (r hub) to a tip defining a 100 percent span position (rtip) and a plurality of span positions therebetween (r ∈ [r hub, r tip]), a leading edge and a trailing edge defining, for each span position, a chord therebetween having a chord length (c), a blade thickness (t) defined for each span position thereof which is normal to said chord and which is defined as a maximum distance between a pressure surface and a suction surface of each blade; wherein a ratio of blade thickness at the 0 percent span position (t hub) to chord length at the 0 percent span position (c hub) is 0.1 or greater, and wherein the fan has, for each span position, a circumferential pitch (s) between each one of said plurality of blades; each of the plurality of blades has a blade inlet angle (β1) which is the angle of a camber line at the leading edge relative to the rotational axis; characterised in that 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·cosβ 1), is 0.25 or greater at the 0 percent span position. a span (r) from a root at the hub defining a 0 percent span position (r hub) to a tip defining a 100 percent span position (rtip) and a plurality of span positions therebetween (r ∈ [r hub, r tip]), a leading edge and a trailing edge defining, for each span position, a chord therebetween having a chord length (c), a blade thickness (t) defined for each span position thereof which is normal to said chord and which is defined as a maximum distance between a pressure surface and a suction surface of each blade; wherein a ratio of blade thickness at the 0 percent span position (t hub) to chord length at the 0 percent span position (c hub) is 0.1 or greater, and wherein the fan has, for each span position, a circumferential pitch (s) between each one of said plurality of blades; each of the plurality of blades has a blade inlet angle (β1) which is the angle of a camber line at the leading edge relative to the rotational axis; characterised in that 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·cosβ 1), is 0.25 or greater at the 0 percent span position.