In order to establish laminar flow on the attachment line (18) of an aerofoil body, a duct entrance (27) is provided on the leading edge of the aerofoil body for receiving spanwise boundary layer flow BLt. The exit (23) of the duct is located spanwise downstream of the duct entrance (27). The boundary layer flow BLt enters the duct (23) and is discharged downstream. The height of the duct entrance (27) above the leading edge of the aerofoil body is greater than the depth of the boundary layer BLt and thus a fresh laminar boundary layer is established on the outer surface (20) of the duct which propagates spanwise along the surface to rejoin the leading edge of the aerofoil body.