Turbofan engine

A turbofan engine 100 comprises a nacelle 121 and an engine core having a core cowling 124. The internal surface of the nacelle 121 and the external surface of the core cowling 124 define a bypass duct 122 having an exhaust end defining a bypass duct exit plane 123 generally normal to the longitudinal axis (X) of the engine 100. The core cowling 124 extends aft of the bypass duct exit plane 123 and has an annular or partly-annular exit ventilation nozzle 128 located aft (downstream) of a first longitudinal position 149 and fore (upstream) of a second longitudinal position 151, the first and second longitudinal positions being respectively fore and aft of (or coincident with) the bypass duct exit plane 123, the core cowling otherwise being free of exit ventilation nozzles fore of the second longitudinal position. The first position is defined as being where a line 148 normal to the core cowling intersects a point 145 on the aft edge of the nacelle 121, and the second position is defined by and angle 143 between it and the bypass duct exit plane 123 being 20% or less than the angle 141 between the bypass duct exit plane 123 and a third position 150 on the core cowling. A further vent 129 may be present aft of the second position. The engine has a lower specific fuel consumption than an equivalent engine having an exit ventilation nozzle in a conventional position 129 aft of the second longitudinal position.