A turbofan engine (100; 200) comprising a nacelle (121; 221) and an engine core having a core cowling (124; 224) and wherein (i) the internal surface of the nacelle and the external surface of the core cowling define a bypass duct having an exhaust end defining a bypass duct exit plane (123; 223) generally normal to the longitudinal axis of the engine; (ii) the core cowling extends aft of the bypass duct exit plane and has an exhaust end defining a core exit plane (119; 219) generally normal to the longitudinal axis of the engine; (iii) the core cowling has an annular or partly-annular exit ventilation nozzle (128; 228) located aft of a first longitudinal position and fore of a second longitudinal position, the first and second longitudinal positions being respectively fore of and either aft of or coincident with the bypass duct exit plane and the core cowling otherwise being free of exit ventilation nozzles fore of the second longitudinal position; and wherein in a plane which includes the longitudinal axis of the engine: (iv) a first straight line passing through a first point on the internal surface of the nacelle in the bypass duct exit plane and a second point on the external surface of the core cowling at the first longitudinal position is normal to the external surface of the core cowling; and (v) a second straight line passing through the first point and a third point on the external surface of the core cowling at the second longitudinal position makes an angle with the bypass duct exit plane which is less than or equal to 20% of the angle between the bypass duct exit plane and a third straight line joining the first point and a fourth point on the external surface of the core cowling in the core exit plane. (i) the internal surface of the nacelle and the external surface of the core cowling define a bypass duct having an exhaust end defining a bypass duct exit plane (123; 223) generally normal to the longitudinal axis of the engine; (ii) the core cowling extends aft of the bypass duct exit plane and has an exhaust end defining a core exit plane (119; 219) generally normal to the longitudinal axis of the engine; (iii) the core cowling has an annular or partly-annular exit ventilation nozzle (128; 228) located aft of a first longitudinal position and fore of a second longitudinal position, the first and second longitudinal positions being respectively fore of and either aft of or coincident with the bypass duct exit plane and the core cowling otherwise being free of exit ventilation nozzles fore of the second longitudinal position; (iv) a first straight line passing through a first point on the internal surface of the nacelle in the bypass duct exit plane and a second point on the external surface of the core cowling at the first longitudinal position is normal to the external surface of the core cowling; and (v) a second straight line passing through the first point and a third point on the external surface of the core cowling at the second longitudinal position makes an angle with the bypass duct exit plane which is less than or equal to 20% of the angle between the bypass duct exit plane and a third straight line joining the first point and a fourth point on the external surface of the core cowling in the core exit plane.