A gas turbine engine comprising a nacelle and a breather duct. The breather duct provides communication between a component of the engine within the nacelle and the exterior of the nacelle and opens at an exhaust port on the external surface of the nacelle. There is a clean air outlet slot located downstream of the exhaust port and arranged to expel, in use, a sheet flow of clean air to form an aerodynamic barrier between the external surface of the nacelle and a flow of breather air expelled from the exhaust port.