A gas turbine engine including core engine is provided. Air may enter the core engine through an inlet and travel through and engine air flowpath extending through the core engine, e.g., generally along an axial direction of the gas turbine engine. The gas turbine engine additionally includes a cooling air flowpath extending outwardly generally along the radial direction of the gas turbine engine. The cooling air flowpath extends between an inlet in flow communication with engine air flowpath and an outlet defined by an opening in an outer casing of the core engine. Moreover, the gas turbine engine includes a heat exchanger positioned at least partially within the outer casing the core engine with the cooling air flowpath extending over or through the heat exchanger.