A gas turbine engine comprising: a core engine defined about an axis (A), the core engine having a primary exhaust flow path ; a fan section driven by the core engine about the axis (A); an upper duct section defined along the axis (A) to at least partially surround the core engine; and a lower duct section defined along the axis (A), the lower duct section mateable with the upper duct section along an axial interface to enclose the core engine, to form a bypass duct which at least partially defines an annular secondary fan bypass flow path around the primary flow path ; characterised in that the gas turbine engine further comprises: an additively manufactured heat exchanger formed into an internal surface of the upper duct section , wherein the additively manufactured heat exchanger comprises a multiple of fins that extend into the annular secondary fan bypass flow path , at least one of the multiple of fins contains at least one of a multiple of additively manufactured passages , at least one of the multiple of additively manufactured passages of a tailored cross-sectional geometry that correspond with the internal surface ; and an additively manufactured tank formed into the lower duct section , the tank in fluid communication with the multiple of additively manufactured passages . a core engine defined about an axis (A), the core engine having a primary exhaust flow path ; a fan section driven by the core engine about the axis (A); an upper duct section defined along the axis (A) to at least partially surround the core engine; and a lower duct section defined along the axis (A), the lower duct section mateable with the upper duct section along an axial interface to enclose the core engine, to form a bypass duct which at least partially defines an annular secondary fan bypass flow path around the primary flow path ; characterised in that the gas turbine engine further comprises: an additively manufactured heat exchanger formed into an internal surface of the upper duct section , wherein the additively manufactured heat exchanger comprises a multiple of fins that extend into the annular secondary fan bypass flow path , at least one of the multiple of fins contains at least one of a multiple of additively manufactured passages , at least one of the multiple of additively manufactured passages of a tailored cross-sectional geometry that correspond with the internal surface ; and an additively manufactured tank formed into the lower duct section , the tank in fluid communication with the multiple of additively manufactured passages . an additively manufactured heat exchanger formed into an internal surface of the upper duct section , wherein the additively manufactured heat exchanger comprises a multiple of fins that extend into the annular secondary fan bypass flow path , at least one of the multiple of fins contains at least one of a multiple of additively manufactured passages , at least one of the multiple of additively manufactured passages of a tailored cross-sectional geometry that correspond with the internal surface ; and an additively manufactured tank formed into the lower duct section , the tank in fluid communication with the multiple of additively manufactured passages . <EOS>