This paper presents the modeling of the irreversible thermodynamics of the Air Frame Subsystem as a component of integrated aircraft design/synthesis. Entropy calculation procedures for complicated geometries in curvilinear coordinates are described, including the effects of turbulence. Both inviscid and viscous calculations are reported and the contributions of the various terms in the entropy equation are investigated. The procedure is validated and then extended to the calculation of entropy generation associated with flow over the B747200 aircraft. Results show that most of the entropy generation is due to turbulence. The viscous dissipation term in the entropy equation dominates compared to the heat transfer term. The implications of the results for design improvement are briefly discussed.