Novel aircraft configurations and synergistic design processes require a closer integration of design and analysis models. For the structural modelling of the aircraft skin, the isogeometric B-Rep analysis (IBRA) of shell structures can provide an effective solution [1]. IBRA facilitates additional design iterations, leading to an optimized product. To achieve a light weight design, the aircraft skin is reinforced with stiffeners, which conventionally consist of orthogonally arranged stringer and frame reinforcements (orthogrids). However, especially for unconventional fuselage concepts, an alternative stiffener layout based on Non-Uniform Rational B-Splines (NURBS) can lead to a favorable weight reduction. Research by [2] has demonstrated how nonlinear isogeometric spatial Bernoulli beams can be embedded in shell elements using nested parameterization. The mesh-independent strong coupling makes this approach well suited for topology optimization. The present study illustrates how embedded beam elements can be combined with growth-based topology optimization to achieve an optimal stiffener layout. Trimmed multipatches and manufacturing constraints are also taken into account. The proposed method is applied to an unconventional rear fuselage design with highly integrated engines required to reduce fuel consumption. Additionally, the CAD-integrated optimization method enables a simple estimation of the influence of the engine integration on the structural weight. Finally, the presented method is compared with a conventional orthogrid semi-monocoque fuselage configuration and Solid Isotropic Material with Penalization (SIMP)-based solutions. REFERENCES [1] T. Teschemacher, A.M. Bauer, T. Oberbichler, Realization of CAD-integrated shell simulation based on isogeometric B-Rep analysis, Advanced Modeling and Simulation in Engineering Sciences (2019): 5, 19. [2] A.M. Bauer, M. Breitenberger, B. Philipp, R.W¨uchner, K.-U. Bletzinger, Embedded structural entities in NURBS-based isogeometric analysis, Computer Methods in Applied Mechanics and Engineering (2017): Volume 325.