The present paper shows the application of the model-based AGILE 4.0 architectural framework currently being developed within the context of the European Union (EU) - funded AGILE 4.0 research project. This architectural framework is conceived to guide designers in the definition, modeling, design and optimization of complex aeronautical systems, by following a Systems Engineering Product Development process. The AGILE4.0 architectural framework supports different activities for the development of systems, such as requirements engineering, functional analysis, system architecting and Multidisciplinary Design and Optimization (MDO). More specifically, the present paper focuses on part of the architectural framework, which is being developed to address the initial activities of a Systems Engineering process, namely the definition and modelling of system stakeholders, needs and requirements. The development of this architectural framework is indeed fostered by all the potential advantages of Model Based Systems Engineering (MBSE) approaches, which in contrast to traditional document-based approaches might bring to several benefits in the design process, as enhancement of systems design quality, more complete and clearer development of system requirements and specifications and improved communications within the design teams. Seven different application cases are addressed in the paper, demonstrating how the architectural framework and its implementation in a MBSE development system can be exploited to streamline, accelerate and improve the definition and modeling of complex systems. Advantages but also some limitations of this model-based approach are identified from the applications and addressed in the paper.