We present the modeling and validation experiments performed with the IFx validation toolset and with the UML profile developed within the IST Omega project, on a representative space vehicle control system: a model of the Ariane-5 flight software obtained by manual reverse engineering. The goal of the study is to verify functional and scheduling-related requirements under different task architecture assumptions. The study is also a proof of concept for the UML-based validation technique proposed in IFx.