UML has, without doubt, become the de-facto technique for the development of IT-type systems. However little concrete information is available concerning its effectiveness (or otherwise) for designing real-time embedded systems. This paper presents Austrian Aerospace’s experiences of applying UML for the design of an avionics system. It describes the complete lifecycle from adoption of UML, through training, CASE tool selection, analysis, design and development of the project itself. It concludes by reflecting on the experiences obtained and the lessons learned.