The aerospace industry makes widespread use of methods and tools to support the later stages of the design process, e.g. finite elements analysis and solid modelling. However, there is a lack of support for the early stages which lead up to the embodiment of design concepts. This is a distinct disadvantage because the decisions made in the early stages of the design process have a profound effect on the technical and commercial success of the product. This paper shows how a systematic approach can be taken during preliminary design of mechanical systems. The results are based on a research project involving British Aerospace and Cambridge University Engineering Design Centre during which this methodology was applied to the design of a flight refuelling probe.