There are many examples of the use of the technique of domain analysis for modeling software systems in the initial stages of their development, although the case studies chosen are often of small systems or of small parts of large systems. In this paper we show that the techniques can be as readily applied to very large domains and how a manageable formal model of the domain can be obtained by abstracting appropriately. We illustrate this with a case study based on the airline business domain. We also discuss how this formal model can be developed towards software support systems for the airline industry which capture a wide range of different requirements, and how it might be applied more generally.