The mission system of an aircraft is a complex real-time distributed system consisting of a mission control computer, different kinds of devices interconnected by a number of serial data buses. The complexity and real-time requirements of mission systems have motivated research into the application of formal techniques to investigate and predict the effects of upgrades on mission system behaviour. This paper reports on a joint research project between the University of South Australia and Australia?s Defence Science and Technology Organisation. In previous work we modelled a generic avionics mission system with Coloured Petri Nets and analysed the model using state spaces. Here, we describe how this model was refined and modified to obtain a Coloured Petri Net model for the AP-3C Orion maritime surveillance aircraft.