Computer simulation of complete aircraft hydraulic systems has seldom been undertaken. The systems can be large and complex and present many problems for both model creation and analysis. This has invariably meant that the traditional testing based (iron bird) approach is adopted for verification of design. This paper describes the approach taken at Honeywell Normalair Garrett Ltd. to manage the complex and varied tasks required for the simulation of such systems. It is illustrated by the Nimrod MRA4 system model which is currently being validated.