This paper describes the methodology developed to determine the size of a new fleet of combat helicopters for the Royal Australian Navy. The Australian Government's 2009 Defence White Paper described "as a matter of urgency" a requirement for a fleet of helicopters "to provide eight or more aircraft concurrently embarked on ships at sea". The objective therefore is to find the minimum fleet size that enables the fleet to meet both this minimum daily embarked requirement, as well as annual requirements for a specified number of flying hours for both embarked helicopters and the remaining ashore-based fleet. The fleet sizing problem incorporates the helicopter fleet, ships, personnel and a home base with maintenance facilities. Individual helicopters (or tails) pass through various states throughout their life. These include the serviceable state when they are able to fly, or a range of maintenance states which will make them unable to fly. Maintenance types include regular inspections, phased maintenance and deep maintenance. Many of these states can occur when a tail is either embarked or ashore. Unscheduled maintenance adds a random element to the problem and influences which aircraft may be serviceable on a day-today basis. A discreteevent simulation approach was chosen to address this problem, due to the requirements to track tails in various states, test for state transitions and incorporate random effects.