Airline reserve fleet capacity is a wide spread phenomenon and is used to limit the impact of delays on the planned flight schedule. This paper focuses on reserve fleet capacity in the form of standby unscheduled air craft. By means of a Monte Carlo simulation on an existing flight schedule using a rule-based delay mitigation model the effectiveness of reserve fleet capacity is analysed. A method of finding a break even point between fleet size and saved delay costs is proposed and the impact of delay duration on the effectiveness of reserve fleet capacity is analysed. Finally, a com parison is made between grouping reserve fleet capacity into a standby aircraft and spreading the reserve fleet capacity over the flight schedule.