The present study relates to a transport organisation operating a fleet of aircraft and aims at determination of the optimal spare float of engines. The flow of the engines is viewed as a queueing problem in a closed loop system with variable demand and supply. The servicing facility consists of three stations in tandem. Two cases of the servicing facility operation are investigated, first when no inter-stage queue is permitted, and second when a queue length of one unit is allowed between the first and second stations. The solution is obtained by the Monte Carlo Simulation technique. The results are compared with those obtained from a reliability theoretical approach and conclusions are drawn.