Computing small-fleet aircraft availabilities including redundancy and spares

Logistics support is a key element of aircraft transportation systems. This paper is concerned with the impact of aircraft spares provisioning decisions on the availability of aircraft. Spares provisioning in this context is complicated by the fact that spares may be shared across aircraft and that aircraft may have redundant systems. In addition, decisions concerning aircraft spares support require a rapid response for safety reasons. Analytical models have proven to provide a quicker response time than corresponding simulation models. There is an existing analytical model that includes the effect of redundancy and spares, but the underlying assumption is that a large number of aircraft are being modeled. In many applications, predictions of the number of times an aircraft can fly each day and the number of aircraft that are ready at any time are applied to a small fleet of aircraft. This paper demonstrates the improvement in computational accuracy that is achieved by reflecting the impact of small numbers of aircraft on availability projections. The approach used is to extend existing finite queuing spares models to including redundancy. Further, the method is used to optimize spares provision with respect to a user specified availability goal. Although the case study for this work is a military combat aircraft application from the Gulf War, the method is applicable to any small system of vehicles or machines where components may be redundant, demand and repairs may be approximated as following an exponential distribution, and limited access to spare parts is the rule.