Controlling multi-indenture repairable inventories of multiple aircraft parts

This paper presents a model for determining stock levels of repairable items supporting a fleet of commercial aircraft operated by a transportation company in the Philippines. The items are characterised by infrequent demand, high cost and a hierarchical (or indentured) structure. The system has three re-supply sources of serviceable parts, namely, the in-house repair shop, the out-house repair shops, and the suppliers. Non-repairable items are scrapped and replaced with new items on a one-for-one basis. The model considers two levels of indenture represented by modules and components. The objective is to minimise the total expected steady-state annual cost of holding inventories and of aircraft delays. A minimum requirement on module availability is also applied. The formulation is such that the regular discrete optimisers available could not be readily used to solve it. The model is implemented on an illustrative problem, employing an integer search for the item stock levels within a limited range.