مقاله سال ۲۰۱۶

In transportation industry, defence sector, oil drilling, telecommunication and other industries, a reliable spare parts supply is a key element to provide service continuity. High inventory levels generally minimize the critical consequences of stock-outs, even in case of remote suppliers and long procurement times. However, a high inventory level generates significant stock cost. Determining the optimum inventory level for spare parts becomes a crucial strategic target for complex systems. In these cases, it is necessary to adopt the so-called system approach, which evolves the classic item approach, as prescribed by Sherbrooke’s METRIC (Sherbrooke, 1968). In details, the item approach aims to define an economic order quantity and period for each item, without considering possible interactions among them in terms of global availability. On the contrary, the system approach permits to define an availabilitycost function with inventory costs and required service levels for the entire system. Although it may indirectly offer measures for the supply system performance (e.g. fill rate and number of backorders), it proposes measures in accordance with the manager or the decision-maker perspective. For example the system approach answers questions such as “What are the costs to ensure a 95% global availability? How much money do we need to spend to have an enhancement in our global availability? What does the optimal system cost-effectiveness curve look like?”

The system approach uses availability and investment targets as inputs to the decision-making process. It presents an availabilitycost curve, in which the manager can easily choose the point that meets the availability constraints within budget limitations. Note that to obtain these outputs it is necessary to solve a series of item approach problems by efficient solving techniques that consider multiple conflicting goals. One of more of these features generally characterize a complex system for which it is generally fruitful to adopt a system approach: – Commonality: different systems may have some parts in common. The manager can decide to stock these parts separately for each system or in a shared inventory; – Service differentiation: systems may require differentiated availability levels, according the criticality levels of the sites; – Multi-transportation modes: it is possible to transport the items in different ways from central to local warehouses; – Multi-echelon structure: a service supply chain generally consists of central and local warehouse, where central warehouses * Corresponding author. replenish the local ones;Lateral transhipment: this aspect concerns the possibility that a local warehouse provides a spare part to another local warehouse that is out of stock. In this case it is necessary a jointly optimization of the warehouses.

In particular, the aeronautical industry is one of the sectors where many of these features characterize the systems. The International Air Transport Association (IATA)’s Maintenance Cost Task Force (IATA, 2011) shows that maintenance cost takes up about 13% of the total operating cost. While aircraft spare parts with very high price are generally not in stock (e.g. engines), and low-cost items are available in every location in a short time, stock levels for medium-range Line Replaceable Units (LRUs) require careful sizing. As a competitive advantage to decrease these inventory costs, some airlines aims to outsource the ownership of spare parts stocks, settling contracts which regulates the performance in terms of spare parts availability. The company that manages the spare parts inventory and maintenance is the maintenance supplier. This latter is commonly another airline that benefits of risk compensation, considering the high unlikelihood of simultaneous breakdowns. Spare parts become a variable cost for the customer airline and a business income for the maintenance supplier. More formally, in these cases, the supplier proposes a Performance Based Contract (PBC) and the airline that decides to submit it, becomes the customer airline. Besides the access to the supplier inventory, generally these contracts allow the customer to stock a reduced subset of spare parts in its warehouse, paying an additional tax to the supplier. Therefore, determining the items to stock and their stock levels acquires a fundamental role in all the supply activities. More generally, a Performance Based Contract is a results-oriented contract focusing on the outputs, quality or outcomes that may tie at least a portion of a contractor’s payment, contract extensions, or contract renewals to the achievement of specific, measurable performance standards and requirements. These contracts may include both monetary and non-monetary incentives and disincentives. What emerged from the literature review is a lack of any specific Performance Based Contract model and related solving procedure. To this extent, the contribution of the paper is to present a functional description, a mathematical modelling and an innovative two steps algorithm, based on METRIC, to fill this gap for a typical Performance Based Contract. The main role of the paper in this research field is to provide a first view on a two-party interaction, looking at the customer airline side. While the main literature on METRIC considers a multi-echelon structure of a single organization, where the optimization of the whole system depends on the complete knowledge of all the parameters, the model developed in this study supports managers to outsource the spare parts management processes, in order to reach a target availability while minimizing costs, under contractual constraints.