Research Article A Reliability-Based Approach to Nonrepairable Spare Part Forecasting in Aircraft Maintenance System Nataša Z. Kontrec, 1 Gradimir V. MilovanoviT, 2,3 Stefan R. PaniT, 1 and Hranislav MiloševiT 1 1 Faculty of Sciences and Mathematics, University of Priˇstina, Lole Ribara 29, 38220 Kosovska Mitrovica, Serbia 2 Serbian Academy of Sciences and Arts, Kneza Mihaila 35, 11000 Beograd, Serbia 3 State University of Novi Pazar, Vuka Karadˇ zi´ca bb, 36300 Novi Pazar, Serbia Correspondence should be addressed to Nataˇsa Z. Kontrec; natasa radenkovic@yahoo.com Received 31 December 2014; Revised 7 April 2015; Accepted 8 April 2015 Academic Editor: Emiliano Mucchi Copyright © 2015 Nataˇsa Z. Kontrec et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In recent times, spare parts inventory systems have been extensively researched, but most inventory models were not fully adequate due to the stochastic nature of inventory environment. is paper proposes an approach that supports decision making process in planning and controlling of spare parts in aircraſt maintenance systems. Reliability characteristics of aircraſt consumable parts were analyzed in order to substantiate this approach. Moreover, the proposed reliability model was used to evaluate characteristics of subassemblies and/or assemblies these parts belong to. Finally, an innovative approach for determining the total amount of parts required in inventory and underage costs, based on observing the total unit time as a stochastic process, is presented herein. 1. Introduction Maintenance techniques have changed over time from cor- rection (breakdown) to prevention by prediction and proac- tive continuous improvement [1]. eir main goal is to ensure that the system performs its function as intended. In order to achieve that, the efficient spare parts management is required. e lack of spare parts when they are needed leads to unexpected downtimes and irreparable loss for a company. e aircraſt are modern high-tech means of transport. ey consist of a large number of components and parts. Each component is exposed to varying degrees of stress, with its own maintenance plan. Downtimes and costs can be reduced if a failure occurrence is forecasted and maintenance planned accordingly. If a spare part needed for replacement is unavailable, additional costs will occur (urgent orders, passenger accommodation costs in case of flight cancellation, etc.). Some consequences such as damage to an aviation company’s reputation due to downtimes are difficult to quantify. is is why the problem of spare parts forecasting in aviation still persists. e main problem is to retain a required level of aircraſt availability, that is, to provide the appropriate spare part at the right time and in the right place. e interest in inventory management has been increased in recent times but as stated in [2] the majority of airline operators and maintenance service companies felt that spare parts forecasting methods are not providing them with realistic results in practice so they tried to outguess the forecast. Uncertainty of events, failure of equipment, and aircraſt components have significant effect on their maintenance. ese failures are random processes. It is not always pos- sible to predict their occurrence. In order to minimize the aircraſt downtime and increase availability, one approach is to maintain the appropriate level of spare parts. If we keep more spare parts than required, unnecessary warehousing costs will occur. In the opposite case, if their level is less than required, certain delay costs will occur. erefore, the goal is to maintain the level of spare parts which provides minimum warehousing and delay costs on one side, while guaranteeing a high level of availability of spare parts on the other. ere are numerous research studies related to determin- ing the required quantities of spare parts with the purpose of reducing delays and lead time. Croston [3] was one of Hindawi Publishing Corporation Mathematical Problems in Engineering Volume 2015, Article ID 731437, 7 pages http://dx.doi.org/10.1155/2015/731437