Optimization of aircraft hangar maintenance planning under MRO outsourcing mode

Abstract

Aircraft Maintenance, Repair and Overhaul (MRO) activities are critical for the aviation industry. The maintenance checks for aircraft must be conducted periodically to ensure its operational safety and reliability. From the perspective of airlines, the cost of MRO activities, especially hangar maintenance checks, takes a large proportion of their annual operating costs, as carrying out hangar maintenances for aircraft is demanding due to the increasing technical sophistication of the maintenances and the inventory cost on the relevant equipment. In this regard, a trend of outsourcing hangar maintenance activities to an aircraft maintenance service provider has emerged and becomes an attractive operation strategy for airlines, which ensures that the MRO operations continue to meet the safety requirements while reducing the maintenance costs. With the rapid adoption of MRO outsourcing mode among airlines, maintenance service providers receive increasing maintenance demands from multiple clients, and fulfilling those outsourcing demands with the resource constraints becomes challenging. In this connection, enhancing the utilization of maintenance resources and developing an effective maintenance planning optimization approach under the MRO outsourcing mode are significant for the maintenance service provider to handle the fast-growing maintenance demands. The objective of this study is to analyse the maintenance planning problem arising from the aircraft hangar maintenance service providers under outsourcing mode then develop efficient optimization approach to tackle the proposed problem. Though aircraft maintenance scheduling and planning problems have received much attentions in the literature, the geometric factor is overlooked while such aspect creates a bottleneck in carrying out the hangar maintenance plan under the outsourcing mode. The maintenance planning problem under such context has to incorporate the variation of the hangar capacity along the planning horizon as well as the impacts of hangar parking layout planning. Specifically, though the dimensions of maintenance hangar are constant, the capacity of the hangar (the number of aircraft that hangar can accommodate) varies according to the physical configurations of respective aircraft along the planning period, while such factor receives little attention in conventional aircraft maintenance scheduling, e.g. aircraft hangar maintenance operated out by airline itself. Therefore, it motivates to analyse the hangar capacity measurement from a spatial aspect under the MRO outsourcing mode. Besides, the additional geometric and manpower resource constraints, including the aircraft's path blocking during the movement operations in the hangar and the multi-skill maintenacne technicain, shall be concurrently incorporated under the outsourcing mode. The outcomes of the integrated solution shall attain trade-offs among aircraft's maintenance scheduling, hangar parking layout planning/movement planning and multi-skill manpower assignment. The interdependent relationships among scheduling, geometric planning and manpower resources shall be carefully analysed and modelled from the MRO outsourcing aspect, which is a pioneer in modeling and optimizing aircraft maintenance scheduling problem. This research mainly focuses on developing a systematic optimization methodology for the aircraft hangar maintenance planning problem, which fills the research gaps mentioned above. The structure of the methodology is mainly divided as four parts as follows: (i) The first part analyses the impact of aircraft parking stand allocation from a two-dimensional space, which is a critical research element in carrying out aircraft hangar maintenance planning. A mathematical formulation characterizing the variation of hangar capacity in two-dimensional space is developed to cater the aircraft in different sizes to be serviced. A proposed MILP-based heuristic algorithm in carrying high quality layout within a reasonable computation time for practical usage is developed for the static parking stand allocation model; (ii) The second part further analyse the hangar space utilization problem. A heuristic algorithm is developed to provide practical solutions, and the intermediate infeasible solutions identified during searching are utilized to develop valid and approximate inequalities, tightening the optimality gap; (iii) With the foundation of geometric analysis on hangar space utilization in the first and second parts, the third parts incorporate the aircraft movement operations and its blocking impact in aircraft maintenance scheduling, which extends the static aircraft parking stand allocation model in the first part. The original model is enhanced by developing an event-based discrete time model to narrow down the domain of the time-related decision variables. In addition, a rolling horizon approach incorporating the enhanced mathematical model is presented to obtain good quality feasible solutions for large scale instances; (iv) The last part incorporates the multi-skill manpower planning, which is identified as another critical resource in aircraft hangar maintenance, to fulfil the integrated maintenance planning model. A two-stage optimization approach is developed to enhance the computational efficiency by decomposing the integrated model, and the two-stage approach is coordinated by the linkage constrains between geometric and numeric decision-making scattering in the decomposed subproblems. Moreover, systematic computational studies are deployed in respective parts of methodology, using on the problem instances generated based on the data collected from an aircraft hangar maintenance company. The managerial insights on carrying out hangar maintenance planning as well as the analysis on computational efficiency are given for both practical and theoretical uses.