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Article Excerpt
We used discrete-event simulation to model the maintenance of fighter aircraft and improve maintenance-related decision making within the Finnish Air Force. We implemented the simulation model as a stand-alone tool that maintenance designers could use independently. The model has helped the designers to study the impact of maintenance resources, policies, and operating conditions on aircraft availability. It has also enabled the Finnish Air Force to advance the operational capability of its aircraft fleet. We designed the model to simulate both normal and conflict operating conditions. The main challenge of the project was the scarcity and confidentiality of data about the fighter aircraft, their maintenance, and various operational scenarios, especially during conflict situations.
Key words: simulation: applications; military: defense systems; reliability: availability, maintenance/repairs. History: This paper was refereed. Published online in Articles in Advance June 4, 2008.
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A fighter aircraft typically requires several hours of maintenance per hour of flight activity. This maintenance involves a diversity of operating policies, processes, people, and materials. In a fleet of fighter aircraft, these elements form a complex maintenance system. The system performance directly affects aircraft availability, i.e., the number of aircraft that can be used in flight missions. Ability to assess how maintenance-related decisions or operating conditions affect the system is critical in maintaining the fleet's operational capability.
We used discrete-event simulation, which has been widely applied in studying manufacturing systems (Law and Kelton 2000), to model the maintenance of the fighter aircraft fleet in the Finnish Air Force (FiAF). It lends itself to analyzing a maintenance system because manufacturing and maintenance share common features, such as workforce considerations, tasks times, material-handling delays, and equipment reliability. We also found simulation to be a suitable method for modeling the FiAF maintenance system because it enabled us to study the system from many aspects that the FiAF maintenance designers were likely to consider. The model describes the essential features of flight operations and maintenance including planned and unplanned maintenance, air bases, aircraft repair shops, and maintenance personnel. Moreover, it describes both normal and conflict operating conditions.
Some earlier studies on military operations also applied simulation to consider the effects of reliability and maintainability on aircraft operational capability. For example, Balaban et al. (2000) and Ciarallo et al. (2005) developed simulation models for availability of cargo and mobility aircraft, respectively. Upadhya and Srinivasan (2004) built a simulation model for availability of generic aircraft and helicopters in combat operations. Rodrigues et al. (2000) used a simulation model to assess the spare-parts management for A-4 aircraft. Kang et al. (1998) considered two simulation models for managing spare-parts and component repairs. In a recent paper, Kladitis et al. (2007) used simulation to analyze the impact of a new avionics system on the availability of B-52H bombers. However, these models either considered different types of flight operations than our model did or considered a more narrowly defined problem; they did not take a system-wide view of maintenance. Pohl (1991) used operational test data to devise a simulation model for operations of the F-15E aircraft. The model described maintenance in much the same way as ours did but limited the discussion to consideration of a fixed-size squadron in a single air base. To the best of the authors' knowledge, no previous simulation models in the open literature have considered the maintenance of a fleet of fighter aircraft at the depth of the model that we present in this paper.
Our primary challenges in constructing the model were scarcity and confidentiality of data. In particular, no data were available for modeling certain elements of conflict conditions such as battle-damage probabilities or repair-time distributions. Some confidential data, which FiAF could not share with the authors, included parts of the contingency plans on conflict-time maintenance policies. We found two approaches useful in overcoming these challenges. First, in situations where data were unavailable, we asked subject matter experts from different organizational levels to provide their opinions. Second, we designed the model such that the confidential information was included in the input data; the maintenance designers who performed the corresponding simulation analysis could thus handle the confidential data independently. Implementing the model as a stand-alone tool with a graphical user interface (GUI) facilitated our second approach because it made the model approachable to the designers. The scarcity of data also affected the validation of the model. We were able to perform limited comparisons between the simulation output and actual performance data from the maintenance system. Therefore, we used subject matter experts on multiple occasions to assess the underlying assumptions as well as the model output.
We introduced the model in the FiAF units that perform aircraft maintenance; it has enabled these units to address many maintenance-related issues. Examples include the forecasting of aircraft availability, the analysis of the resource requirements for international operations, and the feasibility study of a readjusted periodic maintenance program. The project has also provided FiAF with new knowledge about possible applications of simulation techniques. For example, the Finnish Army subsequently devised a simulation model for the maintenance system of newly acquired transport helicopters with collaboration from FiAF.
FiAF Aircraft Maintenance
The FiAF aircraft fleet consists of Boeing F-18 Hornet fighters, BAe Hawk Mk51 jet trainers, and other types of aircraft used in transportation, air surveillance, flight training, and liaison duty. We considered the flight operations and maintenance of the F-18 Hornet aircraft during normal and conflict conditions ("conflict" refers to a situation in which the aircraft fleet is involved in an actual engagement with an enemy). However, because detailed Hornet information is classified, we discuss Hawk maintenance in this paper. At the modeling level, we found that the maintenance principles and the appearance mechanisms of unexpected failures are very similar; in general, they differ only in model parameters. Hence, the principles we report here apply to the Hornet as well.
The FiAF aircraft fleet has three primary operational units that are called air commands (Figure 1). Within each air command, a fighter squadron is responsible for aircraft flight operations and specific maintenance activities. Each air command also has a separate repair shop for more complex maintenance tasks. Depot-level maintenance units of the national aerospace defense industry perform the most demanding maintenance. The organization that Figure I shows remains essentially the same during both normal and conflict conditions, although the decentralization of the units during a conflict may change their geographic locations.
[FIGURE 1 OMITTED]
Normal Conditions
During peacetime, the activities of an air command are centralized at a single air base. The general goals of aircraft maintenance are to assure that sufficient numbers of aircraft are available for training and possible reconnaissance flight operations at all times, and to preserve the long-term operating condition of the entire fleet. An air command should also be able to raise the level of preparedness when necessary.
Daily aircraft maintenance consists of flight-mission-related inspections. The aircraft that perform flight missions undergo a preflight inspection before the first mission, whereas a turnaround inspection is performed after each mission. In these inspections, the aircraft are checked according to given specifications and the necessary replenishments are made.
The aircraft periodically undergo more elaborate maintenance. The frequency of periodic maintenance is based on accumulated flight hours. The maintenance intervals as well as the number and contents of periodic maintenance types depend on the type of aircraft. The Hawk undergoes six different types of periodic maintenance that are referred to as type I, II, ..., VI maintenance. Unplanned maintenance is performed in case of a failure. Some failures are noncritical--the aircraft are repaired only during the next periodic maintenance; however, some failures must be addressed immediately. A repair typically involves diagnosing the defect cause and repairing or replacing the failed component.
The above activities are categorized into different maintenance levels and...
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