## 2 Basic Theory  
### Aircraft Fleet Maintenance Cost Analysis Related to Product Option Selection  
The economy of civil aircraft occupies a decisive role to measure whether aviation corporations or manufacturers have market competitiveness. Only when a model of aircraft masters the ability of continuing airworthiness and, meanwhile, economic benefits can it take the market share sustainably, especially in the field of civil aviation. The cost of a civil aircraft to perform a flight task generally includes fuel, staff salaries, ownership cost, government taxes and fees, maintenance costs and so on. The economy considered in this article mainly refers to the cost related to the performance of products or aircraft, that is, aircraft fleet maintenance cost (AFMC). It is related to the performance of the product itself, unlike taxes and wages, which have nothing to do with selection optimization. The AFMC should be considered during the conceptual design phase by optimizing and evaluating related parameters. The option selection of aircraft is oriented at the parts or product level, which can be replaceable or maintainable on the line, such as electronic equipment, fitting, and substructure component. This class is generally called Line Replaceable Unit (LRU)and Line Maintainable Part (LMP). They are the smallest units to perform a maintenance task. Historical operation data, demand analysis and framework design provide a base for it; this, in turn will allow for a decomposing reliability index as the constraints and select vital design parameters, mean time between failures (MTBF), during the conceptual design phase. Large complex systems, containing thousands of products (whether installed or backed up) experiencing phases of design, manufacturing, sales, using, maintenance, and scrapping, face great challenges in operated strategy and cost. A certain civil aircraft recommended spare parts list (RSPL) disposes up to 757 kinds of Line Replaceable Units and 789 types of Line Maintainable Parts. While the outputs of option selection can assess products' reliability levels, they will also directly change the maintainability. The AFMC is directly related to the performance of products, and it can be considered an objective function of optimization. In this paper, matching the fault causes with different maintenance tasks corresponding to different maintenance costs thus establishes the equations relating reliability for a particular system and aircraft fleet maintenance cost by maintainability as a link. Generally speaking, maintenance tasks are mainly divided into five categories: lubrication/service, operation/visual inspection, inspection/function check, and recovery and scrapping. The first four categories can be collectively called'repair'. This kind of repair task does not require a material cost, but only needs the labor cost. The total cost of carrying out these five maintenance tasks is the AFMC. These can also distinguish between being planned and unplanned by the frequency of occurrence besides categories. The type and frequency of tasks is particularly true for AFMC. Easily speaking, the more reliable the product is, the more complex its manufacturing process is and the more manpower is needed to maintain it; planned maintenance tasks based on product failure characteristics will also increase, resulting in an increase in planned maintenance costs (PMC). However, if we choose products that are relatively unreliable, it may cause sudden or even frequent failures. The resulting costs are unplanned maintenance cost (UPMC). The PMC is directly proportional to the MTBF, and the UPMC is inversely proportional to the MTBF. The mathematical model uses the ratio of the MTBF before and after optimization to quantify the impact on the AFMC in this paper.