# Development of a Maintenance and Spare Parts Distribution Model for Increasing Aircraft Efficiency
In manufacturing, maintenance costs can consume 2-10% of the company's revenue and may reach up to 24% in the transport industry [8]. Most users show concern over the competitive advantage that maintenance can provide to a company. To illustrate the economic importance of maintenance, a recent study of engineering maintenance practices shows that [2]:  
* United States airlines spend 9 billion dollars, approximately 11% of their operating costs, on maintenance;
* The military sector has even higher concern for the maintenance cost that accounts for about 30% of the life cycle cost of weapon system. In 1987/88 the Royal Air Force spent around 1.9 billion pounds on the maintenance of aircraft and equipment. Consequently, maintenance departments are one of the major cost centers, costing the industry billions each year. As such, they have become a critical aspect of the profitability equation for many companies' [6]. An example of the life cycle cost and maintenance in the aviation industry is as follows [6]:  
* The Boeing 777 Airplane has been designed for a useful life of 20 years. This involves defining what minimum scheduled and unscheduled maintenance must be performed in order to continue flying. Scheduled maintenance is performed at certain intervals based on the number of flight hours and, cycles. It consists primarily of inspections followed by maintenance, corrosion prevention, etc. Unscheduled maintenance is performed after a failure occurs. Depending on the nature of the failure, maintenance is accomplished either before the airplane is returned to revenue service or within a specified interval. * The relationship between how often the fleet of airplanes meet scheduled take-off-time versus cost reductions are key parameters for decision-making, knowing that the delivery reliability of take-off-time is 97.8% for initial delivery and 98.8% for fleet maturity. * Critical components of the airplane present double or triple redundancy. * When the total cost is considered over the life cycle, it is evident that the operating and support costs of the airplane will eventually exceed the initial acquisition cost. For Boeing to make the airplane attractive to airlines, engineers must include maintenance cost savings in the design. Increased reliability means fewer failures to repair and an increased maintainability means shorter maintenance times. In this context, design methods can lead to better maintainability, securing lower maintenance costs, better service to customers, and a lower total life cycle cost. The main practices of design to achieve good maintainability are accessibility, modularity, simplicity, standardization, fool-proofing, and inspectability [3]. Therefore, more companies and whole industries have been forced to increase their activities on research, development, and design. In the past, it was believed that every component of a complex system had an age at which a complete overhaul was needed to ensure safety and optimum operating conditions. This was the basis for scheduled maintenance programs. The limitations associated with this approach became clear when it was used to develop the preventive maintenance program for the Boeing 747 in the 1960s. The airlines knew that such a program would not be economically viable and initiated a major study to validate the failure characteristics of aircraft components. The study was concluded with the publication of the _Handbook for the Maintenance Evaluation and Program Development_ for the Boeing 747, and based on it, the United Airlines, sponsored by the US Department of Defense, developed a comprehensive document on the relationships between maintenance, reliability, and safety called _Reliability Centered Maintenance_ (RCM) [3]. Studies in the airline industry revealed that scheduled overhaul did not have much impact on the overall reliability of a complex item unless there was a dominant failure mode. Only 11% of the components exhibited a failure characteristic that justified a scheduled overhaul or replacement.