17.7 Reliability and Maintainability

The Y-axis represents the cost of manufacture, from the existing tolerance level, with current manufacturing costs representing zero savings. Tolerance relaxation results in cost reductions up to the maximum possible level (100%).

Summing tolerance relaxation over an entire aircraft can reduce the manufacturing cost by a small percentage while incurring an increase in excrescence (parasitic) drag. The aircraft DOC reflects the change in cost reduction and drag increase when a trade-off study is conducted. Figure 16.4 shows trends in the tradeoff between cost and tolerance. If the initial tolerance is too strict, the relaxation shows a reduction up to a point at which thereafter the DOC increases as a result of the additional fuel burn due to the drag rise, whereas the aircraft price reduction has leveled out.

Reference [1] is a study of the trade-offs, describing how a midsized jet aircraft can average about 33% tolerance relaxation with a corresponding net savings in DOC of 0.42%. The conservative estimation given herein is a typical aircraft cost reduction through DFM/A studies (a fuel price of $0.75/U.S. gallon is used):

an approximate 1.28% DOC savings due to 2% aircraft costs saving through DFM/A studies involving no drag increase

an approximately 0.42% DOC savings due to 1% aircraft costs saving through tolerance relaxation involving drag increase

This study demonstrates a total of 1.7% DOC savings, which translates into a savings of $530 per sortie for a 150-passenger/3,000-nm range aircraft class. With an annual utilization of 500 sorties, the total is $26,500 per aircraft. For a fleet of 10 aircraft, the savings total $26.5 million in 10 years. For smaller aircraft, the percentage savings is even higher.

This is a good example of how aerodynamic, structure, and manufacturing considerations are needed to conceive designs that result in reduced DOC. Manufacturing cost reductions can be achieved through many other efforts, which is the aim of the DFSS concept. During the trade-off studies of various design parameters, the benefit of cost-estimating activities helps designers investigate and adopt new technologies to advance a product to a competitive edge and generate specification requirements (e.g., tolerance allocations). Designers also analyze the risks involved, balancing the trade-off between cost and performance that eventually leads to affordability for operators as the best buy (i.e., product value), which in turn enables manufacturers to thrive.

17.7 Reliability and Maintainability

Poor reliability is unacceptable. An aircraft as a system must achieve a user’s confidence that it will work as and when required. This entails a multidisciplinary study for an efficient and cost-effective system integration leading to better reliability and maintainability (R&M) during the operational lifespan. In the current economic climate, the role of reliability, maintainability, and recyclability must be scrutinized for cost control – not only the in-house product line but also the supply chain of boughtout items. Even those systems that are perceived as reliable are only reliable due to the significant redundancy built into the system or the vast amount of corrective maintenance that keeps a system running. Despite immense efforts to predict and