improve the components used in the systems, their R&M often remain at the same levels.

The design must guarantee integrity with significant time between failures that repairs can be made in a specific downtime period. An aircraft must have more TBO than the competition, which is linked to the system reliability as a function of the operational environment and length of operational time. Although the avionic and engine suites come with a well-studied R&M status, many other aircraft components (i.e., mainly structures with many built-in redundancies) have yet to evolve to address maintenance issues at the conceptual design stage. Almost all boughtout items and subsystems have reliability figures obtained from rigorous testing. An aircraft as a system maintains a systematic log, recording failures and defects so they can be followed up with modifications to make designs more robust, for those already built and those that are yet to be built.

17.8 Design Considerations

In the chapter overview (see Section 17.1), it is pointed out that the public-domain literature is replete with Design for . . . considerations, including Design for Manufacture, Design for Assembly, Design for Quality, Design for R&M, DFSS, Design for Recycling, Design for Antipollution, Design for Life Cycle, and Design for Cost, all heading toward a generic Design for X. These considerations led to the appearance of new considerations (sixteen listed in this section), with more from the academic circle. The fresh insights of academia may shed new light but may not be amenable to industrial implementation. Only recently have the drive for Design for R&M and DFSS become part of industrial practices and they are still evolving. The industry has yet to address decisively the other costs of LCC (e.g., training and evaluation, logistic supports, and special equipment) at the conceptual design stages of civil aircraft design in order to reduce the ownership costs of operators. Of the various Design for . . . considerations, only a third are applicable to DFM/A considerations. A robust cost model would support trade-off studies to arrive at the best value.

The new challenge for the industry is to examine all aspects of ownership costs at the conceptual design stages of a project. Performance evaluations based on setting individual goals of cost minimization at each design consideration may not result in the global minimum when strong interaction within the multidisciplines exists. In an IPPD design environment, the combined effort of various disciplines provides a better approach to make a product right the first time at a lower cost. The holistic approach suggests the role of cost modeling as a tool to address all considerations simultaneously; this facilitates performance-versus-cost trade-off studies in order to arrive at the most satisfying product line with the widest customer coverage. With this approach, the author introduces the term design for customer as a measuring index for “value for the money” defined in Section 17.9.

The sixteen design considerations appearing as Design for . . . terms are broadly classified in four categories with brief descriptions. They must provide designers with complete product information in the conceptual design stages based on their expertise and technology level. The purpose of this strategy is to make a product

yield the specific benefits of the lowest LCC (or in civil aviation applications DOC) in a unified manner, leading to the Design for Customer.

17.8.1 Category I: Technology-Driven Design Considerations

Design for Performance: Classical aircraft design entails aerodynamics, structures, propulsion, and systems to minimize fuel consumption. Aeronautical engineers strive to make an aircraft light, with low drag and matched engine, low sfc, and bought-out items (e.g., engine, avionics, and actuators) that offer the best value for the money. It is a proven technology for generic, subsonic, commercial aircraft design, with diminishing returns on investment to incorporate advancements.

Design for Safety: Crashworthiness, emergency exits, and so forth are also proven considerations.

Design for Component Commonality: The family concept of derivative aircraft design offers considerable benefits of cost reduction by maintaining several component commonalities within the variants. Derivative designs cover a wider market at a much lower unit cost beacuse amortization of NRC is distributed over larger numbers of units sold. Some of the variant aircraft designs may not be sized for the least fuel burned, but the lower unit cost offsets to a lower DOC. This consideration at the conceptual design stage is crucial to the success of the product range.

Design for Reliability and Maintenance: Currently, significant maintenance resources are planned after the design and then acquired to fit the requirements. This is due to the difficulty of translating statistical feedback from the operational arena, which can be quite abstract. Design attributes – which can make maintenance difficult by demanding additional time and training for highly skilled technicians – must have more detailed considerations to reduce maintenance costs. Cost trade-off studies with the attributes of reliability, repairability, and fault detection and isolation must be investigated more stringently at the conceptual design stage. Reliability issues are most important for improving the support environment – in generic terminology, this is a robust design.

Design for Ecology: Since the 1970s, environmental issues (e.g., antipollution) have been enforced through government legislation on noise and emissions at additional cost. The use of alternative fuels for sustainability is also an issue. The growing stringency of existing requirements as well as additional issues only increase the product cost. This is approaching a matured technology with diminishing returns on investment for improvement.

Design for Recycling: Aerospace technology cannot ignore the emphasis on recyclability, a concern that is gaining strength, as evidenced by the topical agenda of “sustainable development” in recent United Nations summit meetings. The design for stripping is an integral part of the Design for Recycling to minimize the costs of disassembly. New materials (i.e., composites and metals) result in additional disposal considerations. Cost trade-off studies on

LCC versus material selection for recycling may infringe on marginal gains in weight reduction or fabrication costs.

Design for Anti-terrorism: In the offing is a newer demand for Design for Anti-terrorism. In-flight safety features for protection against terrorist activities include an explosion-absorbing airframe and compartmentalization of the cabin for isolation, which incur additional cost.

17.8.2 Category II: Manufacture-Driven Design Considerations

Design for Manufacture: The trade-off study is concerned with the appropriate process required for parts fabrication, including cost-versus-material selection, process selection, the use of NC machines, parts commonality, and modularity considerations to facilitate assembly. A key issue in the conceptual design stage is a low parts count to reduce assembly time. The lowest parts count may not be the least expensive method – compromise may be necessary.

Design for Assembly: This is concerned with the fewest manhours required to assemble parts. Traditional practices in aircraft assembly include numerous components and a complex organizational structure in the engineering, logistics, and management disciplines. This results in an inefficient use of factory floorspace, and quality is compromised due to the unnecessary operations and fasteners required to join mating parts. DFA minimizes manufacturing costs by optimizing engineering methods using innovative best-practice techniques of jigs and tool design, whether a manual or computerized assembly method. Product configuration and the detailed design of parts are important in the assembly process.

Design for Quality: Adherence to the specification requirements is the essence of quality control. One example is meeting the aerodynamic surfacesmoothness requirements through surface-tolerance specifications at the component final assembly. Currently, many quality issues are addressed in the post-conceptual design stage; they should be advanced to the conceptual design stage.

17.8.3 Category III: Management-Driven Design Considerations

Design for Six Sigma: This is an integrated approach to design with the key issue of reducing the scope for mistakes and inefficiencies – that is, make a product right the first time to prevent the waste of company resources (see Section 17.5). A measure of its success is reflected in the final cost of a product; therefore, an estimation method indicates at what cost (i.e., at what efficiency) the Six Sigma approach is working.

Design for Cost/Design to Cost: This is the classical question of Design to Cost (DTC) or Design for Cost (DFC) or a combination of both. The tendency of management to emphasize DTC through a “lean” organizational setup may be counterproductive if it is carried to the extreme application.