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.

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.

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 bought – out 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

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 bought – out 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.

Tolerance Relaxation at the Wetted Surface

Section 17.3 mentions that a best practice to reduce production costs is tolerance relaxation at the wetted aerodynamic surfaces, which contribute to an increase in parasitic drag. This section describes the important DFM/A consideration of toler­ance relaxation, which is a concern of aerodynamicists and structural designers.

Tolerance relaxation during component manufacture could incur problems of the tolerance-chain buildup at the assembly joint. All aspects of tolerance are beyond the scope of this book; only the tolerance allocation at the surface as the aerodynamic smoothness specification is discussed [5].

In current manufacturing philosophy, the main features contributing to excres­cence drag are as follows:

• manufacturing mismatches seen as aerodynamic defects (i. e., discrete rough­ness; e. g., steps, gaps, and waviness)

• surface contamination with fine particles and dirt adhering to it

• damage, wear, and tear during the life cycle

• fatigue deformation

• attachments of small items on the surface (e. g., blisters, antenna, pitot tubes, gaps/holes, and cooling air intakes/exhausts)

The first and last items are the consequences of design considerations; the remainder happens during operational usage. This chapter addresses only the first item, which gives rise to excrescence drag (i. e., parasitic drag). The nonmanufacturing origin of excrescence drag arising from the last item is treated separately for the Copmin estimation. To keep excrescence drag within limits, aerodynamicists specify aircraft smoothness requirements, which then are translated into tolerance allocations at the subassembly joints on the wetted surfaces. If the finish exceeds the tolerance limits, it must be reworked to bring it within the limits and/or obtain concessions to pass the product to the final line. Tolerance specifications affect aircraft manufacturing costs.

Aircraft wetted surfaces are primarily manufactured from sheet metals and composites. At the subassembly joints, there are some mismatches (e. g., steps, gaps, and waviness) that must be kept under strict control by specifying surface – smoothness requirements. Mismatches result in parasitic drag as an excrescence effect. Aerodynamicists specify aircraft surface-smoothness requirements to keep the drag increase within limits. The stricter is the tolerance, the more is the cost of production on account of rework or rejection. Any tolerance relaxation at the wetted surface reduces manufacturing costs at the expense of an aircraft parasitic drag increase, perceived as a “loss of quality function.” It is assumed that the sheet metal and composites at the surface accommodate a certain degree of tolerance relaxation. In addition, cosmetic appeal is perceived as a customer preference. Loss of some cosmetic quality can save on costs without unduly penalizing the parasitic drag. However, with increases in fuel price, aerodynamicists must be careful in spec­ifying surface-smoothness tolerances.

Figure 17.1. Cost-versus-tolerance relation­ship. Manufacturing cost reduces as tolerance is relaxed. Savings = amount reduced from the existing level to a lower level due to tol­erance relaxation

tight Tolerance relaxed

(current value)

17.6.1 Sources of Aircraft Surface Degeneration

In aircraft application, degeneration of the wetted surface area results from surface deviations from the specified level. It has many origins; the important ones are as follows:

1. Lifting Surface (e. g., wing, flaps, and empennage)

• control of LE profile and surface-panel profiles (i. e., aerofoil contour)

• rivet and fastener flushness for skin joints

• component geometry and subassembly joint mismatches

• fitment of access panels on the surface

2. Bodies of Revolution (e. g., fuselage and nacelle)

• control of nose profile and profile of the rest of the body joined in sections

• rivet/fastener flushness for skin joints

• component geometry and subassembly joint mismatches

• fitment of doors, windows, and access panels on the surface

17.6.2 Cost-versus-Tolerance Relationship

The relationship for establishing the manufacturing cost, C, at the assembly is derived by summing all costs involved, as shown:

manufacturing cost, C = (basic work time + rework time) x manhour cost + number of concessions x cost of concessions + nonrecurring costs + cost of support/ redeployment/management (17.1)

Changes in tolerance affect the rework time, number of concessions, and the cost of support. Tolerance relaxation reduces manufacturing costs because more com­ponents and their assemblies are made right the first time. Tolerance relaxation reaches a limit when any further relaxation has no significant benefit because all components and their assemblies require no rework and/or concessions for accep­tance – it is done right the first time. At the limit of relaxation, the cost of manufac­turing levels out to what is required for the “basic” work time and the NRC. Fig­ure 17.1 illustrates the nature of the cost-versus-tolerance relationship, a trend that is common to all features.

The X-axis represents the tolerance variation, from the existing level to the level where any further tolerance relaxation has no further benefit in cost reduction.

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.

Six Sigma Concept

The objective of robust design is to achieve product designs with few defects during manufacture and very few latent defects after a product is delivered to a customer. It focuses on identifying the characteristics of the product that are critical to meeting the product requirements and then seeking the DFSS process capability.

The DFSS 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 [2]. DFSS is a collection of product tools

Table 17.1. Sigma distribution of defects

Sigma distribution process capability

Defects per million











and topics used to assist the design of products for manufacture by processes oper­ating at the Six Sigma capability. It is a management-driven task to facilitate the improvement of labor efficiencies from employees and find new ways to improve on any routine approach so that the product can be manufactured at the highest quality and lowest cost, thereby satisfying all of a customer’s requirements. Six Sigma helps expose the “hidden factory” of waste that robs organizations of profits by using a routine approach to issues with the product and manufacturing process. The vision of Six Sigma is as follows:

• reduce costs and improve margins in a context of declining prices

• surpass customer expectations by a margin few competitors can match

• improve at a faster rate than the competition

• grow a new generation of leaders

Six Sigma is a systematic methodology for eliminating defects in products, services, and processes while also yielding cost and cycle-time reductions. By significantly improving process capability, it can achieve operational excellence in delivering almost defect-free products and services, at the lowest possible cost, and on time.

For manufactured products, the Six Sigma methodology makes use of a variety of managerial, technological, and statistical techniques to change the manufacturing processes, the product, or both in order to achieve the Six Sigma process capability. DFSS is the collection of tools and topics used during the design phase to achieve a Six Sigma product.

One measure of process capability is in the sigma, a, a statistical measure. For example, when a process is operating at Six Sigma capability, the long-term yield is 99.99966%, corresponding to 3.4 defects per million opportunities. The demand for Six Sigma is high, thereby guaranteeing a robust design. Table 17.1 lists the sigma distribution in a statistical histogram of defect levels relative to process capability.

To gain a competitive advantage through customers’ satisfaction, their needs must be understood. One way to capture customer requirements is by using selected quality function deployment (QFD), which consists of a series of interlocking matrixes that are used to translate customer requirements into product functional requirements and process characteristics.

However, development and implementation of DFSS is difficult. It requires employee behavior characteristics such as leadership, commitment, professional­ism, and perseverance to overcome the attitudes heard in phrases such as “no time,” “not invented here,” “doesn’t apply to us,” “we’ve been doing it for years,” “I prefer

design rules,” and “I refuse to use the tools.” DFSS demands a culture change – not easy to achieve but possible.

Manufacturing Practices

Depending on the manufacturing philosophy, jigs and fixtures need to be designed for the type of tooling envisaged for parts fabrication and assembly. Jigs and fixtures are special holding devices for making fast the workpiece for accurate fabrication and assembly of parts. Naturally, jig and fixture design starts early during Phase 2 of a project, along with planning for the facility and process layout. This can be expen­sive, requiring additional production-launch costs; however, there is a payback in saving labor costs when production starts. Investment in the aerospace industry is front-loaded.

Accurate dimensioning during fabrication and assembly is important for reduc­ing manufacture and maintenance costs. The following are used to maintain dimen­sional accuracy (these are not precise definitions but make sense in context):

Tools: This equipment cuts and shapes material in the parts fabrication pro­cess. They can be handheld or fixed in place. Examples include drills, lathes, hammers, riveters, and welders. Tools, jigs, and fixtures work in conjunction with one another.

Gauges: These are measuring devices for accurately locating tools relative to the fixture in which a workpiece is held.

Fixtures: These are special working and clamping devices that facilitate pro­cessing, fabrication, and assembly. Fixtures are fixed frames designed to hold one or several workpieces in the correct position relative to one another. A gauge may be required initially to position a tool for cutting. Fixtures can be large, depending on the size of the workpiece. They should be solid and heavy structures to withstand any vibrations.

Jigs: These have a similar function as fixtures but they also incorporate guides for the tool. Jigs also are fixed items. Jigs typically are used for drilling, ream­ing, and welding.

Given herein are seven of many best-practices techniques that contribute to DFM/A practices. The basic idea of the seven techniques uses a modern manufacturing and tooling philosophy, moving away from the older, manual procedures to digital pro­cessing (see Section 17.10), where most tasks are performed. Modern methods make extensive use of CAD, CAM, and computer-aided process planning (CAPP) to ensure a high standard of accuracy and productivity. Numerically controlled (NC) machines are part of CAM.

1. Jigless Assembly: Designing for ease of assembly should not be restricted exclu­sively to the task of concept-design engineers. Tooling engineers contribute to the reduction of costs through a jigless assembly approach to manufacturing. Jigless assembly is an approach toward reducing the costs and increasing the flexibility of tooling systems for manufacture through minimization of product – specific jigs, fixtures, and tooling. During the development phase, tooling costs are high; consequently, savings in this aspect of aircraft manufacture are signifi­cant and they impact the time from concept to market as well. Jigless assembly does not mean toolless assembly; rather, it means the eradication or at least the reduction of jigs. Simple fixtures still may be needed to hold the parts during specific operations, but other methods are being found to correctly locate parts relative to one another. Assembly techniques are simplified by using precision- positioned holes in panels and other parts of the structure to “self-locate” the panels; here, parts serve as jigs. This process, known as determinant assem­bly, uses part-to-part indexing rather than the conventional part-to-tool systems used in the past.

2. Flyaway Tooling: Within the airframe-manufacturing industry, it is generally accepted that approximately 10% of overall manufacturing costs for each air­frame can be attributed to the manufacture and maintenance of assembly jigs and fixtures. The traditional “hard-tooling” philosophy requires that the desired quality of the finished structure be built into the tooling. The tooling there­fore must be regularly calibrated to ensure build quality. An alternative phi­losophy, “flyaway tooling,” was conceived to reduce tooling costs and improve build quality. This approach envisions future airframe components designed with integral location features with incorporated positional data that transfer to the assembly. This enables in-process measurement and aids in-service repairs. It also may be possible to design an aerospace structure with sufficient inherent stiffness, allowing the assembly tooling to be reduced to a simple, reusable, and reconfigurable support structure.

3. Gaugeless Tooling: This is achieved using a theodolite system linked through a central processor. Coordinated geometry, obtained directly from CAD, is used to establish the “hard points” to meet the build, interface, and interchangeabil­ity requirements. Gaugeless tooling is required for the manufacture and peri­odic inspection of the assembly process.

4. Inline Assembly: This provides a progressive and balanced assembly build sequence, utilizing the maximum number of subassemblies in a cellular-type environment, which improves interchangeability.

5. Automatic Riveting: The assembly is first slave-riveted on the fixture and then moved to the automatic machine. This improves productivity and accuracy; hence, the quality impacts from human error are minimized. The manpower engaged is also reduced.

6. Tolerance Relaxation at the Wetted Surface [2]: Aircraft surface-smoothness requirements are aerodynamically driven with a stricter manufacturing toler­ance to minimize drag – that is, the tighter the tolerance, the higher is the assem­bly cost. Trade-off studies between surface tolerance and aerodynamic drag rise can reduce manufacturing costs (see Section 17.6).

7. Six Sigma and Supporting Methodologies: An important framework in which DFM/A techniques should be conducted is that of concurrent engineering (i. e., IPPD) focusing on improvement of the product-development process by con­centrating on the design stage for the entire life cycle of a product. Management strategies such as DFSS and LAM as well as effective personnel management also must be considered if improvements are to be made in assembly-system profitability. DFM/A should strengthen the team activity in all phases of the design process, thereby ensuring that the technical expertise of the participants is successfully utilized; this is a management tool.

Decisions made during product design have a major impact on cost, defects, and cycle time. In fact, about 70% of production cost is locked in during the design pro­cess. DFM/A helps reduce product complexity through minimization of parts and fastener counts, assembly and manufacturing time, and material costs. Additionally, DFM/A application reduces the potential for defects. Robust design, statistical tol – erancing, and geometric dimensioning and tolerancing actually help reduce defects. A better understanding of DFM/A in reducing the cost of production requires detailed studies in material selection and different process capabilities, which are beyond the scope of this book. The DFM/A concept assists the Six Sigma manage­ment strategy.

Design for Manufacture and Assembly

The public domain proliferates with acronyms, such as DFM and DFA. These do not comprise a standalone concept; there is a relationship between design for man­ufacturing (DFM) and design for assembly (DFA) to meet the objective of lower production costs. In this book, fabrication and assembly are two components of the manufacturing process and are combined as DFM/A. Chart 17.1 shows the typical steps in DFM/A application.

DFM/A is concerned with the design synthesis of parts fabrication and assembly as an integral part of manufacturability. DFM/A analyses involve competition and risk – that is, balancing the trade-off between cost and performance. This eventually

ensures affordability for operators as the best buy. This multidisciplinary study searches for aerodynamic mould lines with surface-smoothness requirements (i. e., tolerance specification) to minimize performance penalties without imposing dif­ficulties in manufacturability. The associated structural-design concepts facilitate parts fabrication and assembly (i. e., low manhours and low parts count, as well as enhanced interchangeability). Bought-out items are selected for efficient and cost- effective system integration leading to better reliability and maintainability during the aircraft’s operational lifespan. Based on an awareness of customer affordability and requirements, designing and manufacturing target costs are established, which measure the objectives of lower production costs, improved quality, and reduced manufacturing cycle times, while increasing the product value without sacrificing design integrity, safety, and established specifications.

As a complex product, an aircraft is constructed of myriad parts. Assembleabil – ity, as a measure of the relative ease of product assembly, plays a prominent role for produceability. Following are the main goals of DFM/A considerations, which reduce parts count and assembly time:

• improvement of the efficiency of individual parts fabrication

• improvement of the efficiency of assembly

• improvement of product quality

• improvement of the assembly-system profitability

• improvement of the working environment within the assembly system

• product’s usefulness in satisfying customer’s needs

• relative importance of the needs being satisfied

• availability of the product relative to when it is needed

• best cost of ownership to the customer

Aircraft Manufacturing Considerations

17.1 Overview

Cost analysis and manufacturing technology are subjects that require specialized instruction in academies, and they are not the main topics of this book. They are included to make readers aware that the classical aeronautical subjects of aero­dynamics, structures, and propulsion are not sufficient for a successful aircraft design. Cost analysis and manufacturing technology must be considered during the conceptual design study and integrated with classical aeronautical subjects. The following terms are used extensively in this chapter; some were referred to previously:

Design Built Team (DBT): This is a team of hand-picked, experienced engi­neers and specialists drawn from various related disciplines, who synthesize design for DFM/A considerations in multidisciplinary interactions with the classical subjects.

Design for Manufacture and Assembly (DFM/A): This is an engineering approach with the object of minimizing costs of production without sacrificing design integrity.

Integrated Product and Process Development (IPPD) (also known as Con­current Engineering): This offers an environment in which DBT uses IPPD to synthesize the trade-off studies in a multidisciplinary study to arrive at the best value for the product as a global optimum, rather than optimizing to a particular design study. DFM/A is part of IPPD.

Design for Six Sigma (DFSS): This is an integrated approach to design with the key issue of reducing the scope of mistakes and inefficiencies – that is, making a product right the first time to prevent the waste of company resources. It is a management-driven task to extract more from employees in order to find new ways to improve on routine approaches. In this way, the product is of the highest quality and lowest cost, satisfying all of a customer’s requirements.

Lean and Agile Manufacturing (LAM): This is a management tool to mini­mize costs by effective personnel management for improvements in the areas of assembly, system profitability, and the working environment.

Product Life-Cycle Management (PLM): This is a business strategy that helps companies share product data, apply common processes, and leverage cor­porate knowledge to develop products from conception to retirement, across the extended enterprise.

Manufacturing Process Management (MPM): This is a management strategy that provides a common environment for manufacturing, preplanning, and cost estimation, as well as detailed production planning, reconciliation analy­sis (i. e., estimate versus actual), and shop-floor work-instruction authoring.

Product, Process, and Resource (PPR): This is the hub environment, which provides a direct use of CAD-based data as a basis for work instructions. Emphasis is on use of single data set feeding all user systems.

Commercial aircraft design strategy is steadily evolving. It was initially driven by the classical aeronautical subjects, but recently it is customer-driven design strate­gies that consider DFM/A problems with the object of minimizing production costs without sacrificing design integrity and specifications. Manufacturing methodolo­gies, jigless assembly, and “flyaway” tooling concepts facilitate DFM/A. Designing for ease of assembly can be improved in the areas of assembly effectiveness and product quality.

Chapter 16 stresses the importance of rigorous costing as an integrated tool embedded in the multidisciplinary systems architecture of aircraft design to arrive at a best value. Cost estimation is used to trade-off studies between the classical aeronautical subjects and DFM/A methodology, with its guiding principles of parts count and manhour reduction, standardization of parts, and emphasis on designing for ease of assembly, which has wider implications for engineers and managers in the manufacturing industry. Whereas specialist groups concentrate on design for their task obligations – whether technology – or manufacture-driven or any other demand – the IPPD environment must synthesize the trade-off studies for the best value of a product as a global optimum rather than optimizing to a particular design study.

The paradigm of “better, faster [time], and cheaper to market” has replaced the old mantra of “higher, faster [speed], and farther” [6]. Aircraft manufactur­ers are meeting the challenges of this new paradigm by assessing how things are done, discarding old methods and working practices for newer, right-the-first-time alternatives. An increase in product value is achieved through improved perfor­mance (better), lower cost (cheaper), and in less time (faster). The paradigm shift from classical aeronautical studies led to new considerations for various types of design for… terms, more so in the academic circle (see Section 17.7). This chap­ter takes a holistic approach to aircraft design by consolidating various design for… considerations. The author suggests the introduction of an index of “Design for Customer” as a measure for establishing a product value.

The digital design and manufacturing process (see Section 17.9) leads to paper­less offices. The advent of the digital-manufacturing process greatly facilitated the DFM/A concept by addressing the role of MPM in the industry. MPM provides a common environment for manufacturing, preplanning, and cost estimation, as well as detailed production planning, reconciliation analysis (i. e., estimate versus actual), and shop-floor work-instruction authoring. It provides a means to integrate across the full product life cycle, ranging from concept to field maintenance to retirement (i. e., “cradle to grave”). Shop-floor execution systems are fed directly from the PPR – hub environment, providing a direct reuse of CAD-based data as a basis for work instructions. As-built data are captured and available for use within the PPR hub for follow-on planning and validation as the product evolves throughout its life cycle.

In some ways, automobile-manufacturing technology is ahead of the aerospace industry by successfully implementing digital-manufacturing technology and ad­vancing to futuristic visions. A successful automobile design can sell a million per year and last for a decade with minor modifications; whereas, in peacetime, fewer than 500 per year of a successful high-subsonic commercial transport aircraft are produced and none has yet reached the 10,000 mark in terms of total aircraft sales. The automobile industry can invest large sums in modern production methods yet keep amortization costs per car low.

17.1.1 What Is to Be Learned?

This chapter covers the following topics:

Section 17.2:

Manufacturing considerations

Section 17.3:


Section 17.4:

Manufacturing practices

Section 17.5:

DFSS concept

Section 17.6:

Tolerance relaxation

Section 17.7:

Reliability and maintainability

Section 17.8:

Designs for consideration: a holistic approach

Section 17.9:

Index of design for customer

Section 17.10:

Digital manufacturing

17.1.2 Coursework Content

Readers may compute the index of the design for customer. However, it is neither essential nor important because the industry is not adopting this system at this stage; more study is needed. However, the DFM/A considerations can be addressed in a second term. Such studies need not alter the finalized and substantiated configura­tion obtained thus far through the worked-out examples (in the industry, DFM/A is carried out in parallel during the conceptual design stage). It is beneficial to have an idea of DFM/A implications in aircraft design and operation. However, if it is a second-term topic, it may not be practical without specialist instructors using real­istic data. This chapter provides only a glimpse of the scope of DFM/A during the conceptual study phase.

17.2 Introduction

Today, it is not the operators who are the only customer. The future trends suggest the entire society as a customer of the high-tech aerospace engineering, which could “make or break” any society depending on how the technology is used. This also is true for other types of technology, including nuclear and bioengineering.

In the past, trade-off studies were limited to the interaction among aerody­namics, structures, and propulsion, as discussed through Chapter 13 of this book.

Subsequently, during the 1990s, the need for DFM/A considerations in an IPPD environment gained credence. The IPPD process continues to evolve for the customer-driven design trends in order to minimize ownership costs without sac­rificing integrity, performance, quality, reliability, safety, and maintainability. The recent economic downturn demands general and significant cost-cutting measures, severely affecting the commercial aircraft industry. In this economic climate, the roles of reliability, maintainability, recyclability, and so forth are design – and manufacturing-process-dependent. This chapter introduces an index of design for customer and incorporates value engineering.

The eventual affordability for operators as the “best buy” (i. e., product value), in turn, will allow manufacturers to thrive. Design considerations should not impose difficulties in their manufacturability. The associated aerodynamic-shape and structural-design concepts facilitate parts fabrication, their assembly, enhanced interchangibility, and so forth. Bought-out items should be selected for efficient and cost-effective system integration that leads to better reliability and maintainability during the aircraft lifespan. Recent events have resulted in the additional constraints of cost-effectiveness and environmental issues, requiring increased attention. The issues of global sustainable-development and anti-terrorism require additional design considerations. The choice of materials from a recycling (i. e., disposal) perspective is an additional issue when the use of composites gains ground over metals.

Worked-Out Example of DOC: Bizjet

Based on the previous formulations, this section works out the DOC of the Bizjet example. Rather than working on an entire fleet, only one aircraft is worked out herein. Input for the DOC calculation of the Bizjet is provided herein; manhour rates are in [3]. All cost figures are in U. S. dollars and are rounded up to the next higher figure. Table 16.14 provides aircraft and engine details.

The DOC is computed for a single aircraft to obtain the trip cost rather than the per-hour cost. A lifespan of 14 years is used with a residual value of 10% of the total investment.

Aircraft Price

• Total investment = $8 million

Utilization (per block hour per annum in hours/year)

• Utilization, U = ((5 з’™ 5) x 5.38 = 637.75 x 5.38 = 3,431 hours per year where t = block time for the mission = 5.38 hours

Fixed-Cost Elements

• Depreciation = О14х3х4306 x 5.38 = $807 per trip

• Loan interest repayment = 0 05| 4зі 106 x 5.38 = $665 per trip

• Insurance premium = 0.005 x [^x^! x 5.38] = $63 per trip

• Crew salary and cost (a) Flight crew

493 x 5.38 = $2, 652 per trip for two crew (b) Cabin crew = 0 because there is no cabin crew

Trip-Cost Elements

• Landing fees = (x 5-38) = $74 per trip

• Navigational charges = ^°1х2,о«ъ<о52^ x у94 x 5.33 = $803 per trip

• Ground-handling charges = (100<8’1) x 5.38 = $110per trip

• Airframe maintenance, material, and labor (a) airframe labor


= (°.°9 < 5 ■52 x 1 .°2 + 6 7 – (5.52 <102 + 75)

x 63 = (1.853) x 1.448 x 63 = $169 per trip

(b) airframe material cost = (4-2+2-2x5C38 °-25)) x 7 x 5.38 = $109 per trip where Cairframe = $7 million

Total airframe maintenance (material + labor) = 910 + 109 = $1,019 per trip

• Engine maintenance, material, and labor (a) engine labor = 0.21 x 63 x 1.018 x 0.89 x (1 + 1.72)0 4 = $17.88 per hour

where T = 17.23 KN = 1.72 tons C1 = 1.27 – 0.2 x 3.202 = 1.018

C3 = 0.032 x 10 + 0.57 = 0.89

(b) engine material cost = 2.56 x (1 + 1.72)0 8 x 1.018 x (0.652 + 0.89) = 5.7 x 1.542 = $8.79 per hour where C2 = 0.4 x (14/20)° + 0.4 = 0.652

Ci and C3 are the same as before.

(c) direct engine maintenance cost (labor + material)

= 2 x (17.88 + 8.79) x (g ^-q^) = $308 per trip

• Fuel charges = 2, 233 x 0.3245 x 0.75 = $544 per trip

The baseline Bizjet DOC is summarized in Table 16.15. Then, DOC per hour = 7,045/5.38 = $1,309.50 per hour.

DOC per aircraft nm = 7°°° = $3.52 per nm per trip

DOC per passenger mile per nm = 2 07°04×510 = $0.352 per nm per passenger

DOC details of a midsize, high-subsonic transport aircraft are in Appendix C.

OC of the Variants in the Family

Large Variant: MTOM = 10,800 kg and 14 passengers Aircraft Price = $9 million Ownership Cost Element = $1,727 Crew Cost = $3,047 Fuel Cost = $625

Maintenance/Operational Charges = $2,650 Total Operational Cost = $8,049 Then, DOC per hour = $8,049/5.38 = $1,496.40 per hour The DOC per aircraft nm = 2°°° = $4.025 per nm per trip

Table 16.15. Bizjet summary of DOC per trip (in U. S. dollars)

Fixed cost elements



Interest on Loan


Insurance Premium


Total Ownership Cost


Flight Crew


Cabin Crew


Total Fixed Cost Elements


Trip Cost Elements

Fuel Charges


Navigational Charges


Landing Charges


Ground-handling Charges


Maintenance (Airframe)


Maintenance (Engine)


Total Trip Cost Elements


Total DOC

$7,045 per trip

The DOC per passenger mile per nm = 2 0004×914 = $0.2875 per nm per passenger.

Smaller Variant: MTOM = 7,900 kg and six passengers Aircraft Price = $6.2 million Ownership Cost Element = $1,228 Crew Cost = $2,201 Fuel Cost = $451

Maintenance/Operational Charges = $1,916 Total Operational Cost = $5,796 Then, DOC per hour = $5,796/5.38 = $1,077.30 per hour.

The DOC per aircraft nm = |200 = $2.898 per nm per trip.

The DOC per passenger mile per nm = 2 00796 6 = $0.483 per nm per passenger.

Formulation to Estimate DOC

The DOC formulation is presented in this section, based on the AEA ground rules [1]. The formulae compute the component DOC per block hour. To obtain a trip cost, the DOC per block hour is multiplied by the block time. Aircraft performance calculates the block hour and block time for the mission range (see Section 13.5.6). The next section works out the DOC values, continuing with the Bizjet example used thus far.

Normally, the DOC is computed for a fleet of aircraft. The AEA suggests a ten – aircraft fleet with a 14-year lifespan and a residual value of 10% of the total invest­ment; these values can be changed, as shown in the next section. Fuel prices, insur­ance rates, salaries, and manhour rates vary with time. Engine-maintenance costs depend on the type of engine; here, only the turbofan type is discussed. For other types of power plants, readers may refer to [1].

Aircraft Price

Total Investment = (aircraft + engine price) x (1 + spares allowance fraction)

Readers must be sure to obtain the Standard Study Price from the manufacturer. The AEA uses the total investment, which includes the aircraft delivery price, cost of spares, any changes in the order, and other contractual financial obligations. In the example, the aircraft and engine price are taken as the total investment per aircraft.

Outstanding Capital = total capital cost x (1 – purchase down-payment fraction)

Utilization (per block hour per annum in hours/year)

TT. Tf 3,750

Utilization, U = —– — x t

(t + 0.5)

where t = block time for the mission.

For the flight crew, the AEA uses $493 per block hour for a two-crew operation. For the cabin crew, the AEA uses $81 per block hour for each crew member.

Trip-Cost Elements

• Landing fees = (7■8xMTOtWintons) where t = block time for the mission

• Navigational charges = (°-5xraJeinkm) x умтоуши™ where t = block time for the mission

• Ground-handling charges = (100xPaylotad in tons) where t = block time for the mission

The landing and navigational charges are MTOW-dependent and the ground­handling charges are payload-dependent. In practice, the crew salary is also MTOW – dependent but the AEA has kept it invariant.

• Airframe maintenance, material, and labor

(a) airframe labor

where Wairframe = the MEW less engine weight in tons

R = labor manhour rate of $63 per hour at the 1989 level t = block time for the mission

(b) airframe material cost

where Cairframe = price of aircraft less engine price in millions of dollars

• Engine maintenance, material, and labor

(a) engine labor

0.21 x R x C1 x C3 x (1 + T)0 4

where R = labor manhour rate of $63 per hour at the 1989 level T = sea-level static thrust in tons C = 1.27-0.2 x BPR02 where BPR = bypass ratio

C3 = 0.032 x nc + K where nc = number of compressor stages K = 0.50 for one shaft = 0.57 for two shafts = 0.64 for three shafts

(b) engine material cost

2.56 x (1 + T)0 8 x C1 x (C2 x C3)

where T = sea-level static thrust in tons

C2 = 0.4 x (OAPR/20)13 + 0.4

where OAPR is the overall pressure ratio; C1 and C3 are the same as before.

(c) direct engine maintenance cost (labor + material)

Ne x (engine labor cost + material cost

where Ne = number of engines

block fuel x fuel cost
block time

Table 16.14. Bizjet data for DOC estimation

Aircraft details

Turbofan details (two engines)

Conversion factors

MTOW – 9,400 kg

TSLs/Engine -17.23 kN

1 nm = 1.852 km

OEW – 5,800 kg

Dry Weight – 379 kg

U. S. gallon = 6.78 lb

MEW-5,519 kg

Bypass Ratio – 3.2

1 lb = 0.4535 kg

Payload -1,100 kg[29]

No. of Compressor Stages -10**

1 ft = 0.3048 m

Range – 2,000 nm Block Time – 5.38 hr Block Fuel – 2,233 kg

Overall Compressor Ratio -14

No. of Shafts = 2

Fuel Cost = $0.75 per U. S. gallon

1 kg fuel = 0.3245 gallon


* 10 passengers

** It has one high-pressure compressor, four-stage low-pressure compressor, and one fan. Aircraft price = $7 million Engine price = $1 million

Total aircraft acquisition cost = $8 million (total investment per aircraft;price includes spares)

Then, DOC per hour = (fixed charges + trip charges)per_hour and DOC per trip = t x (DOC)per_hour and DOC per aircraft mile = DOCx10Q”~t’i°cktime and DOC per pas­senger mile per nautica. l rnle = гаП^х^^І^^^ ■