Commercial Gas Turbine Engine Platform Strategy and Design by
Habs M. Moy
M.S. Aerospace Engineering, University of Cincinnati, 1991 B.En. Mechanical Engineering, Cooper Union, 1989
Submitted to the System Design & Management Program In Partial Fulfillment of the Requirements for the Degree of
Master of Science In Engineering & Management
at the ASSACHUSETTS INSTITtE OF TECHNOLOGY Massachusetts Institute of Technology
February 2000 LIBRARIES @ 2000 Habs M. Moy, All Rights Reserved The author hereby grants to MIT permission to reproduce and to distribute publicly and electronic copies of this thesis document in whole or in part.
Signature of Author_ I Habs M. Moy System Design & /nagementProgram January 14, 2000
Certified by 1. ff - 1 ' Kevin N. Otto Robert N. Noyce Associate Professor of Mechanical Engineering Product Portfolio Definition Thrust Leader, Center for Innovation in Product Development Thesis Supervisor
Accepted bv______-______Thomas A. Kochan LFM/SDM Co-Director George M. Bunker Professor of Management
Accepted by rau P. Lagace LFM/SDM Co-Director Professor of Aeronautics & Astronautics and Engineering Systems 2
Commercial Gas Turbine Engine Platform Strategy and Design
by
Habs M. Moy
Submitted to the System Design & Management Program on January 14, 2000 in Partial Fulfillment of the Requirements for the Degree of Master of Science in Engineering & Management
Abstract
Product development challenges companies to produce designs that meet customer requirements yet, that are within their technological and financial means to do so. The proliferation of customized or unique designs may tax the resources of a firm if product variety cannot be achieved in a cost-effective manner. A product platform strategy allows a set of core elements or subsystems to be shared across all or part of a company's product portfolio, while design flexibility allows differentiated functions to satisfy specific customer needs. A framework for identifying potential platform elements from among key system design variables is provided. This framework supports the hypothesis that system design variables with low normalized coupling and low normalized variation across a set of conceptual product designs should be considered as potential platform elements. A system level approach for identifying the coupling and variation of these elements is facilitated through the formulation and use of a modified quality function deployment (QFD) mapping procedure. Normalized coupling is quantified as the relative importance of relationships between stakeholder needs, system requirements and system design variables, divided by a ranking of the difficulty in their achievement. Normalized variation of system design variables from a sample of parameter data is calculated as the standard deviation divided by the mean. The proposed framework and hypothesis is validated with a case study of the Pratt & Whitney PW4000 family of commercial gas turbine engines where predicted platform elements were consistent with actual design choices.
Thesis Advisor: Kevin N. Otto Title: Associate Professor of Mechanical Engineering 3
Acknowledgments
I cannot begin to acknowledge all the people who have made this work possible. It certainly includes all the people and events that have shaped the last two years in the System Design & Management Program, but I would be remiss if I did not also acknowledge those who have been a constant anchor in my personal life.
Thanks go to UTC/Pratt & Whitney for sponsoring me to this program and to the following individuals who supported the work that went into this research: Franklin Gattis, Karl Hasel, Jeffrey Hathaway, Billie Jones, Craig Lewis, Kent Lyons, Walter Malkauskas, Ben Mancuso, James Panaia, Joe Presing, Thomas Rogers, Robert Saia, Austin Smith, Paul Smith, Reid Smith, Yasar Tanrikut, William Taylor, George Titterton and Barry Wood. Special thanks go to George Aronstamm who spent many a late afternoon passing on the gas turbine engine knowledge he has accumulated with over 30+ years of service at Pratt & Whitney. Thanks also go to Michael Chemerynski and Frank Gass for supporting me through two years of two shift workdays.
Thanks go to MIT and the Center for Innovation in Product Development for fostering research that is pertinent for today's industry. Special thanks go to my advisor, Kevin Otto, for his guidance and vibrant attitude towards this effort, and to Javier Gonzalez-Zugasti for all the philosophical discussions on platforms that gave me the perspective with which to look at gas turbine engines. Best wishes to Javier for a successful doctoral defense!
Perhaps the richest part of this entire learning experience was working with and learning from my fellow colleagues in the SDM program. Learning from all of you first hand about the inner workings of the various industry leading companies you represent, is far more valuable than reading about it in a case study or business journal. Thanks to all of you for making this experience come alive. Best wishes for your continued success.
I want to thank my family for their continuous support. Thanks to Yvette, Evelyn and Yvonne for taking care of things on the home front while I was occupied. I want to thank my significant other, Ying, for her support, patience and heartwarming smile through two years of distance relationship held concurrently with two years of SDM distance learning. We are finally at the end of this part of the journey. The next part is about to begin.
My most heartfelt acknowledgments go to my parents for their investments in me since the day I was born. Those investments of care, guidance and support have paid back handsomely with 3 university degrees ... all tuition free. How about that for a measure of return on investment? Thanks Mom and Dad. This third one was a charm. 4
Table of Contents
1 Introduction...... 8 2 Related W ork...... 12 2.1 Product PortfolioArchitecture...... 12 2.2 Examples of Product Platforms ...... 14 2.3 Product Architecture Concepts ...... 15 3 System Architecture of a Commercial Gas Turbine Engine...... 17 3.1 Airplane System and Engine Subsystem ...... 17 3.2 Modularity and Integrality ...... 18 3.3 Mechanical & Aerothermodynamic Coupling ...... 20 4 Q uality Function Deploym ent (QFD)...... 23 4.1 What is QFD? ...... 23 4.2 Applying QFD to Identify Platform Elements ...... 25 4.3 Elem ents of the Platform QFD ...... 28 4.3.1 Stakeholders and their Needs ...... 28 4.3.1.1 Airplane Mission ...... 29 4.3.1.2 Reliability ...... 30 4.3.1.3 Cash Operating Cost...... 30 4.3.1.4 Environmental ...... 31 4.3.1.5 Recurring Cost (Manufacturing)...... 31 4.3.1.6 Non-Recurring Cost Spent to Launch (Technology)...... 31 4.3.1.7 Non-Recurring Cost Spent from Launch to Certification (E&D)...... 31 4.3.1.8 Entry into Service (EIS) Date ...... 32 4.3.2 System Requirements ...... 33 4.3.2.1 Airplane Integration ...... 34 4.3.2.2 Performance...... 34 4.3.2.3 Reliability...... 35 4.3.2.4 Environmental ...... 36 4.3.2.5 Cost ...... 37 4.3.2.6 Design ...... 37 4.3.2.7 In-Service Operations...... 37 4.3.3 System Variables at the Module Level ...... 38 4.3.4 Module Flowpath Aerothermodynamic Variables...... 40 4.4 QFD Platform Mapping ...... 42 4.4.1 Mapping Stakeholder Needs to System Requirements ...... 42 4.4.1.1 Relative Importance of System Requirements...... 44 4.4.1.2 Conflicts Between and Among System Requirements ...... 44 4.4.1.3 Stakeholder Needs for Different Market Segments ...... 47 4.4.1.4 System Requirement Difficulty and Core Competencies...... 48 4.4.1.5 Deriving the Normalized Coupling Measure ...... 48 4.4.2 Mapping System Requirements to System Variables ...... 50 4.4.3 Mapping System Variables to Module Flowpath Aerothermodynamic Variables.....51 4.4.3.1 Relative Importance of Module Flowpath Aerothermodynamic Variables ...... 53 4.4.3.2 Relative Module Ranking ...... 54 4.4.3.3 Module Development Difficulty Ranking ...... 54 5
4.4.4 Normalized Variation ...... 55 4.4.5 Defining Boundaries for High and Low Normalized Coupling and Variation.....55 5 Pratt & W hitney Case Studies ...... 57 5.1 Sample of 8 Engines...... 58 5.2 PW4000 Engine Fam ily...... 62 5.2.1 Background...... 62 5.2.2 PW 4000-94" Platform Strategy...... 63 5.2.3 PW 4000-1 00" and PW 4000-112" Growth Strategy...... 65 5.2.4 Validating the Hypothesis with the PW 4000...... 66 6 Sum m ary and Conclusions ...... 74 7 Recom m endations ...... 76 7.1 Conceptual Design Tool...... 76 7.2 Extensions of QFD Mapping ...... 78 7.2.1 Mapping to Support Structure Part Characteristics...... 79 7.2.2 Mapping to Key Process Operations...... 80 7.3 Other Applications...... 80 7.3.1 Value Engineering ...... 80 7.3.2 Military, Small Commercial and Industrial Engines ...... 81 7.4 Multi-Project Management as a Portfolio Planning Strategy ...... 81 7.4.1 Push versus Pull Market...... 82 7.4.2 Product Lifetime & Certification Costs...... 83 7.4.3 Production Volume ...... 83 7.4.4 Level of Technology Capability...... 84 7.5 Strategic Analysis...... 84 7.5.1 Core Competencies & the Organization...... 84 7.5.2 Porter's Five Forces Model...... 85 7.5.2.1 Customers ...... 86 7.5.2.2 Suppliers ...... 87 7.5.2.3 Competitors...... 87 7.5.2.4 Substitutes...... 88 7.5.2.5 Barriers to Entry ...... 89 References ...... 92 G lossary ...... 95 6
List of Figures
Figure 1.1: Hypothesis for Assessing Platform Elements ...... 8 Figure 3.1: Airplane Passenger and Range Capabilities ...... 17 Figure 3.2: PW 4000-94" ...... 19 Figure 4.1: Quality Function Deployment (QFD) Mapping Framework ...... 24 Figure 4.2: Modified QFD Framework for Platform Analysis...... 26 Figure 4.3: Exam ple of QFD Mapping ...... 27 Figure 4.4: Phase I - Mapping Stakeholder Needs to System Requirements ....43 Figure 4.5: Conflicts Between and Among System Requirements...... 45 Figure 4.6: Phase I - Sample Mapping of System Requirements to System V ariables ...... 50 Figure 4.7: Phase Ill - Sample Mapping of System Variables to Module Flowpath Aerothermodynamic Variables...... 52 Figure 5.1: Normalized Coupling and Normalized Variation for the 8 Engine Sample (Module Flowpath Aerothermodynamic Variables)...... 59 Figure 5.2: Module Flowpath Aerothermodynamic Variable Classifications for the 8 Engine Sam ple ...... 60 Figure 5.3: Normalized Coupling and Normalized Variation for the PW4000 (Module Flowpath Aerothermodynamic Variables)...... 67 Figure 5.4: Module Flowpath Aerothermodynamic Variable Classifications for the PW 4000 ...... 68 Figure 5.5: Normalized Coupling and Normalized Variation for the PW4000 with a 4.5% Normalized Variation Threshold (Module Flowpath Aerothermodynamic Variables) ...... 70 Figure 5.6: Module Flowpath Aerothermodynamic Variable Classifications for the PW4000 with 4.5% Normalized Variation Threshold ...... 71 Figure 7.1: Extension of Modified QFD Mapping for Platform Elements...... 79 7
List of Tables
Table 2.1: Types of Modular Architectures ...... 16 Table 4.1: Stakeholders and their Needs...... 29 Table 4.2: FAR Part 33 Aircraft Engine Certification Tests ...... 32 Table 4.3: Propulsion System Requirements...... 33 Table 4.4: System Variables...... 39 Table 4.5: Module Flowpath Aerothermodynamic Variables...... 41 Table 5.1: PW 4000 Family of Engines...... 62 Table 7.1: Porter's Five Forces...... 86 Table 7.2: Travel Alternatives Between Hartford, CT and Washington, DC...... 88 Table 7.3: Collaborations in Commercial Gas Turbine Engine Development .... 90 8
1 Introduction
Product development challenges companies to produce designs that meet customer needs, yet that are within their technological and financial means to do so. The proliferation of customized or unique designs may tax the resources of a company if product variety cannot be achieved in a cost-effective manner. One strategy to minimize the costs associated with unique designs is to share elements or subsystems across all or part of a company's product portfolio, while design flexibility allows differentiated functions to satisfy specific customer needs. The grouping of these shared elements comprise a platform. The key is to determine which elements or subsystems comprise the platform. The objective of this investigation is to provide a framework for identifying potential platform elements from among key system design variables. The proposed framework is validated with a case study of commercial gas turbine engines that confirms the hypothesis that system design variables with low normalized coupling and low normalized variation from design to design should be considered as potential platform elements. This proposed hypothesis is illustrated in Figure 1.1, where platform candidates would cluster in Quadrant .
Quadrant III Quadrant IV
s High Do Not .4isk Platform
S Quadrant I Quadrant 11
08 Low dr. Platform Risk
Low High
Normalized Variation (Standard Deviation / Mean)
Figure 1.1: Hypothesis for Assessing Platform Elements 9
A system level framework for identifying the normalized coupling and normalized variation of these elements was facilitated through the formulation and use of a modified quality function deployment (QFD) mapping procedure. Normalized coupling was assessed by quantifying the relative importance of relationships between stakeholder needs, system requirements and system design variables, and dividing these rankings by a ranking of the difficulty in their achievement. Normalized variation of system design variables from a sample of parameter data was calculated as the standard deviation divided by the mean. Normalized coupling implies that a product attribute or function, which has low coupling and low difficulty, is as likely to be considered a platform element as one that has high coupling and high difficulty. The motivation for platforming an element that has high coupling and high difficulty is to leverage the higher development cost and effort associated with this more difficult element across multiple applications, rather than developing costly, unique solutions over and over for each new product [Robertson and Ulrich, 1998]. Sharing platform elements across multiple products may lead to lower manufacturing costs from economies of scale, lower development costs, and faster time to market to name a few benefits. In contrast, system design variables that have high normalized coupling and high normalized variation may be poor candidates as platform elements. These variables are predicted to cluster in Quadrant IV of Figure 1.1. Because of their high level of coupling with upstream stakeholder needs and system requirements, keeping them at a fixed level as platform elements may adversely affect many other system variables. The cost of keeping Quadrant IV variables constant in a platform scenario is the high overall system impact due to the high coupling. This system cost may outweigh the elemental cost savings benefit. Since these variables are not difficult to achieve anyway, it may be beneficial and cost effective to allow them to vary as appropriate, so that overall needs and system requirements can be met. 10
For cases where there is low normalized variation, but high normalized coupling as in Quadrant Ill, there is risk in considering these elements for a platform. There is a possibility that fixing them in a platform scenario is risky should some future growth potential or unanticipated condition force them to be changed, moving these elements from Quadrant Ill to Quadrant IV. Since these elements are highly coupled, changing them could have a large impact on the overall system. For cases where there is high normalized variation, but low normalized coupling, there is less risk than the opposite case described in the preceding paragraph because of the low coupling. These variables would cluster in Quadrant 11. Fixing these parameters at a given level for a platform may have a small overall system effect due to the low coupling. Again, there is always risk that a change in requirements may increase the coupling, moving these variables into Quadrant IV. The proposed framework and hypothesis was validated with a case study of the Pratt & Whitney PW4000 family of commercial gas turbine engines where platform elements predicted by the model were consistent with actual design choices. The results identified a set of system design variables with low normalized coupling and low normalized variation that could serve as elements of a commercial gas turbine engine platform and be shared across multiple products. Chapter 2 begins with an overview of product portfolio architecture and methods of defining them. A platform is a type of product portfolio architecture and some examples are given to provide the reader with a perspective of existing platform strategies. The chapter ends with a brief discussion of product architecture concepts as a lead into Chapter 3, which discusses the system architecture of the gas turbine engine and how certain aspects of the engine's architecture may or may not lend themselves to platform considerations. Chapter 4 introduces QFD and summarizes the methodology used to adapt the traditional QFD framework to perform platform analyses. 11
To validate the hypothesis set forth above, two case studies involving recent Pratt & Whitney engine designs are discussed in Chapter 5. Following some concluding statements in Chapter 6, Chapter 7 discusses recommendations for extending the framework presented in this investigation as well as interrelationships and implications between the strategic management of a company and the company's product strategy. Finally, a glossary is included at the end of the document for those who desire further explanation of terms used in this work. Scattered throughout the text are descriptive examples and what if scenarios, which are provided to clarify some of the concepts and issues surrounding commercial gas turbine engine platform strategy and design. They are by no means exhaustive explanations, but are primarily included to provide the reader with an appreciation of the issue(s) and to highlight key points. Analogies to other products are also provided, not only to help explain issues specific to gas turbine engines, but also to provide some basis for comparison as to how these analogies apply or could be applied to gas turbine engines. 12
2 Related Work
To establish a basis for the platform framework presented in this investigation, it is important to review related work concerning product platforms from both a design perspective as well as a product strategy perspective. The literature contains a number of studies that have been conducted to classify product portfolio architectures such as platforms and recommend ways to define these architectures. Product platforms such as the Sony Walkman and Ford automobiles are examples of how platform strategies have been successfully implemented and which can provide additional perspectives on what is achievable. The discussion begins with an overview of product portfolio architecture.
2.1 Product Portfolio Architecture Product portfolio architecture entails defining the way in which members of a portfolio of products share or do not share features. Yu [1998] defines three categories of portfolio architecture: fixed, platform and adjustable. A fixed portfolio architecture is where a single option for a feature is offered across an entire set of products. An example of a fixed architecture is a videocassette case. A platform portfolio architecture is where multiple options for a feature are offered across an entire set of products. An example of a platform architecture is Chrysler's LH platform where the Intrepid, Eagle, Concorde and LHS all share a common body frame construction, but have different styling features for different market segments. An adjustable, mass customization portfolio architecture is where multiple options are offered through a single design, which can be customized by the user. An example of an adjustable portfolio architecture is a hair dryer with multiple heat settings. Recent research has focused on customer needs as a basis for product portfolio definition and planning. Moore [1999] proposes conjoint analysis as a way to quantify customer preferences for different combinations of product 13 attributes. Yu [1998] defines a methodology for product portfolio definition of instant film cameras based on customer needs and accounting for the possibility that these needs may change over time. Roberson [1998] proposes a product attribute clustering technique to define appropriate combinations of automobile platform elements. These investigations all seem to have focused on consumer products where variety is needed to fulfill customer needs. Another product portfolio architecture strategy is based on some measure of product performance. Product performance can be defined as how well a product implements its intended functions [Ulrich, 1995]. Some general examples of product performance characteristics are speed, efficiency, life, accuracy and noise. Krishnan [1998] proposes a model based approach for planning and developing a product family where customers choose products based on some measure of performance. In another product performance based example, Gonzalez-Zugasti [1998] proposes a methodology for optimizing the product portfolio architecture of a family of future spacecraft fielded by the Jet Propulsion Lab. The methodology begins with a point design calculation for each of the different spacecraft missions. The proposed hardware and system performance characteristics of all the point designs are then reviewed and areas of similarity or commonality are identified. These particular components or actual design values are held constant as each of the point designs is then re-evaluated in terms of being able to meet their specific mission requirements. If mission requirements cannot be met, then a negotiation process may take place to arrive at a mutually optimal solution in light of different mission constraints. If mission requirements are met, then those elements can be considered part of a platform. The framework proposed in this investigation is based on an approach similar to that of the JPL case where the variation of key system design variables from a sample of engines is calculated. A modified QFD mapping procedure is implemented to quantify the degree of coupling between stakeholder needs, system requirements and system design variables as well as the difficulty in their 14 achievement. Platform elements are then identified as those variables with low normalized coupling and low normalized variation.
2.2 Examples of Product Platforms The idea of platforms as a strategy for defining product portfolio architecture is not new. Examples of product platforms include the Sony Walkman [Sanderson and Uzumeri, 1995], and Ford's 4.6L SOHC V-8 engine [Hagen, 1990] and vehicle platforms [Nelson et al., 1998]. The benefits of product platforms include reduced engineering and development costs, quicker time to market, economies of scale due to increased volume of standard parts, and common design concepts. The case of the Sony Walkman and Ford V-8 engine illustrate two different product platform strategies, where the former is based on topological design changes, while the latter focuses on fundamental, internal design changes. In the early 1980's, Sony developed 3 basic platforms on which all subsequent Walkman models were built. These platforms focused on two key areas including miniaturization, which affected battery size, and high sound quality systems. With these 3 platforms, Sony offered as many as 20 new models each year and almost 250 US models in the 1980's. Approximately 85% of these 250 models were the result of topological design changes, or cosmetic changes to the outside case and minor re-arrangement of existing features. Sony's success with the Walkman was the result of providing product variety to several market niches. In fact, they offered more models than the competition during this period. This platform strategy focused on providing product variety through topological design changes, while only incrementally improving the performance of the basic platforms [Sanderson and Uzumeri, 1995]. The case of Ford's V-8 engine platform is more analogous to that of the gas turbine engine, where the primary means of providing product variety is not necessarily with topological design changes as in the case of the Sony Walkman, but with design changes to the internal workings of the machinery to enhance performance. Ford's engine platform strategy was to design a family of engines 15 that were to be used in a variety of large and luxury vehicles based on common combustion chambers, valvetrains and basic structure to allow ease of interchangeability. The use of the same component in multiple products can be defined as component standardization [Ulrich, 1995]. Similar components within engine families were also shared such as cylinder blocks, aluminum cylinder heads, camshafts, water and oil pumps, and fasteners. Maintaining key characteristics of a particular engine platform, such as bolt patterns, bore spacing and journal sizes were also part of the platform strategy. As a result, it was estimated that the family of engines would share 75% of all parts [Hagen, 1990]. Maintaining key characteristics is also the basic strategy for Ford's Global Architecture Process (GAP) for entire vehicle platforms [Nelson et al., 1998]. Hardpoints are defined for each platform and consist of master location holes and surfaces, weldlines, and wheelbase and overhang variation ranges. Even with these hardpoints, there is still flexibility to build variety into products of a given platform family. The rationale for maintaining hardpoints is to support high volume vehicle production with flexible manufacturing lines. This vehicle platform strategy is more analogous to the Sony Walkman case than it is to the V-8 engine case, because product variety is provided by topological design changes like body panels, cabin size as well as other attributes distinguishable by the consumer. The Sony Walkman and Ford V-8 engine/vehicle cases illustrate different product platform strategies. Chapter 5 discusses the Pratt & Whitney case study of the PW4000 engine family and similarities to the Sony and Ford cases. The next section discusses some basic concepts of product architecture that may or may not lend themselves to a platform strategy.
2.3 Product Architecture Concepts Given the examples of product platform strategies and how they may be defined, it is important to understand some fundamental concepts of product architecture that may or may not lend themselves to a platform strategy. Product architecture can be categorized as either modular or integral [Ulrich, 1995]. An 16
architecture that is modular has functional elements that have a one-to-one mapping to the physical components of the product and where interfaces between components are decoupled. Two components are considered decoupled if a change made to one component does not require a change to the other component in order for the entire product to work correctly. In contrast, an architecture that is integral has functional elements that have more than a one-to- one mapping to physical components and/or have coupled interfaces between components. There are various types of modular architectures, namely slot, bus and sectional [Ulrich, 1995]. These are summarized in Table 2.1.
Table 2.1: Types of Modular Architectures
TypeDefinition E xamples A car radio versus speedometer. The do not Slotinterface and cannot be car radio has interfaces that plugged into the same interchanged. allow it to be interface as the speedometer. Dell Cpi laptop computer where both Various components have the same the 3.5" disk drive and CD-ROM drive Bustype of interface and can connect to a have the same interface that allows common component. one to be interchangied with the other in the same rece tacle. Components all have the same S cin l interfaces with no sing e element to Ppnscinlsfsadofc Sectional which all the other components prtin. attach.
These examples illustrate how some products lend themselves to one form of platform architecture over another. The next chapter sets the stage for the discussion on how platform elements are identified with the modified QFD mapping by providing perspectives on the system architecture of a gas turbine engine. Included in this discussion is a description of basic system architecture, issues of modularity and integrality, and coupling. 17
3 System Architecture of a Commercial Gas Turbine Engine
3.1 Airplane System and Engine Subsystem From the perspective of an air transportation vehicle, the gas turbine engine is a subsystem of an airplane system. All airplane engines have the same basic functionality of producing thrust to propel an airplane into the air and over a design range with a specified payload. Some secondary engine functions that support airplane functions include providing cabin air, electrical power to airplane systems, and pressurization for airplane hydraulic systems through airplane/engine interfaces. Figure 3.1 illustrates the range and passenger capacities of Pratt & Whitney powered narrowbody and widebody airplanes [Jackson, 1995 & 1997].
500
450 x B747-400/PW4056 400 B777-2OO/PW4084. B777-300/PW4098 IA 350 A330-300/PW4168 B777-2001GW/PW4090 300 A300-600/PW4158 B767-300/PW4056 A330-200/PW468 E- 250 A31 O-300/PW41 52 Z + 9 B767-300ER/PW4060 200 A321/V2633-A5A B757-200/PW2040 @ B767-200ER/PW4056 150 A A31 9N2522-A5 A A320N2525-Al 100
50
0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10 Range (nautical miles)
Figure 3.1: Airplane Passenger and Range Capabilities 18
Engines can provide different thrust levels for different airplane applications. Thrust variation is achieved as a result of the aerothermodynamic and mechanical design of the engine's turbomachinery that includes the rotating blades and stationary vanes, as well as the associated support structure. Support structure includes major parts such as disks, cases, seals, bearings and shafts. Airlines can choose between and among engines offered by different manufacturers, since the engine is an option on the airplane. For example, an airline that purchases a Boeing B777 has the option of choosing either Pratt & Whitney PW4000, General Electric GE90 or Rolls Royce Trent 800 series engines. Here, all three engine manufacturers supply engines with roughly the same rated takeoff thrust, but that may be differentiated by their degree of fuel efficiency, weight, and reliability as examples. What allows three different engine types to interface and be used on the same airplane is the engine buildup unit (EBU).
3.2 Modularity and Integrality A commercial gas turbine engine has attributes of both modular and integral architectures. The modular construction of the engine is such that the major components, typically referred to as modules, can be bolted to each other to form the entire engine. Major engine modules include . Fan . Low Pressure Compressor (LPC) . High Pressure Compressor (HPC) . Combustion Chamber (also referred to as the burner) . High Pressure Turbine (HPT) . Low Pressure Turbine (LPT) Figure 3.2 is a cutaway illustration of the PW4000-94" showing the major modules and their relative position to one another [http://www.pratt- whitney.com/engines/galery/g.pw4000.94cut.htm]. 19
LOW PRESSURE FAN COMPRESSOR
ILET CASE
Figure 3.2: PW4000-94"
The high pressure turbine, as an example, is a module that is the assembly of its constituent parts including blades, disks, seals and a case. The high pressure turbine module as a whole is attached to the diffuser case, which houses the combustion chamber on the upstream end, and the low pressure turbine module on the downstream end. Although the engine is modular in construction, its functionality on the module level is integral both in terms of the many to one mapping of functional elements to physical components, as well as coupled interfaces between modules. In terms of basic engine functionality, compression is accomplished by the fan, LPC and HPC, fuel-air mixing and burning is accomplished by the combustion chamber, and expansion is accomplished by the HPT and LPT to all to create thrust. However, there is a many to one mapping of functions to a 20 particular module. For example, functions of the HPC not only include compressing air, but also providing secondary flow to other parts of the engine as well as to the airplane, providing airflow acceptable to the downstream burner module, providing support for internal turbomachinery, accepting torque from the shaft connected to the HPT, and driving an accessory gearbox with assorted pumps and generators. In terms of coupled interfaces, the engine is integral in the sense that a change to one module affects other engine modules. Ulrich [1995] refers to different types of coupling including those of geometry and heat. These types of coupling found in gas turbine engines are discussed in the next section.
3.3 Mechanical & Aerothermodynamic Coupling Geometric or mechanical coupling occurs where certain modules are mechanically connected to the same shaft and so turn at the same speed. The high pressure compressor (HPC) module and high pressure turbine (HPT) module are connected to the same shaft, which rotates at high speed. The combination of the HPC and HPT modules is typically referred to as the high spool or engine core. Sandwiched between the HPC and the HPT is the combustion chamber which is also considered to be part of the core. An example of mechanical coupling in the core is where a change in the exit diameter of the HPC case requires a change to the inlet of the diffuser case to which it is connected. As in the core, the combination of the fan, LPC and LPT modules, typically referred to as the low spool, are connected to a different shaft that rotates at a speed slower than that of the high spool. The combination of the fan, LPC and LPT is typically referred to as the low spool. Although the low and high spools can be considered mechanically decoupled, since each spool is connected to a different shaft and turns at a different speed, there are still interactions between the spools due to aerothermodynamic coupling. Aerothermodynamic coupling comes from the fact that air and exhaust gases travel through a continuous flowpath formed by the turbomachinery of all 21 the engine modules, from the inlet to the exit of the engine. The exit conditions of mass, momentum and energy in the form of pressure, temperature and flow from one module serve as the entrance conditions for the following module. In addition, a change in a flowpath condition for a module on one spool may affect another module on the same or the other spool because of this continuous flow from one module to the next. Coupling effects are not necessarily bad. Quantification of module to module parameter coupling can be used during the engine development process to optimize overall system performance. The coupling between modules is typically quantified by what are referred to as influence coefficients or trade factors. For example, if an engine test reveals that fuel efficiency goals are not being met, influence coefficients generated from powerplant performance simulations can be used to compare actual parameter shifts with predicted parameter shifts in order to determine which module(s) are key contributors to this deficiency. This information can then be used to determine what module improvements are needed in order for the engine to meet overall system requirements. In contrast, coupling can also be detrimental when a change to one aspect of the engine adversely affects one or more aspects of another part of the engine. For example, although the low pressure compressor (LPC) and high pressure compressor (HPC) are not mechanically coupled because they are connected to different shafts, they are still aerothermodynamically coupled because they share a common interface. LPC exit conditions of pressure, temperature and flow serve as the entrance conditions to the HPC. Good engine design will minimize the coupling between these modules such that a surge condition in one does not exacerbate a surge condition in the other. An engine surge is where the compression system has lost its ability to compress air and there is a momentary reversal of flow towards the front of the engine instead of rearward. This is an example of the desire to minimize the coupling between modules. 22
Coupling between and among engine modules both mechanically and aerothermodynamically complicates the issue of a platform. Swapping a module from one engine type to another in a building block philosophy first requires that the mechanical interface is compatible, e.g. bolt locations, diameters and shaft size. Even if the mechanical interface is compatible, the aerothermodynamic coupling between and among modules may prevent this swapping strategy from allowing the entire engine to meet system requirements. Because the traditional approach of defining platform elements as those which have little or no coupling at the interface or have a one-to-one mapping of form to function, are not entirely appropriate for a commercial gas turbine engine which is functionally integral, as well as mechanically and aerothermodynamically coupled, an alternative approach is needed. The next chapter introduces quality function deployment (QFD) as a framework for assessing a form of system level coupling that not only captures the physical coupling described above, but also the relationships between key system design variables and the stakeholder needs and system requirements that drive them. Quantifying the degree of coupling between needs, requirements and system design variables as well as their difficulty in achievement, will help to identify the system level effect of keeping key design variables constant or within a certain range of variability in a platform scenario. Identifying these key design variables is then the first step in identifying potential platform elements in an integral and coupled architecture such as the gas turbine engine. 23
4 Quality Function Deployment (QFD)
4.1 What is QFD? This investigation utilized QFD to systematically identify key elements of a gas turbine engine product platform. QFD was reported on by Hauser and Clausing [1998], but was originally based on the quality tables developed by Professor Mizuno at the Tokyo Institute of Technology for Mitsubishi Kobe Shipyards in 1972. QFD is a means to ensure that high level needs and requirements flow down or are deployed to the design and manufacture of various product components. QFD has been used as a system engineering tool for requirements management, tracking and traceability. It has been used in the design of complex systems such as spacecraft and military airplanes [Boppe, 1998]. Xerox used QFD in the design of their successful Lakes digital document platform to "deploy the voice of the customer to the factory floor' [Paula, 1997; Elter, 1998]. Figure 4.1 [Quality Function Deployment Implementation Manual, 1989] shows the QFD framework beginning with customer wants or needs and progressing to a series of 4 mappings first to design requirements, then to part characteristics, key process operations, and finally to production requirements. 24
Conflict Design Part Key Process Production Requirements Characteristics Operations Requirements
E g 0
Important Important Important Difficult Difficult Difficult
Phase I Phase 11 Phase III Phase IV Product Planning Part Deployment Process Planning Production Planning
Figure 4.1: Quality Function Deployment (QFD) Mapping Framework
Each deployment phase in Figure 4.1 is a matrix mapping of relationships between row and column categories. A relationship is indicated at the intersection of a row and column and is interpreted as the importance of a column category in achieving the row category, relative to the influence of other column categories in influencing that same row category. The relative importance of these relationships is typically captured on a 1 to 10 scale, with 1 denoting low importance and 10 denoting high importance. An organization's experts are consulted to provide the relative importance relationships. In the end, the relative importance of each column category can be obtained. These relative importance rankings can then be used as a roadmap to indicate where the organization should focus its resources and attention at each phase. A difficulty ranking can also be assessed against each of the column categories. This can be used to highlight areas that may require additional resources or attention. Difficulty assessment combined with relative importance rankings can then be used to guide the organization's strategy during the product development process. Phase I of the QFD mapping framework is referred to as the House of Quality. It is at this stage where customer wants are translated into design 25 requirements. The proverbial "roof" of the House of Quality captures the conflicts between design requirements, where achieving an optimal level for one design requirement can lead to a suboptimal level for another requirement. For example, if one considers the generic requirements of performance and cost, a high performance product may cost more to develop than a low performance product because of extra features and capabilities. Likewise, low cost may imply low performance. Thus, a requirement to achieve better performance comes at the expense of cost and vice versa. These requirements work in opposite or conflicting directions. Each successive phase of deployment is driven by the preceding set of requirements or variables. In other words, the requirements or variables are deployed to successive phases. An example of how QFD can be used during detailed design is when a key process operation in Phase Ill of Figure 4.1 cannot be accomplished due to the limitations of an existing manufacturing process. The mapping will indicate what key part characteristics in Phase II are affected and may need to be altered so that the part can be manufactured, as well as what design requirements in Phase I may be affected. One can thus trace the upstream or downstream effects of such changes.
4.2 Applying QFD to Identify Platform Elements The reason why QFD was chosen as a framework to analyze platform elements was because of its ability to capture not only physical coupling, but also the system level coupling of customer wants, design requirements, part characteristics, key process operations and production requirements. The traditional QFD framework discussed in the previous section was modified for this investigation and was subsequently used to identify potential platform elements based on system level normalized coupling and normalized variation. The modified QFD framework is presented in Figure 4.2. 26
Conflict
System System Module Requirements Variables Flowpath Aero
0 E E)
00 C~CO
Importance Importance Importance Difficulty Variation Difficulty Variation Variation
Phase I Phase Il Phase il Requirements System Variables Module Flowpath Deployment Deployment Variables Deployment
Figure 4.2: Modified QFD Framework for Platform Analysis
The system level coupling between stakeholder needs, system requirements, system variables and module flowpath aerothermodynamic variables is quantified through the identification of relationships between successive mappings and the importance of each relationship. The ranking schemes discussed in Chapter 4.4 allow the relative importance of each relationship to be captured. Phase I of the modified mapping illustrated in Figure 4.2 is consistent with the traditional QFD mapping found in the previous section where stakeholder needs are mapped to requirements. For this investigation, stakeholder needs are deployed to what is referred to as system requirements. The difficulty associated with each system requirement is also assessed. Because of the modular construction of the engine, system requirements are allocated to each of the modules, hence Phase 11 mapping from system requirements to system variables. For example, a typical system requirement may be for a certain level of thrust specific fuel consumption (TSFC), which 27 satisfies an airplane mission stakeholder need for airplane range as illustrated in Figure 4.3. Although TSFC is a system requirement, each module is expected to operate at a certain level of efficiency so that the entire engine can meet the TSFC requirement. In this way, the system requirement of TSFC is allocated to the system variable of module efficiency across all the engine modules.
Stakeholder Requ emen Sysem Variable- Aerothemn narn c
Figure 4.3: Example of QFD Mapping
Unlike Phase 1, Phase I does not include a separate assessment of system variable difficulty in this investigation, because system variables at the module level essentially inherit the difficulty deployed from the system requirements. Phase Ill of the modified QFD mapping is from system variables to module flowpath aerothermodynamic variables, which are key design variables that are associated with each of the engine modules. Extending the example of the TSFC system requirement cited above, the system variable of HPC efficiency is achieved by defining module flowpath aerothermodynamic variable levels such as number of stages, blade aspect ratios, flow coefficients as well as others. System variables from Phase 11 therefore drive module flowpath aerothermodynamic variables in Phase Ill. A level of difficulty is assessed for each engine module in Phase Ill and applies to all the flowpath aerothermodynamic variables associated with that particular module. This difficulty ranking can be based on resource requirements 28 for personnel, as well as module hardware and non module hardware required during development. As discussed in the introduction, the degree of difficulty is used as a discriminator for identifying platform elements so that they can be leveraged across multiple products. This can reduce subsequent product development effort and cost. What differentiates the modified QFD framework for platform analysis from the traditional QFD is the quantification of parameter variation at each phase of the mapping. Later sections discuss how the normalized variation of actual aerothermodynamic design data is calculated and how potential platform elements are identified for Phase Ill of this mapping process. Knowing the level of normalized variation as well as normalized coupling then allows platform elements to be identified. A detailed description of different elements for each phase of the modified QFD mapping used to identify platform elements is described in the next section. These include stakeholder needs, system requirements, system variables, and module flowpath aerothermodynamic variables.
4.3 Elements of the Platform QFD
4.3.1 Stakeholders and their Needs Pratt & Whitney uses a balanced scorecard approach to design engines [Kaplan and Norton, 1996]. Engine designs are driven by the needs of many stakeholders including airlines, airplane manufacturers, regulatory agencies as well as Pratt & Whitney. Airlines are the end user of Pratt & Whitney engines. Airplane manufacturers consider the engine to be an airplane subsystem. Regulatory agencies like the Federal Aviation Administration (FAA), International Civil Aviation Organization (ICAO), and Environmental Protection Agency (EPA) define policies and guidelines to protect the public and the environment. Finally, Pratt & Whitney, like any other company, is in the business to make a profit. Table 4.1 lists the stakeholder needs used in Pratt & Whitney's balanced scorecard approach and who the primary and secondary stakeholders are. Below is a detailed description of stakeholders and their needs. 29
Table 4.1: Stakeholders and their Needs
4.3.1.1 Airplane Mission From Pratt & Whitney's perspective, the airplane manufacturer is the primary stakeholder in ensuring the airplane system satisfies its defined mission so that the airline as the secondary stakeholder is satisfied. Elements of the airplane mission which are made possible in large part by the propulsion system include the design range, typical mission length, takeoff gross weight, and the amount of fuel burned. Pratt & Whitney is a secondary stakeholder in the sense that in order for the airplane manufacturer to even consider it a viable contender for an airplane application, its proposed engine offering has to be competitive in achieving the airplane mission. Not being competitive could mean exclusion from consideration. The airplane manufacturer does not have to offer a particular manufacturer's engine as an option to the airline. 30
4.3.1.2 Reliability Airlines are the primary stakeholder for engine reliability, although the FAA may become a more vocal stakeholder when flight safety issues have the potential to adversely affect the flying public. Reliability is the ability of the engine to operate safely and according to its original design intent. Engine reliability is typically measured in terms of in-flight shutdowns, unscheduled engine removals, and delays and/or cancellations. When engine reliability poses a severe safety hazard, the FAA may intervene and mandate that certain rectifying actions be taken to minimize risk to the flying public. Poor reliability also increases an airline's direct and indirect operating costs when it has to fix these problems. It may also lower their revenues when flight delays or cancellations decrease passenger satisfaction. In many ways, Pratt & Whitney is also a secondary stakeholder, because the reliability of the engine influences the amount of post certification engineering (PCE) effort required to resolve these problems. Given limited budgets, this may impact the funding available for new engine development programs. Poor engine reliability may also influence an airline's decision not to buy current or future engines from a given manufacturer, because reliability problems like delays and cancellations can result in lost revenues from low customer satisfaction.
4.3.1.3 Cash Operating Cost Airlines are the primary stakeholder for cash operating cost (COC). COC includes costs associated with operating the engine such as total maintenance cost (TMC) and the cost of fuel burned. The stakeholder need is to minimize COC via low maintenance costs and fuel efficient engines. Pratt & Whitney can also be considered a stakeholder when it offers fixed price maintenance contracts to airlines. This is where Pratt & Whitney maintains an airline's fleet of engines and charges a certain maintenance rate based on the number of hours the engines are operated. If actual maintenance costs exceed negotiated contract levels, Pratt & Whitney stands to lose profit. In addition, if 31 maintenance costs are too high relative to the competition, Pratt & Whitney stands to lose market share for these maintenance contracts.
4.3.1.4 Environmental Regulatory agencies such as the FAA, ICAO and EPA are the primary stakeholders acting on behalf of the public for ensuring that engines are environmentally friendly when they are operated, as well as when they are manufactured or repaired. There are published guidelines for allowable emissions and noise levels. Airlines are secondary stakeholders because they are penalized for operating engines that violate local emission and noise restrictions.
4.3.1.5 Recurring Cost (Manufacturing) Recurring cost is the cost for Pratt & Whitney to manufacture each engine. As such, Pratt & Whitney is the primary stakeholder for ensuring that recurring costs are minimized in order to maximize profit margins.
4.3.1.6 Non-Recurring Cost Spent to Launch (Technology) Again, Pratt & Whitney is the primary stakeholder for this need. Non- recurring costs include the development of technologies that will allow the engine to achieve the airplane mission. New technologies must demonstrate a certain level of maturity before they can be considered for inclusion in a new engine program. It is the cost associated with the maturation of these technologies that comprises this cost.
4.3.1.7 Non-Recurring Cost Spent from Launch to Certification (E&D) Engineering and development (E&D) costs include that for manpower, engine hardware, non-engine related equipment, and testing to ensure the engine meets airplane mission requirements as well as passes FAA tests to be certified as airworthy. Listed below are FAA tests prescribed by Federal Aviation Regulations (FAR) Part 33, Airworthiness Standards: Aircraft Engines [http://www.faa.gov/avr/AFS/FARS/far-33.txt]. 32
Table 4.2: FAR Part 33 Aircraft Engine Certification Tests
E&D is essentially an affordability issue for Pratt & Whitney. There may be instances when a development program may be technologically ready for launch into full scale development and certification, but may end up being delayed because of limited resources.
4.3.1.8 Entry into Service (EIS) Date This is the date when the launch airline begins operating airplanes in revenue service carrying passengers. This date is mutually agreed to by the launch customer, airplane manufacturer and various suppliers, of which Pratt & Whitney is an engine supplier. During the elapsed time between formal program 33 launch and EIS date, the development organizations must not only develop and test hardware that meets airplane mission requirements, but also ensure the engine passes FAA tests to be certified as flightworthy. EIS date influences what technology can be incorporated into an engine design, whether or not sufficient resources are available during the given development period, and whether or not the engine can meet its requirements when airlines begin revenue service operations. Although there are various stakeholders, Pratt & Whitney is the primary stakeholder.
4.3.2 System Requirements To ensure stakeholder needs are fulfilled, system requirement categories are defined and target levels are set prior to program launch. Pratt & Whitney uses the system requirement categories and subcategories listed in Table 4.3.
Table 4.3: Propulsion System Requirements
During the product development process, actual system requirement levels are tracked relative to target levels. Shortfalls are identified and action plans are 34 implemented to ensure the engine meets all requirements. For example, the engine's thrust specific fuel consumption (TSFC) is a performance requirement, which is a measure of how efficiently the engine burns fuel to produce a given thrust level. A target TSFC level is defined such that the stakeholder needs for the airplane to fly a certain range with a given payload, low cash operating cost due to fuel burned, and low recurring and non-recurring costs can all be achieved. As discussed in Chapter 3.3, a deficiency in TSFC can be isolated to certain parts of the engine, so that specific hardware changes can be implemented to address the problem. This may mean changing the aerodynamics of the turbomachinery airfoils or reducing tip clearances as example solutions. The sections below describe in greater detail each of the requirements listed in Table 4.3.
4.3.2.1 Airplane Integration Requirements in this category involve system level interface issues between the engine and the airplane and include engine weight, drag, diameter limits and length limits. The engine is mounted to the airplane via the pylon. Because the pylon is designed to support a certain load, the engine weight must be kept within these limits. In addition, the diameter of the engine is constrained for wing mounted engines because of the required clearance between the bottom of the engine and the ground. This clearance is necessary to minimize ground vortex as well as foreign object ingestion which may adversely affect engine operation. Length limits are important since they affect how and where the engine is mounted to the pylon.
4.3.2.2 Performance Requirements in this category involve the primary function of the engine, which is to generate thrust to propel an airplane in flight. Thrust is the force that propels an airplane at a specified speed and altitude throughout its flight envelope. Other requirements include thrust specific fuel consumption (TSFC) and performance deterioration rate. TSFC is a measure of how efficiently the engine burns fuel in terms of rate of fuel burned per pound of thrust generated. 35
Performance deterioration rate is how quickly an engine's fuel efficiency changes over time and is typically measured in %TSFC change per a given number of cycles. Worse TSFC means that the engine is operating less efficiently and has to burn more fuel to achieve the same thrust level. For long range airplane missions, fuel burn is critical given the finite amount of fuel the airplane is designed to carry.
4.3.2.3 Reliability Requirements in this category are associated with the engine's ability to operate according to its original design intent. Engine reliability is measured in terms of in-flight shutdown rate (IFSD), unscheduled engine removal rate (UER), and delay and cancellation rate (D&C). Both the in-flight shutdown and unscheduled engine removal rates are measured as events per one thousand flight hours. The delay and cancellation rate is measured as events per one hundred airplane departures. Although these reliability metrics are applicable to airplane related problems as well, the descriptions below focus on engine related problems. An in-flight shutdown is when the pilot terminates fuel flow to the engine. A pilot may elect to shutdown an engine when its continued operation after an anomalous operating condition is deemed to have the potential of causing further damage to the engine or creating a safety hazard for the airplane. Examples of conditions that may cause an in-flight shutdown include a bearing failure which may cause an oil filter clog indication and high vibration, compromised bearing compartment seal which may cause an indication of low oil pressure, low oil level and/or high oil temperature, and fractured airfoils that may cause a surge and high vibration. Although an engine may be shutdown, the airplane can still continue the flight if the other engine(s) are operating normally. An unscheduled engine removal occurs when the engine's inability to continue functioning within normal operating guidelines causes it to be removed for repair or refurbishment. This can result from an in-flight shutdown, the inability to correct a problem even after on-wing troubleshooting, as well as an 36 engine durability problem where a part deteriorates or fails before reaching its predicted design life. An unscheduled engine removal is in contrast to a scheduled removal where an airline deliberately plans to remove an engine for scheduled maintenance or rotation purposes. Engines may be rotated on or off wing for the same reason that tires are rotated on an automobile, so that they wear evenly. In the case of engines, they are rotated on or off wing so that all the engines in an airline's fleet accumulate similar flight hours and cycles and have similar levels of performance. A delay may be caused when an engine problem prevents a flight from departing within 15 minutes of its scheduled departure time. A cancellation is caused when an engine problem prevents the flight from taking off at all. Delays and cancellations may be caused by an in-flight shutdown or an unscheduled engine removal.
4.3.2.4 Environmental Requirements in this category relate to how friendly the engine is to the environment during its operation as well as during its manufacture and repair. Requirements include emissions levels, noise levels and whether or not the design utilizes environmentally friendly materials and processes. Regulated engine emissions include nitrous oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), and smoke. NOx, CO and HC emissions are measured as grams per kilo-newton of maximum thrust generated. Smoke is identified as the matter in exhaust emissions that obscures the transmission of light and is measured in terms of a dimensionless smoke number. There are often local airport restrictions or guidelines on emissions where penalty fees are imposed on airlines that operate engines which exceed these limits. Noise levels are measured in decibels (EPNdB) and are also regulated. There are typically local airport restrictions on cumulative noise generated by the engine at three reference conditions including sideline during the takeoff roll, cutback when engine power has been reduced from takeoff power on the climb out from the airport, and approach for landing. 37
So called "green" engines are designed to take advantage of environmentally friendly materials used in anti-gallants, anti-seize materials, primers, adhesives, coatings, corrosion protection and wear resistance. Hazardous materials pose a health risk not only to production and maintenance personnel, but also to the environment.
4.3.2.5 Cost Requirements in this category have to do with Pratt & Whitney manufacturing and development costs and include recurring cost, non-recurring cost of technology development required for program launch, and non-recurring cost of engineering and development. Low recurring or manufacturing cost is desired for business profitability. Non-recurring costs for technology, and engineering and development are primarily affordability issues and impacts whether or not a development program can be launched or completed with given resources and within a given time frame. For example, development of high temperature materials for high performance engines may be costly to develop from a technology as well as manufacturing standpoint.
4.3.2.6 Design Requirements in this category have to do with the general design of the engine. Of primary concern is the cyclic design life of critical parts that are exposed to extreme temperature and stress conditions. For example, extreme temperature conditions typically occur during takeoff when the engine operates at its hottest temperature. The engine is therefore designed to operate at these conditions for a specified number of takeoff cycles.
4.3.2.7 In-Service Operations Requirements in this category represent issues that are important to an airline as it operates the engine. Requirements include total maintenance cost (TMC), time required to replace externals and accessories while the engine is installed on an airplane, refurbishment interval and the engine being service ready at EIS. Total maintenance costs include that for parts and labor and is 38 measured as cost per engine flight hour. Time required to replace externals and accessories is measured in terms of minutes and is important for minimizing maintenance labor costs when it comes to performing on-wing maintenance. Short part replacement times may also help to reduce the frequency of delays and cancellations when an engine problem needs to be corrected between flights. Refurbishment interval refers to how often parts in the engine need to be replaced because of wear and is measured in terms of engine flight cycles. Finally, an engine is considered service ready at EIS if all documentation and support equipment needed to operate and maintain the engine are in place and available for an airline to use.
4.3.3 System Variables at the Module Level Due to the modular construction of Pratt & Whitney engines, many of the system requirements described in the previous section are achieved by specifying levels of system variables at the module level. In other words, many system requirements are achieved by summing the contributions from each major engine module. For example, one system requirement that is the sum of the module contributions is that of engine weight. Not all system requirements are merely the cumulative total of all module contributions, as in the case of TSFC. From the earlier example illustrated in Figure 4.3, it was shown that TSFC is a system requirement that is influenced by the design efficiencies of the various engine modules including the fan, LPC, HPC, HPT and LPT. In reality, the efficiencies of some modules have a greater impact on TSFC than others. For example, a one percent change in HPT efficiency has a greater impact on TSFC than does a one percent change in LPC efficiency. These non-linear effects are captured in the QFD mappings. Listed below in Table 4.4 are the system variables associated with each of the major engine modules, as well as a set of high level system variables that may span across more than one module. These high level system variables are commonly used in turbomachinery design. A detailed description of their use and relevance may be found in the literature [Cohen et al., 1987; Dixon, 1978]. 39
Table 4.4: System Variables 40
Table 4.4: System Variables, concluded
4.3.4 Module Flowpath Aerothermodynamic Variables System variables at the module level listed in the previous section influence module flowpath aerothermodynamic variables. A detailed description of the use and relevance of these module flowpath aerothermodynamic variables commonly used in turbomachinery design may be found in the literature [Cohen et al., 1987; Dixon, 1978]. The module flowpath aerothermodynamic variables considered in this investigation are listed below in Table 4.5 by engine module. 41
Table 4.5: Module Flowpath Aerothermodynamic Variables 42
Module flowpath aerothermodynamic variables listed above comprise the last level of mapping addressed in this investigation. It is at this level where potential platform elements are identified. The next section discusses the interrelationships between stakeholder needs, system requirements, system variables and module flowpath aerothermodynamic variables and how these interrelationships are captured via the QFD mapping. This will provide insight into the coupling of module flowpath aerothermodynamic variables to stakeholder needs, system requirements and system variables. Again, quantification of this coupling is necessary to validate the hypothesis that system design variables with low normalized coupling and low normalized variation are potential candidates as platform elements.
4.4 QFD Platform Mapping In this section are discussed the modified QFD mapping process illustrated in Figure 4.2 and how a quantitative assessment of platform elements is derived from the mapping of importance relationships from Phase I through Phase Ill. The mapping of importance relationships was made possible through consultation with system engineers, technical experts and marketing specialists that support Pratt & Whitney's Advanced Engine Programs. Individuals were asked to offer their subjective, yet expert opinion on the relative importance of relationships among stakeholder needs, system requirements, system variables and module flowpath aerothermodynamic variables.
4.4.1 Mapping Stakeholder Needs to System Requirements Various elements contributed to the mapping of stakeholder needs to system requirements to derive the system requirement coupling ranking for Phase I of the modified QFD mapping. These elements included the relative importance of system requirements, stakeholder needs for different markets, and the difficulty associated with achieving system requirements. The following 43 sections discuss these elements and the equations used to calculate the normalized system requirement coupling ranking shown at the b'ottom of Figure 4.4. This figure is Phase I of the modified QFD mapping of stakeholder needs to system requirements.
System Requirements Relative Importance Ranking (10=ligh ... 1=Low) mwkd
Airplane Integration Performnence Reliability Environmentel Cost Dsn n In-Service Operations (10-Ngh.. 1=b"
Stakeholder- Needs
Ak plsne Mlsalon 8 9 10 10 7 4 7 10 8 7
RelIabIlIty 9 9 10 10 10 3 7 5 5 5 3
CashOperatingCostteholdy) 9 7 5 5 3 2 7 10 8 3 8 3 2 8 5 3 2 8 10 4 8 4
Environmental 6 8 7 10 10 7 5
CoshRecurring Cost 7 5 5 3 2 7 10 6 3 a 32 8 5 3 2 0 4 a 4 (Mnfatr 9g 9 5 8 5 5 5 5 5 2 10 3 7 1 5
Non-Racunring Cost Speit to Launch 3 3 8 8 8 4 5 10 5
Spentcrom Launch to 5 5 5 5 5 3 3 7 10 3 7 Certification (E&D) __ _
Entry Into Service (EIS) 3 3 8 5 Date Difficulty (9wHih,S=MedIum,3=Low) 9 3 9 6 8 9 6 9 9 9 9 6 8 9 8 9 3 6 9 6 9 t 8 x :ti . h1 E I E II 8 I t I ii
Nornulized Systemn Requirement Copling Rnkg Shari Range Missioni 58% 191% 132% 118% 145% 179% 144%154% 187% 54% 862% 188% 186% 181% 1100%187%1 18% 182%153% 139% 147% 170% Long FtRge Misskx 82% 1108%138% 122% 148% 182% 149%152% 82% 152%182% 188% 114% 157% 105% 88% 112% 155%147% 135% 142% 88ff%I
Figure 4A4 Phase I - Mapping Stakeholder Needs to System Requirements 44
4.4.1.1 Relative Importance of System Requirements The stakeholder needs and system requirements shown in Figure 4.4 were discussed in Chapters 4.3.1 and 4.3.2 respectively. System engineers responsible for performing conceptual engine design studies were asked to rank the relative importance of system requirements to stakeholder needs on a scale of 1 to 10 with 1 denoting low importance and 10 denoting high importance. The final rankings shown in Figure 4.4 were reached by consensus for the system engineers surveyed. The relative importance rankings answer two questions, namely (a) which system requirements need to be met in order for a given stakeholder need to be satisfied, and (b) how important is one system requirement relative to another in meeting a stakeholder need. The second question of the relative importance between system requirements and stakeholder needs is captured with a ranking scale of 1 to 10. For example, a rank of 10 at the intersection of the thrust specific fuel consumption requirement and airplane mission indicates that this requirement is extremely important in meeting the need. Likewise, the thrust specific fuel consumption requirement is equally important in meeting cash operating cost needs with a rank of 10. The first question of which system requirements have to be achieved to meet a particular need is answered with a relative importance rank at the intersection of a system requirement and a stakeholder need. Figure 4.4 illustrates that in general, a given stakeholder need is satisfied only after meeting several system requirements. For example, the stakeholder need of airplane mission is driven by system requirements of weight, low drag, diameter limits, length limits, thrust, thrust specific fuel consumption, performance deterioration rate and non-recurring cost of technology development.
4.4.1.2 Conflicts Between and Among System Requirements More often than not, more than one system requirement is needed to satisfy a particular stakeholder need as illustrated by the previous example of the airplane mission. Tradeoffs between conflicting system requirements may be 45 necessary in order to meet a stakeholder need once a particular requirement's minimum threshold has been met. The roof of the House of Quality depicted in Phase I of Figure 4.2 captures the conflicts between system requirements. Figure 4.5 shown below captures the conflicts between system requirements pertinent to this investigation.
'Performn" Refebility Enonmsnfal Coat DesignflS Inerto " 4I I Operations
1=
w Co w S 5. to b IL I I S S CI a I I ZI I 5. IUF A Co I1 ILLI Ij I Ii 2 Ii I
Weight x x x x x x Arplene Low Drag x integrtlon Diameter Uxnits; x LengtLh Urnits x
Thrust x x x x x Thrust Specific Fuel CnsuIon (TSFC)
Performnce Deterioration Rale x
Infight ShudoWn Rate (IFSD) x x x x
RelIab lItyRelmit kwaheduled EngineUEJF) Renvsl Rats
Delay & Cancelation Rate x x x
Enissions x x x
Environnntal Noise x x x x
EnAronmsntey Friendly Design (Green Engine)
Recurring Costs (Manufacturing) x x x x x x
Launch (Technology) I I I I I I Non-Rcrng Coat Spentfromx x x x Launch toCertification(E&D)__ x x Transportabilty x x Cyclic Design Life x x
Total Maintenance Costs (TMC) x x x
Une Rsplaceable Unit (LF) In-Service Required Raplacermnt Timre Operations efurbishrent Interval x x
SerAce Ready at Entry Into Serv4ce x
Figure 4.5: Conflicts Between and Among System Requirements 46
A possible scenario where system requirements might be traded off against each other in order to meet a stakeholder need is when system requirements of weight and recurring cost are traded off to meet airplane range, which falls under the stakeholder need of airplane mission. Suppose that an engine weighs more than originally intended and is predicted to burn more fuel to meet a given range. Assuming the fuel consumption capability of the engine is limited, a weight reduction effort may be implemented to ensure airplane range can be met. This may necessitate the use of more exotic and higher strength materials that weigh less than the materials currently being considered, but that cost more to manufacture. Here, goodness is low weight, but low weight translates into higher recurring costs, which is undesirable. The opposite is also true in that the desire for lower recurring costs, which is goodness, can translate into higher weight with less expensive, less exotic, lower strength materials which is undesirable. The relationship between weight and cost is thus reflexive. This is true for the other relationships considered in Figure 4.5. The decision of what to do then becomes a question of the relative benefit and cost of weight versus recurring cost. A further complicating factor in this example is the pylon weight limit imposed by the airplane manufacturer. This implies that the tradeoff between weight and recurring manufacturing cost is constrained by a minimum weight threshold. If the engine is heavier than this specified limit, there may be no alternative in this hypothetical example but to use lighter, higher strength materials that cost more to manufacture and then implement a cost reduction plan. Timing is also an issue and whether or not there is sufficient development time prior to EIS for a materials solution to be obtained. Improving fuel consumption may also be required to resolve this requirement conflict. In this case, both weight and fuel consumption may need to be improved simultaneously. 47
4.4.1.3 Stakeholder Needs for Different Market Segments The relative importance of stakeholder needs may vary depending on the market segmentation. Traditionally, the engine market has been segmented by airplane payload and range, with payload referring primarily to passenger capacity and fuel carried. Figure 3.1 illustrated various airplane/engine combinations and their passenger capacities and ranges. Although there may be finer gradations for markets of different combinations of payload and range, this investigation assumed two general markets consisting of short and long range airplanes. The relative importance of stakeholder needs for different market segments was assessed on a 1 to 10 scale with 1 denoting low importance and 10 denoting high importance. Although four stakeholders are considered in Pratt & Whitney's balanced scorecard approach, a single consolidated rank for each need was chosen based on historical trends. Both marketing specialists and value engineering experts at Pratt & Whitney were consulted for the rankings used in this investigation. An example of how stakeholder needs may vary according to short and long range market segments is in the case of total maintenance cost (TMC), which is a component of cash operating costs (COC). Engines designed for short range missions are typically influenced more by TMC due to the greater number of takeoff and landing cycles flown per year. In contrast, engines designed for long range missions are driven more by the airplane mission and the engine's fuel consumption to meet range requirements, and less by TMC. An analogy to the automobile industry is city driving versus highway driving. City driving with short trips is analogous to short range missions with many takeoff and landing cycles. Highway driving is analogous to long range missions where low fuel consumption is more important to allow the airplane to fly its design mission without running out of fuel. Although TMC is still important for long range missions, greater priority is placed on airplane mission needs and range. Differences in stakeholder needs tend to drive engine design choices and basic system architecture decisions. 48
4.4.1.4 System Requirement Difficulty and Core Competencies Some system requirements may be perceived as harder to achieve than others in terms of development cost and effort. For example, weight and thrust specific fuel consumption requirements may be more difficult to achieve than the engine's transportability or time required to replace LRU's as shown in Figure 4.4. Difficulty in achieving system requirements can certainly influence the architecture of the design. If an organization excels in particular areas, the system architecture and design may in turn reflect these core competencies. In contrast, if an organization does not excel in areas important for achieving critical requirements, it may need to develop or acquire the relevant core competencies to be successful. The difficulty in achieving a system requirement in this investigation was based on the following scale: . 9 = High Difficulty . 6 = Medium Difficulty . 3 = Low Difficulty Again, Advanced Engine Programs system engineers involved in conceptual design studies were surveyed to obtain the difficulty rankings shown in Figure 4.4. Although the difficulty in achieving a system requirement is primarily used in the determination of the coupling effect described in the next section, ranking each system requirement by its perceived difficulty also provides a roadmap for guiding the development of key core competencies of the firm. The recommendations in Chapter 7.5.1 discuss how this can be done.
4.4.1.5 Deriving the Normalized Coupling Measure Assessing the importance relationships between system requirements and stakeholder needs, the relative importance of market segment stakeholder needs, and the difficulty in achieving system requirements were all used to derive a measure of normalized coupling between system requirements and stakeholder needs. This section explains how the normalized coupling in Phase I was calculated. 49
The values in the market segment importance ranking columns for short and long range missions were independently multiplied with the corresponding row values under each system requirement in Figure 4.4. The sum of these products divided by the difficulty of a given system requirement is the measure of the normalized coupling between a particular system requirement and the associated stakeholder needs. The normalized system requirement coupling ranking was calculated using Equation 1.
Normalized System Requirement Coupling Ranking (i, j) = I {Market Segment Importance Ranking(i, k)
* System Requirement Relative Ranking (j, k) / Difficulty(j)} (1) for i = 1 to total number of market segments j = 1 to total number of system requirements k = 1 to total number of stakeholder needs Each of the normalized system requirement coupling rankings of both the short and long range mission segments was non-dimensionalized relative to the maximum normalized coupling level of the short range mission and is shown at the bottom of Figure 4.4 as a percentage. A normalized system requirement coupling ranking with a higher non-dimensionalized value implies that the system requirement either helps to meet many stakeholder needs having a medium to high importance relationship where the requirement is difficult to achieve, or helps to meet a fewer and less important stakeholder needs and is less difficult to achieve. The normalized system requirement coupling rankings are allocated to the next level of mapping from system requirements to system variables in Phase II of the mapping, so that the system variables will inherit the coupling relationships from Phase I. Recall again that the goal is to categorize a variable's coupling with its variability so that variables with low normalized variability and low normalized coupling are identified as potential platform elements. The next 50 section discusses the methodology of Phase Il mapping from system requirements to system variables.
4.4.2 Mapping System Requirements to System Variables Figure 4.6 is a sample of Phase I mapping of system requirements to system variables. It does not contain all the system variables described in Chapter 4.3.3. The actual mapping included all the system variables listed in Table 4.4. The sample of normalized system variable coupling rankings shown at the bottom of the figure illustrates how system requirements drive system variables across all the engine modules.
System Variables Relative Irnportance Ranking (10=1-Igh ... 1=Low) Nornafked Burner / System Fan LPC HPC HPT LPT Requiement Diffuser
a S S -I 3 3 .2 Ii I I I I I is E 6 U - U .3 W I System r i i Requirements -I Weight 56% 62% 8 7 2 5 6 8 2 2 3 I 4 I3 4 2 5 5 3 6 Airplane Low Drag 91% 108% 10 integration Diameter Units 32% 36% 10 4 _____ Length Uits 18% 22% 5 Thrust 45% 48% 10 8 Performance Thust Specific Fuel 79% 82% 6 2 8 6 3 2 8 6 3 7 7 10 7 Coeun'plon (rSFC) I II I I III Performance Deerloraion Rake 44%- 49% 8 2 4 6 tnfight Shrlw Flats (I FSO) 54% 52% 2 1 2 ____ 58 RelIabilityRe~blily UnscdedFRais Eng(UEO ne Reno 67% 62% 2 1 1 5 2 8 Deka &Cancelsion Pats 54% 52% Emissions 62% 82% 5 8 1 Environmental Noise 88% 88% 5 8 Erwronmentaiy Friendly Design 18% 14% (Green Engine) I Recurring Cosle (Mnufacturing) 61% 57% 3 7 5 1 7 1 3 4 3 8 1 3 5 1 6 5 1 6
Costs Non-ecwng CostSpent> 100% 105% 7 3 2 2 2 1 1 4 2 3 2 3 3 2 4 2 2 2 Launch (Technology)I ______NLaunch -ecurring CostSpentfrom 67% 88% 4 t Certfic aion (E&D) 5 5 4 2 4 10 5 3 3 3 8 4 8 3 _ _ _ Tr li 16% 12% 10 Cycic Desin Life 62% 55% 5 5 Total Maidnenance Costs MC) 53% 47% 1 6 2 2 2 4 5 5 1 3 5 7 8 2 1 3 In-Service Une Replacedde Unt (Tm11e 39% 35% Operations RFlurblahment interval 47% 42% 7 2 2 4 4 7 2 Service Ready at Entry rinto 70% 88% 7
Normalized System Variable Coupling Ranki Short Range Mission 82% lO%138%145%132%18%118%113%134%188% 34%15%125%124%139%173%131% 1%137%12 Long Range Missionl87% 101% 137%147%132%110%118%113%133%188%133%15%125%124% 39%171% 30%142%137% 28%
Figure 4.6: Phase I1- Sample Mapping of System Requirements to System Variables 51
The normalized system variable coupling ranking is calculated using Equation 2. Normalized System Variable Coupling Ranking (i, m) = I {Normalized System Requirement Coupling Ranking (i, j)
* System Variable Relative Ranking (m, j)} (2) for i = 1 to total number of market segments m = 1 to total number of system variables j = 1 to total number of system requirements As in the mapping of system requirements to stakeholder needs, the values in the body of the matrix denote the relative importance of each system variable in achieving the corresponding system requirement with 1 denoting low importance and 10 denoting high importance. As in Phase 1,Advanced Engine Programs system engineers were consulted for appropriate ranking levels. Note that the normalized system requirement coupling rankings used in Equation 2 and which appear as columns in Figure 4.6 are the same as the rows at the bottom of Figure 4.4 from the Phase I mapping. This illustrates the deployment of system requirements to system variables from Phase I to Phase II. The normalized system variable coupling rankings of Figure 4.6 thus inherit the coupling characteristics associated with system requirements and stakeholder needs. The next section discusses the methodology for Phase IlIl of the mapping from system variables to module flowpath aerothermodynamic variables.
4.4.3 Mapping System Variables to Module Flowpath Aerothermodynamic Variables Figure 4.7 is a sample of Phase IllI mapping of system variables to module flowpath aerothermodynamic variables. It does not contain all the module flowpath aerothermodynamic variables described in Chapter 4.3.4. 52
Module Flowpath Aerothermodynamic Variables Relative Importance Ranking (10=High ... 1=Low) Normalftod Fwr Burner / HPT varhbm Fan LPC HPC Difsr HT Lr Couplng ADiffuser Raning -- A 0
.C 2 (_ 10..8 i C 2c
- --
System Variables Total Fan Corrected 62% 67% 9 System 100% 101% 9 1 1 10 10 _-igh-Low Spool Work S 't 10% 11% 6 8 -8 8 5 Desion EfficIenc 36% 37% 6 8 8 8 Fan I 45% 47% 6 3 8 3 Recurring Costs (Manufacturina' Desian EfficIenc 9% 10% 3 LPC a 18% 18% 3 2 5 2 Recurring Costs (Manufacturia' 13% 13% 9 2 EMfcec 34% 33% 6 2 2 1 1 3 4 HCDesio 68% 66% 9 2 2 _ ecurring Costs (Manufcturing) 34% 9 Burner /o Total Pressure Loss 5% 5% 6 8 25% 25% 3 6 6 6 Dfume I'ecurrin Costs (Manufacturina 24% 24% 3 5 10 4 HIRTDesio Effic"ec 39% 39% 6 5 73% 71% 6 4 8 8 Rcrrin Cot Manufacturina 31% 30% 6 Desio Efficien, 41% 42% 9 5--- LPT V 37% 37% 6 4 8 8 1 ecurrinu Costs (Manufacturina) 27% 26% 6 10 2 Module Development Difffculty Ranking (I 0=Hgh, 1=Low) 5 4 10 3 8 8
Normalized Module Flowpath Aerodynamic Variable Coupling Ranking Short Range Missiol 28%144%128%112%14% 12% 15%0%157 1011%8 Lnnx Qasws- UlMinjni IOWIrVI0QdoW-Ij0,q-.AV- 1 o~W 901 IO-12 113wl awlv [igo.w. I6%i2S~1w 1%15s 2w%183%
Module Flowpath Aerothermod ynamic Variable Levels Engine 1 x x x Lx x x x x x x x x x x x Engine 2 x x x x x x x x x x x x x x x x x x Engine 3 x x x x x X x x x x x x x x x x x x Engine 4 x x x x x x x x x x x x x x x x Engine 5 x x j lx x x x x x x x x x x x x x x Engine 6 x x x x x x x x x x x x x x x x x x Engine 7 x x x x x x x x x x x x x x x Engine 8 x x x x rtxox x x n x f x x ox Ix x x x x x
Module Flo pth Aerothermodynamic Variable Normalized Variation Mean x I x x x x x x x x x x x x x x x x Standard Deviation x I x I x I x I x I x Ix I x x IxIx x x x IxI x Ix x] Normalized Variation 1-x I x I x I x I x I x I x I x I x I x I x I x I x I x I x I x I x x
Figure 4.7: Phase IlIl - Sample Mapping of System Variables to Module Flowpath Aerothermodynamic Variables
The actual mapping included all the module flowpath aerothermodynamic variables listed in Table 4.5. The sample of normalized module flowpath aerothermodynamic variable coupling rankings shown at the bottom of the figure 53 illustrates how system variables drive module variables across all the engine modules. The normalized module flowpath aerothermodynamic coupling ranking is calculated using Equation 3.
Normalized Module Flowpath Aerothermodynamic Variable Coupling Ranking (i, n) = {I {Normalized System Variable Coupling Ranking (i, m) * Module Flowpath Aerothermodynamic Var. Rel. Ranking (n, m) * Relative Module Ranking (m)}} / Module Development Difficulty Ranking (p) (3) for i = 1 to total number of market segments n = 1 to total number of module flowpath aerothermodynamic variables m = 1 to total number of system variables p = 1 to total number of engine modules
4.4.3.1 Relative Importance of Module Flowpath Aerothermodynamic Variables The values in the body of the matrix denote the relative importance of the module flowpath aerothermodynamic variable in achieving the corresponding system variable with 1 denoting low importance and 10 denoting high importance. Advanced Engine Program system engineers provided initial ranking values, which were later validated by appropriate module technical experts. Note that the normalized system variable coupling rankings used in Equation 3 and which appear in the columns of Figure 4.7 are the same as the rows at the bottom of Figure 4.6 from the Phase 11 mapping. This illustrates the deployment of system variables to module flowpath aerothermodynamic variables. The normalized module flowpath aerothermodynamic variable coupling rankings of Figure 4.7 thus inherit the coupling characteristics associated with upstream system variables, system requirements and stakeholder needs. 54
As alluded to earlier in Chapter 3.3, the modified QFD mapping captures coupling effects between modules. One example illustrated in Figure 4.7 is where both HPT and LPT turbine cooling air impact HPC design efficiency, since the source of the cooling air is the HPC.
4.4.3.2 Relative Module Ranking Another way that the modified QFD mapping captures the coupling effect between modules is via the relative module ranking factor. This factor captures the impact that a particular module system variable may have relative to another module. A good example is the case described in Chapter 4.3.3 of how the HPT's design efficiency, with a rank of 6 in Figure 4.7, has a greater effect on the engine's TSFC than the LPC's design efficiency, which has a rank of 3. In this example, the relative module impact was derived from powerplant performance influence coefficients described in Chapter 3.3. Other relative module rankings were similarly derived from other information sources for weight, reliability, performance deterioration, refurbishment interval, total maintenance cost, non- recurring cost, and recurring cost as examples. The relative module ranking used in this investigation was based on the following scale . 9 = high relative importance . 6 = medium relative importance . 3 = low relative importance
4.4.3.3 Module Development Difficulty Ranking As discussed in the introduction, the difficulty in achieving different aspects of a product design may motivate an organization to consider platforming these elements to leverage the resources already expended across multiple products. For gas turbine engine design, the module development difficulty ranking factor is used to capture this difficulty on a module by module basis. Rankings of the assessed difficulty appear in Figure 4.7 and are based on resources expended during recent engine development programs. A rank of 1 denotes low difficulty while a rank of 10 denotes high difficulty. 55
4.4.4 Normalized Variation The quantification of coupling and difficulty through all 3 phases of the modified QFD mapping is now complete. The variation in actual levels of the selected module flowpath aerothermodynamic variables is needed in order to validate the hypothesis that system design variables with low normalized variation from design to design and low normalized coupling are potential candidates as platform elements. For the purposes of this investigation, the normalized variation of each variable for a set of designs was calculated using Equation 4. Normalized Variation (n) = Standard Deviation (n) / Mean (n) (4) for n = 1 to total number of module flowpath aerothermodynamic variables
For example, in the PW4000 case study discussed in Chapter 5, one of the parameters analyzed is HPT turbine cooling air level. The mean and standard deviation of HPT turbine cooling air levels is calculated for the four PW4000 engines considered in the case study. Dividing the standard deviation by the mean yields the normalized variation of this variable for the set of engines. Similarly, for the 8 engine case study, normalized variation of HPT turbine cooling air is calculated for the sample of eight parameter values.
4.4.5 Defining Boundaries for High and Low Normalized Coupling and Variation Once the normalized coupling and normalized variation are calculated for the set of module flowpath aerothermodynamic variables, they can be plotted in a manner consistent with Figure 1.1. This requires that the boundaries between low and high normalized coupling and low and high normalized variation be properly defined. For the purposes of this investigation, a first approximation of the boundary defining low and high normalized coupling can be calculated using Equation 5. 56
Normalized Coupling Arithmetic Mean (i) = {I Normalized Module Flowpath Aerothermodynamic Variable Coupling Ranking (i, n)} / Total Number Of Module Flowpath Aerothermodynamic Variables (5) for i = 1 to total number of market segments n = 1 to total number of module flowpath aerothermodynamic variables
This first approximation is merely the arithmetic mean of all the normalized module flowpath aerothermodynamic variable coupling rankings shown near the bottom of Figure 4.7. Likewise, a first approximation of the boundary defining low and high normalized variation was calculated using Equation 6 as the arithmetic mean of all the module flowpath variation factors shown at the bottom of Figure 4.7 for the engines considered in a particular sample study.
Normalized Variation Arithmetic Mean = {I Module Flowpath Aerothermodynamic Variable Normalized Variation (n)} / Total Number Of Module Flowpath Aerothermodynamic Variables (6) for n = 1 to total number of module flowpath aerothermodynamic variables
This completes the description of the modified QFD mapping framework that was used to determine potential platform elements for a commercial gas turbine engine. Chapter 5 discusses the results of two case studies performed to validate the hypothesis that module flowpath aerothermodynamic variables with low normalized coupling and low normalized variation should be considered as potential platform elements. 57
5 Pratt & Whitney Case Studies
To demonstrate the viability of using the modified QFD framework to determine potential platform elements, a four-step approach was undertaken. The first step involved surveying appropriate experts supporting conceptual design studies for Pratt & Whitney's Advanced Engine Programs for appropriate relative importance rankings needed to populate the QFD mappings described in Chapter 4.4. These rankings were required to assess the coupling and difficulty of module flowpath aerothermodynamic variables and upstream stakeholder needs, system requirements and system variables. The second step involved obtaining and calculating the normalized variation of values of stakeholder needs, system requirements, system variables, and module flowpath aerothermodynamic variables from a sample of 8 engine designs. The third and fourth steps involved performing two case studies, one for an 8 engine sample, and the other for a subset of engines comprising the PW4000 family of engines. The 8 engines were chosen for their wide variety of thrust level capability from 24,000 to 98,000 pounds for both short and long range missions. In contrast, the PW4000 family provided thrust capabilities in the high end range between 52,000 and 98,000 pounds, and exclusively for long range missions on widebody airplanes. The goal of performing these two case studies was to compare and contrast predicted platform elements in each case and to validate that the model appropriately predicted platform elements in the PW4000 case, where deliberate system architecture decisions were made to platform certain elements of the engine. Platform elements were identified by plotting normalized coupling versus normalized variation of module flowpath aerothermodynamic variables and using the methodology discussed in Chapter 4.4.5 to determine the boundaries between low and high normalized coupling, and low and high normalized variation. Potential platform elements were predicted to cluster in a region of low normalized coupling and low normalized variation, while non platform elements in 58 a region of high normalized coupling and high normalized variation according to Figure 1.1. Discussed next are the results from the first case study of the 8 engine sample.
5.1 Sample of 8 Engines This case examines potential platform elements in the 8 engine sample. Because there was a mix of both short and long range mission engine designs, a weighted average of the module flowpath aerothermodynamic coupling rankings was used. In other words,
Weighted Average of Normalized Module Flowpath Aerothermodynamic Coupling Ranking (n) = Short Range Mission Normalized Module Flowpath Aerothermodynamic Coupling Ranking (n) * {Number of Short Range Engine Designs / Total Number of Engines in Study}
Long Range Mission Normalized Module Flowpath Aerothermodynamic Coupling Ranking (n) * {Number of Long Range Engine Designs / Total Number of Engines in Study} (7) for n = 1 to total number of module flowpath aerothermodynamic variables
Short and long range mission normalized module flowpath aerothermodynamic variable coupling rankings used in Equation 7 were obtained from the Phase IlIl mapping. Figure 5.1 illustrates the normalized coupling and normalized variation of the 56 module flowpath aerothermodynamic variables for the 8 engine sample. 59
100% Arithmetic Mean of all Module Flowpath Aerothermodynamic 90% Variables' Normalized Variation = 17% 0 80%
C 70% 60% 50% " Arithmetic Mean of all Module Flowpath 40% * Aerothermodynamic Variables' Normalized Coupling = 16% 0 2. 30% 20%
10% * t . * * no/ 0% 10% 120% 30% 40% 50% 60% 70% 80% 90% 100% Normalized Variation (Standard Deviation / Mean)
Figure 5.1: Normalized Coupling and Normalized Variation for the 8 Engine Sample (Module Flowpath Aerothermodynamic Variables)
The boundary between low and high normalized coupling was calculated to be 16% using Equation 5, while the boundary between low and high normalized variation was calculated to be 17% using Equation 6. At first glance, the levels of normalized variation appeared to be rather high, but not unexpectedly so, since this case study involved engines providing a wide variety of thrust levels between 24,000 and 98,000 pounds for both short and long range missions. Figure 5.2 lists the actual module flowpath aerothermodynamic variables found in the four quadrants of Figure 5.1. 60
Figure 5.2: Module Flowpath Aerothermodynamic Variable Classifications for the 8 Engine Sample
Design variables that appear in Quadrant I are those that the model would recommend as platform elements because of their low normalized coupling and low normalized variation. Recall that normalized coupling is defined as the quotient of a particular variable's coupling and how difficult it is to achieve that 61 variable level. Elements with low coupling and low difficulty are as likely to appear in Quadrant I as elements with high coupling and high difficulty. Figure 5.2 shows that a large number of design variables evenly distributed across all the engine modules appear in Quadrant I and are predicted to be part of a platform, with no significant clustering in one particular module versus another. In contrast, module flowpath aerothermodynamic variables in Quadrant IV are those that the modified QFD framework would classify as non platform elements. The high normalized variation of these parameters and their high normalized coupling suggest that they not be considered as platform elements and be allowed to vary as appropriate in order for each engine to meet its particular stakeholder needs and system requirements. For example, the number of LPC, HPC, HPT and LPT stages are Quadrant IV variables that directly allow engine thrust variety, and as such, are correctly predicted by the model as variables not to be platformed. Although this case illustrates trends for why certain module flowpath aerothermodynamic variables are not to be considered as platform elements in Quadrant IV, there is no compelling evidence of why the variables in Quadrant I should be considered as platform elements, especially since they are evenly distributed across all engine modules and their average level of variation is 17%, which seems high. In addition, it is not clear that even if these variables and their average levels are assumed constant for a platform scenario and each engine design is re-evaluated assuming these average levels, that each design would still satisfy its respective stakeholder needs and system requirements. Again, the normalized variation levels may be too high for this to be feasible. Intuitively, the best case for platform elements is where there is no coupling, low difficulty and no variation. Consequently, the criteria for defining an appropriate boundary between low and high normalized coupling and normalized variation can certainly be further refined by specifying lower arithmetic means. However, an actual product platform implementation is needed as a benchmark to provide the basis for this refinement. The PW4000 family of engines provides such a benchmark perspective on an appropriate range of normalized coupling 62 and normalized variation for platform elements. The PW4000 is a case where deliberate system architecture decisions were made to platform certain elements of the engine.
5.2 PW4000 Engine Family
5.2.1 Background Pratt & Whitney implemented a product family strategy for the PW4000 engine series that provides thrust between 52,000 and 98,000 pounds. The PW4000 is the only family of engines that powers all current widebody airplanes. Table 5.1 lists all current PW4000 powered airplanes.
Table 5.1: PW4000 Family of Engines
Airplane (A=Airbus, Rated Fno maiain gt tEngine Tske Diameter, Engines MD=Douglas Thrust' an ses per Div. of poundsea plais Boeing) A300 PW4158 58,000 94 2 A31 0 PW4152 52,000 94 2 Prat Witey'A330 ai taeyfrmeingPW4168 68,000 hihero aefftrsfeeswst100 2
B767 P46 2009 B777 P40 4/09 84-98,000 12
Pratt & Whitney's basic strategy for meeting higher takeoff thrust needs was to maintain commonality to the extent possible in the engine core or high spool consisting of the HPC, burner and HPT, while modifying as appropriate, the low spool consisting of the fan, LPC and LPT. The growth strategy involved increasing fan diameters and the number of LPC and LPT stages. Since the high and low spools are mechanically decoupled in the sense that each spool is connected to a different shaft, these changes were possible even though there was still aerothermodynamic coupling between the spools. 63
Part of the rationale for the common core strategy had to do with the effort associated with developing various modules of the engine. The motivation for platforming elements with high difficulty is the potential savings for not having to go through some or all of the costly development process for each successive member of the engine family. Because the engine core, and especially the HPC, required extensive development cost and effort, the PW4000 strategy was to leverage a common core across multiple products. From a product development perspective, the difficulty factor used in this modified QFD framework was quantified in terms of development effort and cost for each of the engine modules and is captured by the module development difficulty ranking illustrated near the bottom of Figure 4.7. By not having to develop new cores for every new engine model in the family, the company would be able to reduce non-recurring development costs, reduce recurring costs from common parts and economies of scale, decrease time to develop, reduce time to market, and leverage common design knowledge to produce evolutionary designs with higher reliability. The reader should recall that these benefit categories appear as Pratt & Whitney's stakeholder needs listed in Table 4.1 and are important drivers in its product strategy and system architecture decisions. Other stakeholders could also benefit from this platform product portfolio strategy. Airline customers would benefit if they owned multiple PW4000 powered airplanes by having common parts, which would result in lower tooling costs, lower inventory carrying costs, lower TMC from common maintenance procedures, and more reliable operation with derivative products. More reliable operation would also satisfy regulatory agencies such as the FAA. Many stakeholders would thus benefit from a product platform strategy.
5.2.2 PW4000-94" Platform Strategy The PW4000-94" is the original engine in the family and is an example where the engine's turbomachinery and support structure can be considered the platform because it is common regardless of whether it powers the A300, A31 0, 64
B747, B767 or MD1 1. Differences in the engine design arise in external components that interface with different airplanes such as mount locations and air system off-takes. Variety is thus provided by externals design and the EBU for each airplane installation, while the engine's internal turbomachinery and support structure can be considered the platform. This platform portfolio strategy is analogous to the topological design strategy of the Sony Walkman. In the case of the PW4000-94", topological design changes providing product variety are achieved via different external designs required for different airplane applications. In addition to the EBU, the ability of the PW4000-94" to provide thrust variety with the same turbomachinery and support structure for different airplane applications is accomplished by what is known as a programming plug in the engine's full authority digital electronic control (FADEC). The modular architecture of the FADEC and programming plug is a bus type [Ulrich, 1995] and allows the PW4000-94" to generate between 52,000 and 62,000 pounds of thrust with minimal physical changes to the engine. This programming plug contains information used by the FADEC to direct appropriate engine operation depending on the airplane installation. For example, if a pilot of a PW4152 powered A31 0 advances the throttle to full rated takeoff power, the engine will generate 52,000 pounds of static takeoff thrust. In contrast, if a pilot of a PW4056 powered B747 advances the throttle to full rated takeoff power, the engine will generate 56,000 pounds of static takeoff thrust. The use of the programming plug to provide variety in thrust level can be considered an enabler for a mass customization portfolio architecture, where the programming plug is the adjustment variable that allows variety in thrust level. This should not be confused with the topological design change platform portfolio strategy discussed above that allows the engine to be used on different airplane installations with different EBU's. Clearly, the PW4000-94" used a combination of topological design change and mass customization portfolio architecture strategies to satisfy market needs. 65
In the example of the PW4152 versus the PW4056, the same turbomachinery will generate different levels of thrust via FADEC control system commands for different amounts of fuel to be supplied to each engine. The PW4000-94" was originally certified to provide 60,000 pounds of static takeoff thrust. Relative to the original certified thrust level, lower thrust levels such as that required for the PW4152 and PW4056 are achieved through thrust derate, where less fuel is consumed relative to the 60,000 pound thrust class engine. In contrast, a higher thrust of 62,000 pounds of static takeoff thrust delivered by a PW4062, which powers the B767, is achieved through throttle bending, where more fuel is consumed to produce higher thrust. Throttle bending results in higher rotational speeds, increased airflow and hence increased thrust. An undesirable side effect of throttle bending is shorter engine part lives due to the higher operating temperatures and stresses. Likewise, thrust derate results in longer part lives and lower TMC. There are limitations to this portfolio architecture strategy of providing thrust variety with more or less fuel addition. One limitation is the materials technology that allows the engine to operate at hotter flowpath temperatures given the additional fuel that is burned. Higher operating temperatures require more exotic and costly materials. In addition, higher strength shaft materials are needed to handle the increased torque. Increased thrust capability can be achieved by throttle bending only up to a certain point. After this point, it may become an unattractive strategy considering available materials technology and high recurring costs that may be associated with the advanced materials, hence the tension between system requirements as discussed in Chapter 4.4.1.2.
5.2.3 PW4000-1 00" and PW4000-112" Growth Strategy Another alternative for achieving higher thrust levels is to increase the diameter of the fan. This increases the amount of flow that can be used to generate thrust. In the case of the PW4000-94", increasing fan diameter to achieve higher thrust was not a viable strategy for the airplane applications it was being considered for, because as a replacement for the Pratt & Whitney JT9D- 66
7R4 which powered existing B747 and B767 airplanes, the new engine was constrained to fit within an existing nacelle. The nacelle constrained the fan diameter, while having to use existing pylon mounts constrained the engine length at certain locations. Increasing fan diameter was, however, the design strategy for the PW4168, which was based on the same core as the PW4000-94". The PW4168 has a 100 inch fan diameter. Compared to the PW4000-94", it has an additional LPT stage to provide power to drive the larger fan as well as an additional LPC stage to provide a higher overall pressure ratio across the entire compression system of both the LPC and HPC. Adding an additional LPC stage is referred to as supercharging where more flow is pumped from the LPC to the HPC. The strategy was to keep the PW4168 core (HPC, burner, HPT) common with the PW4000-94" core and provide additional thrust capability with the larger diameter fan and additional LPC and LPT stages. A similar strategy was followed for the PW4084 which has a 112 inch fan diameter.
5.2.4 Validating the Hypothesis with the PW4000 The PW4000 engine family is a case where there was a deliberate effort to implement a platform portfolio architecture strategy. To validate the hypothesis that system design variables with low normalized coupling and low normalized variation are potential candidates as platform elements, PW4000 module flowpath aerothermodynamic design data were analyzed and normalized variation was calculated using Equation 4. Equations 5 and 6 were used to define the boundaries between low and high normalized coupling and normalized variation, respectively. If the model is indeed valid, then a plot of normalized coupling versus normalized variation should reveal platform elements consistent with the design choices that were made. Figure 5.3 is a plot of normalized coupling versus normalized variation of the 56 module flowpath aerothermodynamic variables for the PW4000 case. 67
100% Arithmetic Mean of all Module Flowpath 90% - -Aerothermodynamic Variables' Normalized Variation = 7.8%/ 80%
C 70% 60% 0 50% C + Arithmetic Mean of all Module Flowpath Aerothermodynamic[ 40% e 4 Variables' Normalized Coupling = 16% 30% N 0 20% .5 10% 0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Normalized Variation (Standard Deviation / Mean)
Figure 5.3: Normalized Coupling and Normalized Variation for the PW4000 (Module Flowpath Aerothermodynamic Variables)
The boundary between low and high coupling was calculated to be 16%, while the boundary between low and high normalized variation was calculated to be 7.8%. Figure 5.4 lists the actual module flowpath aerothermodynamic variables found in the quadrants of Figure 5.3. 68
Figure 5.4: Module Flowpath Aerothermodynamic Variable Classifications for the PW4000
As in the 8 engine sample study discussed in the previous section, the model clearly differentiates module flowpath aerothermodynamic variables in Quadrant 69
IV associated with the LPC and LPT that should not have been considered as platform elements. Indeed, this result is consistent with the case of the PW4000 growth strategy for the PW4000-1 00"/112" where higher thrust levels were achieved by adding LPC and LPT stages. In addition, because burner exit temperatures were also increased to improve fuel efficiency at the higher thrust levels, HPT and LPT turbine cooling air levels had to be increased to protect the airfoils from the higher operating temperatures. The issue of the number of fan blades being in Quadrant IV is misleading because of the use of shrouded fan blades for the PW4000-94"/1 00" versus the use of hollow, shroudless fan blades in the PW4000-112". Fewer blades are required for a shroudless fan configuration than for a shrouded fan configuration, hence the high normalized variation. If the normalized variation is ignored, then the high normalized coupling suggests that the number of fan blades could still be a risky Quadrant Ill variable. Quadrant I of Figure 5.4 shows several module flowpath aerothermodynamic variables that could be considered as platform elements. As in the case of the 8 engine sample, the fact that these variables span the entire engine is not entirely unexpected due to the mechanical and aerothermodynamic coupling between and among modules as well as the system level coupling between these variables and upstream needs and requirements. Upon closer analysis, a number of HPC variables were found to have less than 4.5% normalized variation. Again, in hindsight, this was due to the deliberate strategy to maintain a common HPC and core. If the boundary between low and high normalized variation is set to this 4.5% level, rather than the arithmetic mean of 7.8%, then Figure 5.3 becomes Figure 5.5. 70
100%
90% ---- Absolute Level of Normalized Variation = 4.5%] 80% Arithmetic Mean of all Module Flowpath 70% Aerothermodynamic Variables' Normalized Variatin = 1% Gb 60% 0I 50% Arithmetic Mean of all Module Flowpath Aerothermodynamic 40% -- 4 4 Variables' Normalized Coupling = 16% 0 30%
'OAO/L -I E 10% 0 Z 0% 0 % 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Normalized Variation (Standard Deviation / Mean)
Figure 5.5: Normalized Coupling and Normalized Variation for the PW4000 with a 4.5% Normalized Variation Threshold (Module Flowpath Aerothermodynamic Variables)
Figure 5.6 illustrates that with the assumed 4.5% normalized variation threshold, a number of fan, LPC, HPT and LPT variables in Quadrants I and Ill move to Quadrants I and IV, respectively. 71
Figure 5.6: Module Flowpath Aerothermodynamic Variable Classifications for the PW4000 with a 4.5% Normalized Variation Threshold
Figure 5.6 also indicates like Figure 5.4, that there are only 3 HPC module flowpath aerothermodynamic variables that are considered at risk in Quadrant Ill because of their high normalized coupling. A closer analysis of the actual levels of their normalized coupling indicates that HPC variables of airfoil gap/chord ratio and corrected tip speed are at most, 2.5% greater than the arithmetic mean of 16%. This suggests that except for the number of HPC stages, these two variables along with all the other HPC module flowpath aerothermodynamic 72 variables found in Quadrant 1,can be considered as platform elements with less than 18.5% normalized coupling and less than 4.5% normalized variation. The model thus presents compelling results which recommend that the HPC should be the platform. As far as the number of HPC stages, its high normalized coupling ranking suggests that it is critical to several stakeholder needs, system requirements and system variables. Although the number of HPC stages was kept the same for all the PW4000 engine models, the modified QFD mapping framework predicted that it was risky to do so, since the number of HPC stages was categorized as a Quadrant III variable. Recall that there is risk for such a variable to move from Quadrant IlIl to Quadrant IV due to some unanticipated growth potential or condition. In the case of the 8 engine sample illustrated in Figure 5.2, the model predicted that the number of HPC stages is also categorized as a Quadrant Ill variable that should not be considered a platform variable. These two results are consistent. Not as many burner/diffuser and HPT variables comprising the rest of the core appeared as predicted platform elements in Quadrant I as anticipated. The majority of HPT variables appeared in Quadrant 11 with low normalized coupling, but high normalized variation. A plausible explanation for this is that these module variables acted as slack or adjustment variables for the entire core. Recall that the HPC, burner and HPT are coupled modules that comprise the core. Given that conditions upstream of the HPC have changed in the PW4000- 100"/112" due to supercharging with additional LPC stages and given the strategy to keep the HPC as similar as possible, the burner and HPT tended to act as slack modules which allowed nominal operation. The normalized variation arithmetic mean for the PW4000 case was nearly half that for the 8 engine sample. The trend of lower normalized variation for the PW4000 case was expected because of the deliberate choice to platform certain elements of the engine. The PW4000 case analysis also suggests that an appropriate level at which to set the boundary for a platform scenario is 4.5% for normalized variation and 18.5% for normalized coupling. 73
It is interesting to note that if the 4.5% normalized variation threshold level is applied to the 8 engine sample, only 6 out of the 56 variables in that case are recommended by the model to be platform elements compared to the 19 out of 56 platform variables in the PW4000 case, with 10 out of the 19 being from the HPC. Again, the lower number of platform variables for the 8 engine sample is expected, since no historical attempt was made to platform them. 74
6 Summary and Conclusions
This investigation was based on the premise that traditional methods for identifying platform elements could not be appropriately applied to commercial gas turbine engines because of the high degree of mechanical and aerothermodynamic coupling between and among the various engine modules. Therefore, an alternative framework was employed to quantify a truly system level coupling, which accounted not only for mechanical and aerothermodynamic coupling, but also coupling between design variables and both stakeholder needs and system requirements. A modified QFD mapping process was used to identify platform elements from among these design variables that exhibited low normalized coupling and low normalized variation. Actual design data from a sample of 8 Pratt & Whitney engine designs that included the PW4000 engine family were used to exercise and validate the model. Actual design choices in the case of the PW4000 validated model predictions that the HPC should be considered the platform for a commercial gas turbine engine based on the low normalized coupling and low normalized variation of its associated module flowpath aerothermodynamic variables. Although the burner and HPT are also considered part of the engine core along with the HPC, the model suggests that the former modules be classified as slack modules, whose module flowpath aerothermodynamic variables are allowed to vary so that the engine system can meet stakeholder needs and system requirements. Results suggest that an average normalized coupling level of less than 18.5% and an average normalized variation level of less than 4.5% be used to define the boundaries for potential platform elements. The fact that model predictions agreed with the design choices made for the PW4000 suggests that the model accurately represents Pratt & Whitney's design philosophy. The design philosophy is captured by the importance relationships and difficulty rankings in the modified QFD mapping. A change in design philosophy would presumably change the normalized module flowpath 75 variable coupling rankings and perhaps predicted platform elements. The methodology presented here can thus capture the effect of design philosophy changes on platform recommendations. Although the framework presented here was exercised with existing conceptual design data and validated with historical design choices, it does illustrate the potential for such a methodology to be used in designing new product platforms based on stakeholder needs and system requirements. Again, the key is to consider system design variables with low normalized coupling and low normalized variation as potential platform elements. The next chapter suggests how this methodology could be implemented in the conceptual design process to determine elements of a product platform along with other recommendations on how the strategic management of a company can influence its product strategy. 76
7 Recommendations
Recommendations are provided for future work that may help Pratt & Whitney refine its product strategy and design philosophy. A methodology for using the framework presented here as a conceptual design tool for further platform analyses is discussed. In addition, extending the analysis to include additional QFD mappings to key support structure part characteristics and manufacturing processes is recommended to identify platform elements at those levels. An extension of this framework to other Pratt & Whitney business segments such as military, small commercial and industrial gas turbine engines is recommended to explore the potential for platform elements to be leveraged across a wider variety of products, and not just large commercial engines. Product platform strategy can also be viewed from the overall strategic management of the company. It is from this perspective that other factors influencing system architecture and product platform strategy can be analyzed. Multi-project management is discussed as a portfolio planning strategy where products developed concurrently can also share development resources and platform elements. Finally, recommendations are made for strategic analyses of the company's core competencies and in terms of Porter's five forces model.
7.1 Conceptual Design Tool The modified QFD mapping proposed in this investigation captures the basic design philosophy of Pratt & Whitney based on stakeholder needs and system requirements. Stakeholders include not just Pratt & Whitney, but also airlines, airplane manufacturers and regulatory agencies. Presumably, changes in stakeholder needs would manifest themselves as changes in the design philosophy and the relative importance and difficulty rankings in the modified QFD mapping process. Assuming a constant design philosophy, the coupling rankings can be used as a tool during the conceptual design process to evaluate elements of new product platforms. 77
By considering actual conceptual design data for a set of new product designs, the normalized variation for the key module flowpath aerothermodynamic design variables identified in this investigation can be calculated and plotted against the corresponding normalized coupling ranking as in Figure 1.1. For potential platform elements predicted by the model, an average or weighted average value can be calculated for each variable that will be later used to re-evaluate the designs. An appropriate weighting factor for each variable level, w(q), could be based on the expected number of engine sales multiplied by some profitability factor per engine using Equation 8.
w(q) = s(q) *p(q) (8) where w = weighting factor s = expected number of engines sold p = profitability factor per engine for q = 1 to number of engine designs in sample study
Each of the conceptual designs would then be re-evaluated assuming a constant weighted average platform variable level, X(r), calculated from Equation 9 to determine whether or not individual system requirements are still satisfied.
X(r) = X{x(q,r) * w(q)} / I w(q) (9) where X = weighted average level of module flowpath aerothermodynamic variable x = nominal level of module flowpath aerothermodynamic variable for q = 1 to number of engine designs in sample study r = 1 to number of module flowpath aerothermodynamic variables being considered as part of the platform 78
If system requirements cannot be met, then a negotiation process may take place to arrive at a mutually optimal level for the particular module flowpath aerothermodynamic variable in light of different mission constraints. If system requirements are met, then those elements can be considered part of a platform. An iterative process of platform analysis and re-evaluation of system requirements can then lead to common platform elements across multiple products.
7.2 Extensions of QFD Mapping The modified QFD mapping methodology used in this investigation focused on the three phases shown in Figure 4.2. Platform elements were then investigated at Phase IlIl for module flowpath aerothermodynamic variables. In contrast, the traditional QFD suggests two additional phases of mapping as shown in Figure 4.1, namely for key process operations and production requirements. It is recommended that the modified QFD mapping introduced in this investigation be extended to key process operations. Mapping to production requirements which includes detailed information about quality control charts, preventive maintenance, job instruction availability and others is not recommended because this information is geared towards managing the production process and not particularly useful for platform analyses during the conceptual design process. The recommended mapping strategy is illustrated in Figure 7.1. 79
Conflict System System Module supwn, soucum Part Key Process Requirements Varables Flowpath Aero Cho Operations
-- S
Importance Imp or t --I Importance L Importance g n Importance n ingkey pa Difficultya tics Difficulty Diff iculty Variation T r pon Variati variation
[eomed Phshase IV Phase IV Phase V 7..CapnPhRequirements I oSupr.tutr5ar System Variables Module Flowpath hrceitc Support Structure Part Key Process elome n Deployment Variables Deployment Deployment Operations
Figure 7.1 : Extension of Modified QFD Mapping for Platform Elements
7.2.1 Mapping to Support Structure Part Characteristics It is important to point out that the traditional QFD mapping in Figure 4.1 involving key part characteristics has actually been decomposed into two phases in the modified approach illustrated in Figure 7.1. One phase was for module flowpath aerothermodynamic variables, which was completed in this investigation. The other phase is support structure part characteristics that is recommended as Phase IV in Figure 7.1. Phase IV involves mapping both system variables and module flowpath aerothermodynamic variables to support structure part characteristics for disks, seals, shafts, cases and bearing compartments that support the flowpath turbomachinery. Included as a support structure part characteristic is materials selection, which is a key aspect of gas turbine engine design, since parts must be capable of operating in environments of extreme temperatures, pressures and stresses required to achieve system requirements. The use of different materials may in turn impact the manufacturing processes used. Mapping to key process operations in Phase V of Figure 7.1 is therefore also recommended. 80
7.2.2 Mapping to Key Process Operations Mapping to key process operations occurs in Phase V and involves manufacturing processes that are required for both flowpath and support structure parts. Because successive levels of mapping inherit coupling rankings, the relationship between key process operations all the way back to stakeholder needs is defined. This information is critical because it means that upstream needs and system requirements such as TSFC, can be shown to influence support structure characteristics such as material selection, case thicknesses, surface treatments as well as others. Flowpath and support structure part characteristics subsequently drive key process operations like grinding, laser drilling, broaching and others. A successful platform strategy will presumably include identical parts, similar parts or a family of parts with similar key characteristics that allow them to be manufactured with the same capital equipment and tooling. Ideally, using the same capital equipment and tooling to produce identical or similar parts would result in economies of scale and reduce piece part costs. Hence, having identical or similar parts of a platform that are shared among many products can help reduce overall recurring costs of manufacturing. The key then, is to determine which parts or group of parts can be kept common or similar while stakeholder needs are still satisfied. This determination can be made if the mapping strategy shown in Figure 7.1 is followed.
7.3 Other Applications
7.3.1 Value Engineering Although the purpose of the modified QFD mapping process in this investigation was to serve as the framework for identifying potential platform elements, it can also be used as it was originally intended, which is to deploy the voice of the customer to the factory floor. In this case, it may fall under the purview of value engineering. However, before it can be used in this way, Phases IV and V mapping discussed in the previous section must be completed. 81
The mapping may also need to be completed on a finer scale than the system level approach used in this investigation. For example, Phase IV mapping to support structure part characteristics should be accomplished at the detailed piece part level rather than on an overall module level. The same is true for key process operations, which needs to be defined on the part level.
7.3.2 Military, Small Commercial and Industrial Engines This investigation focused exclusively on large commercial gas turbine engines. A similar mapping of stakeholder needs and system requirements illustrated in Figure 7.1 can be completed for Pratt & Whitney's other business segments, such as military engines, small commercial engines and industrial gas turbine engines. Stakeholder needs could be so varied in these different segments that they could drive system requirements and hence design variables to different recommendations for what should be considered part of a platform. However, there may be areas of similarity across large and small commercial engines, military engines and industrial engines that could lead to a synergistic system architecture for Pratt & Whitney's entire product portfolio. Being able to leverage a platform across a wide variety of product offerings would provide many benefits.
7.4 Multi-Project Management as a Portfolio Planning Strategy Cusumano and Nobeoka [1998] performed a study of the automobile industry and examined various aspects of multi-project management where sharing resources as well as key common components across different product development efforts allowed savings in development and production costs. This represented a shift from previous trends of single-project management where there was little sharing from one product to the next. They discussed how multi-project management is significant in an environment of slowing growth and lowered profits where companies can no longer afford frequent investment in new product designs. In fact, Cusumano and Nobeoka found that companies that utilized concurrent technology transfer were not only able to increase their market share at a rate 2.5 times that of 82 companies that practiced sequential technology transfer, but they were also able to achieve a 35% higher product introduction rate as well. Concurrent technology transfer refers to the practice of platform teams sharing technology across multiple products that are developed concurrently, while sequential technology transfer is where one product is completed, and the next development program begins and attempts to use elements common to the previous product. A good example of concurrent technology transfer at Pratt & Whitney was the PW4168 and PW4084 development programs which overlapped in the early 1990's. Development engineers for the PW4000 program were collocated with each other to facilitate this concurrent technology transfer. As a result, there are many commonality aspects between the PW4168 and PW4084 engines. Given the demonstrated benefit of multi-project management and concurrent technology transfer, it would be worthwhile to examine similarities and analogies between the automobile and gas turbine engine industries. One must be aware, however, of the differences between the two industries. A few differences are listed below for consideration.
7.4.1 Push versus Pull Market The automobile industry can be characterized as a push industry where automobiles are produced and pushed onto a consumer mass market. A new automobile program is launched without necessarily having customers sign up for firm purchases, although marketing studies would have presumably confirmed the level of demand prior to launch. In contrast, the gas turbine engine industry can be categorized as a pull industry where new products are introduced only when there is sufficient demonstrated demand. Typically, a gas turbine engine development program will be launched only after a specified number of firm orders have been received from airline customers. Unlike automobiles, airplane/engines are not manufactured and stored on a lot until a customer purchases them, hence the phrase "order backlog." 83
With the gas turbine engine business being a pull type industry, it may be challenging for a company to deliberately execute concurrent product development for a platform strategy since program launch is contingent upon the timing of sufficient demand. Sequential product development may occur as a result of the lag between successive engine development programs. This may precipitate a tendency to diverge from the basic platform and infuse the latest technology into engines. Given these market conditions, defining and executing a platform strategy is indeed a challenge.
7.4.2 Product Lifetime & Certification Costs Gas turbine engine product lifetimes are on the order of twenty to thirty years, while automobile model lifetimes are less than half that long. Life cycle cost thus plays a key role in gas turbine engine platform strategy and design. Because of the long product lifetimes, there may be a motivation to get the latest technology into the engine at entry into service because it needs to last for the next twenty to thirty years. Post certification engineering (PCE) budgets for product improvements are limited, since the large majority of funds go into new engine development. Product improvements after the engine is originally certified by the FAA as flightworthy need to be re-certified before airlines are allowed to incorporate these improvements into their fleets. Engine testing, validation and re-certification of a product improvement is a costly process which may motivate a divergence from a platform strategy.
7.4.3 Production Volume Current annual commercial gas turbine engine production is on the order of less than one thousand units for all of Pratt & Whitney's commercial gas turbine engine offerings, while annual automobile production may be on the order of 100-300,000 vehicles for a single model alone. There is a high level of automated assembly for automobiles, while assembly of gas turbine engines is entirely manual. Production levels as well as level of automated assembly will have implications on the benefits of platform thinking. 84
7.4.4 Level of Technology Capability Gas turbine engine technology has allowed thrust capabilities to almost double in a span of a little over 10 years. The original PW4000-94" certified in 1986 could achieve 60,000 pounds of thrust, while the PW4098 certified in 1998 could achieve 98,000 pounds of thrust. This was due in large part to advances in hollow, shroudless fan technology for larger diameter fans, advances in materials technology allowing higher operating temperatures for better performance, manufacturing processes as well as advanced analytical design tools. With such an advancement in thrust variety, it is a challenge as well as an opportunity to share common platform elements across these engines as discussed in this investigation. It would be interesting to understand the level of technology capability infused into automobiles over the recent past and how a platform strategy emphasizing shared use is reconciled with technology advances or improvements in new products over time.
7.5 Strategic Analysis Perspectives related to the strategic management of the firm can also provide insights into other factors that can influence system architecture and product platform strategy. Discussed below are the concepts of core competencies and Porter's five forces model.
7.5.1 Core Competencies & the Organization The quantification of system requirement difficulty discussed in Chapter 4.4.1.4 points to the importance of identifying an organization's core competencies [Prahalad and Hamel, 1990]. Meyer and Utterback [1993] proposed an equally viable method for identifying a firm's core competencies within four basic dimensions: product technology, understanding of customer needs as reflected by products sold at that time, distribution and manufacturing. Sustained success with a product platform strategy is fostered by the firm's underlying core capabilities and its continuous renewal. 85
Once the firm's core competencies have been assessed, an appropriate organizational strategy can be formulated in light of three perspectives, namely strategic design, political and cultural. The premise is that analyzing the organization from these three perspective can provide an understanding of the interrelationships between the way an organization is structured, its politics, and its culture [Ancona et al., 1996]. Knowing the state and dynamics of an organization is the first step in positioning it to successfully execute a platform strategy. In the case of product platform strategy, it would be beneficial to investigate whether or not a company is organizationally prepared to execute such a strategy. One particular area that might be of interest is Pratt & Whitney's newly formed module centers. Pratt & Whitney recently reorganized the development organization into what are known as module centers to bridge the gap between design and manufacturing. Each module center is responsible for all aspects of the design, development and manufacture of a particular module, such as the compressor or turbine, across all the engines in Pratt & Whitney's product portfolio. An analysis of the new organization could reveal opportunities for promoting platform strategies and optimizing their benefits.
7.5.2 Porter's Five Forces Model Pratt & Whitney's balanced scorecard approach for defining appropriate stakeholder needs includes airlines, airplane manufacturers, regulatory agencies and Pratt & Whitney itself. This certainly makes decision making and product design tradeoffs more challenging because of the multiple perspectives that need to be taken into account. There are however other perspectives from the strategic management of the company that are equally important and are embodied in Porter's five forces model [Oster, 1994]. A detailed five forces analysis of the firm can help illustrate strategic issues which may influence product platform architecture decisions. 86
The five forces which can influence a company's strategy are listed below in Table 7.1 along with the appropriate constituents in the gas turbine engine industry. Table 7.1: Porter's Five Forces
Five For ces Constituents Customers Airlines, Airplane Manufacturers Suppliers Materials, Piece Parts, Modules, Externals, Accessories
Competitors General Electric, Rolls Royce Substitutes Trains, Buses, Automobiles
BrIrie a n t r T e ch n o l e M ,fiINrRt m o M e , E nt I nt D o fvc
Below is a brief description of each category and recommendations on how each can be analyzed in the context of influencing system architecture and platform strategies.
7.5.2.1 Customers Customers include airlines as the end user as well as airplane manufacturers as the system integrator of the entire airplane where the engine is a subsystem. Both airline and airplane manufacturer needs were discussed earlier in Chapter 4.3.1, while Figure 4.4 illustrated how needs drove system requirements. A more accurate assessment of customer needs and market segment be importance rankings used in the Phase I mapping shown in Figure 4.4 could accomplished via a conjoint analysis [Dolan, 1990; Green and Wind, 1975]. This methodology allows customer preferences for different product attributes or one performance levels to be captured. The customer's utility or preference for set of attributes or performance levels over another provides insight into how much more valuable one is relative to another and over what range of levels he or she would be indifferent. One pertinent example is the issue between airplane mission and cash operating cost. Would a customer tolerate an engine design 87 that may fall short of the design range, but be cheaper to operate? How much of a range shortfall would they be willing to tolerate until being cheaper to operate is no longer an attractive tradeoff. Answers to questions like these could provide insight into avenues of design flexibility that would make a platform strategy viable.
7.5.2.2 Suppliers Suppliers are becoming a more important part of the business model as firms concentrate on their core competencies and rely on outsourcing for non- core items. To meet aggressive development milestones as well as help reduce costs, subcontractors are playing a much larger role in developing the PW6000 engine for the Airbus A318 airplane [Kandebo, 1999]. Perhaps suppliers can be considered in a platform study for the components they are responsible for. Pratt & Whitney primarily relies on suppliers for external and accessory equipment including solenoids, actuators, metering devices, pumps, harnesses and others. Although these items were not considered in the current investigation, there are potential platform opportunities for these components across different engines. The modified QFD mappings presented in this investigation can easily be extended to include external and accessory equipment and linked to stakeholder needs and requirements.
7.5.2.3 Competitors Competitors certainly influence Pratt & Whitney's decision making process. Stakeholder needs are typically evaluated and tracked relative to the competition. In terms of product platform strategy and design, both General Electric and Rolls Royce have product platforms of their own. General Electric has the CF6, CFM56 and GE90 engine families, while Rolls Royce has the RB21 1 and Trent engine families. Understanding and keeping abreast of competitive platform strategies is a part of the overall product strategy as it affects Pratt & Whitney's positioning relative to time to market, new product offerings and the installed engine base......
88
Part of the normal conceptual design process at Pratt & Whitney includes benchmarking relative to the best in class. The traditional QFD allows one to competitively track both stakeholder needs and requirements so that shortfalls can easily be identified. Phase I of the modified QFD mapping presented in this investigation can easily be expanded to track competitor levels in satisfying stakeholder needs and system requirements.
7.5.2.4 Substitutes Although air transportation may appear to be in a class by itself, other forms of public transportation including trains and buses may be viable substitutes for air travel. Three factors that may influence the degree of substitution among planes, trains and buses are travel time, roundtrip cost and departure frequency. A comparison of these alternatives for Monday through Thursday travel from Hartford, Connecticut to Washington, DC are shown in Table 7.2. Reservations offices of Southwest Airlines, Greyhound and Amtrak were consulted for the information summarized in Table 7.2 [Southwest Airlines, Greyhound, Amtrak].
Table 7.2: Travel Alternatives Between Hartford, CT and Washington, DC
One Way .Number o TraelSource Travel Time, RudrpDaily Iternative hours Cost, $ Departure Plane Southwest Airlines 1.2 88 8 Bus& f Greyhound 72
In the case of Southwest Airline's recent introduction of service between Hartford's Bradley International Airport and Washington, DC's Baltimore- Washington International Airport, the clear advantage to air travel is both in shorter travel time and lower roundtrip cost. Although Greyhound offers nearly three times as many daily departures as Southwest Airlines, the 7-hour travel time by bus could be unattractive. At the time this research was conducted, the 89 roundtrip airfare was even cheaper than either bus or train fares. In the past, this was often not the case. Airlines lead by Southwest Airlines [Perry, 1995] who offer frequent service with low cost fares are presenting serious competition to buses and trains as low cost travel alternatives. Although buses and trains have historically offered lower fares, Colleen Barrett, an executive vice president was quoted as saying that Southwest's real competition was the automobile [Hallowell, 1993]. As a result, airlines are increasingly driven to low cost business models to compete with viable substitutes like buses, trains and automobiles. Many of the larger airlines are creating subsidiaries to compete in the short haul, regional markets. Some examples include United Airlines' United Express and Delta Air Lines' Delta Express services. From the stakeholder needs identified in Figure 4.4, a key need for airlines is low cash operating costs which include total maintenance costs and fuel costs. This need in turn drives the system architecture towards low cost attributes. From a strategic analysis perspective then, developing an understanding of a product's substitutes can help define a system architecture that best addresses the appropriate stakeholder needs. Issues like ultra low cost, safety and reliability for frequent short haul flights may significantly affect an engine's system architecture. Perhaps further study of these substitutes and identification of key attributes for comparison can be used to develop platform concepts.
7.5.2.5 Barriers to Entry Barriers to entry refer to what prevents a new or existing competitor from entering a market. In the case of commercial gas turbine engines, there have been no new single company entrants to the market for some time. However, a number of alliances between existing players have been formed as summarized in Table 7.3......
90
Table 7.3: Collaborations in Commercial Gas Turbine Engine Development Collaborations Products
BMW and Rolls Royce jointly produce the BMW-Rolls Royce BR700 engine family for the Boeing B717, Gulfstreamn V and Bombardier Global Express
Snecma and General Electric jointly produce CFM International the CFM56 engine family for the Boeing B737
General Electric and Pratt & Whitney Genera Eeic nd Prat W n inefiy fr EnginAllancethe Airbus A3XXX
Pratt & Whitney, Rolls Royce, Daimler Chrysler Aerospace - MTU Munchen and Japanese International Aero Engines Aero Engines Corporation jointly produce the V2500 engine family for the Airbus A319/320/321 and Boeing MD90
One of the reasons for these alliances is because individual companies are finding it increasingly cost prohibitive to shoulder engineering and development (E&D) programs on their own. Costly development programs can be considered a key barrier to existing competitors. Sharing the development risks with partners allows those already in the market to stay in the market. E&D is clearly a stakeholder need for Pratt & Whitney. Presumably a platform architecture would result in E&D savings due to the reuse of existing capital equipment and tooling, accumulated learning, not having to start from scratch, and not having to re-perform certain FAA certification tests that are listed in Table 4.2. Quantifying the amount of savings is complicated due to the integral functionality of the engine, which is one of the basic theme's for this thesis. Because of the engine's integral functionality, even a small change to a module may require the same rigorous testing because in the end, it's the system performance and operation that is tested and validated. Depending on the level of change or similarity, costly engine testing may still need to occur. As one source cites: "Don't confuse functioning of the parts for the functioning of the 91 system" [Rechtin and Maier, 1997]. A study to accurately quantify these savings is thus recommended. Time to market is another barrier to entry of an existing competitor to a particular market. In the case of the PW4000-112", and in particular the PW4084, this engine was the launch engine for Boeing's B777 airplane, meaning it was the first of three competitive engines to be certified for airline operations. This was due in large part to the platform strategy that allowed quicker time to market with derivative engine technology. As a result, many of the early B777 sold were Pratt & Whitney powered. The sooner a competitor can come to market and lock in engine sales, the less market remains for the other competitors. This is especially critical given the 20-30 year product lifetimes discussed in Chapter 7.4.2 where airlines do not necessarily order engines every year. Surely, the timing of technology development and insertion, product development and entry into service have an impact on product strategy where platforms are a way to address these timing considerations. Perhaps a study of Pratt & Whitney's technology strategy can be completed to provide an integrated framework of how to effectively address time to market as a barrier to entry. 92
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Glossary
(* denotes source from http://www.pratt-whitney.com/engines/terminology.html)
Term Definition Source Airflow Measured in pounds of air moved through * the engine per second. The more airflow, the more thrust. Bleed Air Air taken from the cooler compressor * section that is passed through various ducts to provide air for air conditioning of the airplanes as well as cooling air for the hot sections, the combustor and turbine. Bypass Ratio (BPR) The ratio of air ducted around the core of a * turbofan engine to the air that passes through the core. For example, in a 6 to 1 bypass ratio engine, six parts of air pass around the core compared to one part that passes through it. In a high bypass ratio engine, the fan at the front of the engine develops the bulk of the engine's total thrust. The air that passes through the core or basic engine is called primary airflow. The air that bypasses the core is called secondary airflow. Bypass ratio is the ratio between secondary and primary airflow. High bypass ratio turbofans were developed for fuel efficiency. It is more efficient to accelerate a large mass of air moderately through the fan to develop thrust than to greatly accelerate a smaller mass of air through the core to develop the equivalent thrust. Combustor or Burner This is the section of the engine where the * air passing out of the compressor is mixed with fuel, typically kerosene-based, and ignited. Fuel is introduced through an array of spray nozzles that atomize the fuel as in a home heating oil burner. An electric igniter is used to begin combustion. The combustor adds heat energy to the core engine air stream and raises its temperature, which can reach 3,500 degrees Fahrenheit. This energy is I extracted by the turbines and used to drive 96
the compressors and fan. Any energy not extracted by the turbines is expanded through the exhaust nozzle to produce thrust. Compressor IThe combustion of fuel and air at sea level -I * pressure will not produce significant thrust. In order to produce thrust the air must be compressed or squeezed before the fuel is added. In a car engine this is done by the pistons inside the engine's cylinders and is referred to as compression ratio. In most jet engines a compressor is used. This is a series of spinning blades that continually compress the engine air stream and speed it up before it enters the combustor. A way to visualize this is to imagine a household fan with a long shaft and several rows of fan blades all turning together. As the air is compressed, it is forced into a smaller and smaller area as it passes through the compressor's stages, thus raising the pressure ratio. In the automotive world the compression ratio is typically 10-to-1. In a jet engine the compression ratio can be as high as 40-to-1. In most modern engines the compressor is divided into low pressure (LPC) and high pressure (HPC) sections which run off two different shafts. Compressor Pressure The ratio of the air pressure exiting the * Ratio compressor as compared to that entering. It shows the amount of compression the air experiences as it passes through the compressor. Cycle (Interval) Wear and tear on an engine occurs neither * during cruising nor through flight time, but each time high power settings are used to accelerate and decelerate, such as during take off or reversing while landing. Throttle movements change the speed of the rotor, causing fatigue. Each such high power setting is called a cycle. The amount of time between inspections for wear and tear is determined by the number of these cycles a plane goes through, rather than the number of hours it has been in the air. Diffuser The diffuser is a large round structure * 97
immediately behind an engine's compressor and immediately in front of the combustor. It slows down compressor discharge air and prepares the air to enter the combustor at a lower velocity so that it can mix with the fuel properly for efficient combustion. Delay and/or A delay caused by an engine problem Cancellation Rate (D&C) occurs when the airplane is not able to pushback from the gate within 15 minutes of its scheduled departure time. A cancellation is when a flight has been cancelled due to engine problems. The number of delays and/or cancellations for a particular engine model is measured as events per 100 airplane departures. Direct Operating Cost Airline costs associated with operating the (DOC) airplane. Includes pilot wages, fuel costs, total maintenance costs. Electronic Engine Control The EEC, also know as the FADEC (Full- * Authority Digital Electronic Engine Control) is an advanced computer attached to the engine and used to control with great precision many functions inside the engine. For instance, the EEC controls fuel flow, the position of various mechanical parts such as bleed valves and compressor vanes and overall pressure ratios. It gives a much more precise control than previous mechanical systems. This eases pilot workload and greatly improves engine performance and efficiency. The EEC is equivalent to electronic fuel injection in modern cars. The EEC also monitors the engine and sends messages to the cockpit or to ground crews for maintenance action. Engine Build Unit (EBU) The EBU is equipment typically supplied by * the airplane manufacturer that is attached to the basic engine. It can include ducting for environmental control systems, wiring packages for connection to the airplane cockpit, electrical and hydraulic pumps and engine mounting hardware. Engine Pressure Ratio A method of measuring the thrust or power * (EPR) of a Pratt & Whitney engine. It is not used by all engine manufactures. EPR I (pronounced Eeeper) is the ratio of the 98
pressure of the engine air at the rear of the turbine section as opposed to the pressure of the air entering the compressor. For instance, in a typical wide-body commercial airplane engine, EPR might be 1.55 at takeoff and 1.39 at cruise. Exhaust Gas The temperature of the engine's gas stream * Temperature (EGT) at the rear of the turbine. It is one of the most critical of engine variables and is used to monitor the mechanical integrity of the turbine section as well as the engine's overall operating condition. A sudden rise in EGT usually indicates a problem. In a modern gas turbine EGT would typically range between 1,000 degrees Fahrenheit at take off to 700 degrees at cruise. Externals Includes components that are attached to the engine case external to the flowpath that are needed in fuel delivery, air flow control within the flowpath, lubrication system, heat exchange, gearboxes. Fan The large disc of blades, resembling an * automobile fan, at the front of a turbofan engine. The fan takes in vast amounts of air and provides most of the engine's thrust. Flowpath Part of the engine where air and exhaust gases travel through. Includes the fan, rotors and stators of the compressors, and blades and vanes of the turbines. Also referred to as gaspath. Full Authority Digital See EEC. Engine Control (FADEC) High Pressure See Compressor. Compressor (HPC) High Pressure Turbine See Turbine. (HPT) Indirect Operating Cost Airline costs not directly related to airplane operations. Includes cost of delays and cancellations (putting passengers up in hotel rooms, tickets, ferrying engine back to a maintenance base, sending a maintenance crew to service the engine at a remote location away from a maintenance base), variable passenger servicing costs, airplane servicing costs. In-flight Shutdown Rate A measure of the reliability of an engine, * 99
(IFSD) expressed as the number of times per thousand flight hours an engine must be shut down in flight. A modern airplane/engine combination like the Airbus A330 and Boeing 777 must demonstrate an in-flight shutdown rate of .02 or lower per thousand flight hours to gain Extended Twin Operations (ETOPS) certification. This is one shutdown in 50,000 hours of flight. In normal commercial service that equates to once every 10 years. Launch Customer First airline to order and operate a new airplane/engine. Line Replaceable Unit A part or component that can be replaced * (LRU) fairly easily on the flight line at an airport. Low Pressure See Compressor. Compressor (LPC) Low Pressure Turbine See Turbine. (LPT) Mach The speed of sound is approximately 762 * mph at sea level. Jet-powered airplanes fly at speeds measured in Mach numbers, or multiples of the speed of sound. Nacelle The cylindrical structure that surrounds an * engine on the airplane. The nacelle protects the engine and improves aerodynamics. It contains the engine and thrust reverser and many other mechanical components that run airplane systems. The nacelle and engines along with the EBU make up the propulsion system. N1 The rotational speed of the engine's low- * pressure compressor and low pressure turbine measured in revolutions per minute (RPM). N2 The rotational speed of the engine's high- * pressure compressor and high pressure turbine measured in RPM. Nozzle The rear portion of a jet engine in which the * gases produced in the combustor are accelerated to high velocities. Overall Pressure Ratio The pressure ratio achieved by both the low (OPR) pressure compressor (including fan root) and high pressure compressor. Performance Performance deterioration means that the Deterioration Rate engine has to burn more fuel to achieve the 100
same thrust level. Burning more fuel means that the gaspath temperatures are hotter than nominal. Although an engine is certified to operate over a range of temperatures, there is a limit as to how high the gaspath temperature it is allowed to operate. Operating above this temperature limit is not allowed for safety reasons. Therefore, when an engine has deteriorated to the point where its operating temperature has exceeded FAA certified levels, it must be removed and its parts replaced or repaired. Pounds of Thrust The measure of how much propulsion a jet * engine generates - literally, how many pounds it can move. Surge Surge is a disturbance of the airflow * through the engine's compressor. It can be caused by a number of factors. It has also been called a stall, but this is an aerodynamic stall, not like the stall in a car's engine. In a surge the compressor blades lose their lift, much like an airplane wing when it stalls. Surges occur for a wide variety of reasons and usually result in loss of power for only a fraction of a second, although they can damage an engine. They are sometimes accompanied by a loud bang and a puff of smoke. They have been likened to a car engine's backfire. Thrust Thrust is the measurement of engine * power. Although it is difficult to equate this directly with the commonly used term "horsepower," multiplying an engine's maximum thrust rating by .62 will give a rough equivalent horsepower. Thrust Specific Fuel The pounds of fuel used per hour for each * Consumption (TSFC) pound of thrust an engine produces. Total Maintenance Cost The cost to maintain the engines, including (TMC) parts and labor. Measured in $ / engine flight hour. Turbine The turbine consists of one or more rows of * blades mounted on a disc or drum immediately behind the combustor. The turbine extracts energy from the hot gases I coming out of the combustor. The spinning I 101
of the turbine turns the shafts which run the compressors and the fan, as well as engine accessories such as generators and pumps. Like the compressor, the turbine is divided into a low- pressure and a high- pressure section. The high-pressure turbine (HPT) is closest to the combustor and drives the high-pressure compressor through a shaft connecting the two. The low-pressure turbine (LPT) is next to the exhaust nozzle and drives the low-pressure compressor and fan through a different shaft. The low-pressure shaft is the longest and fits through the hollow high-pressure shaft. Temperatures at the entrance to a turbine can be as high as 3,000 degrees Fahrenheit, considerably above the metal's melting point. Complex cooling schemes are required to keep turbine blades from melting. Many turbine airfoils are hollow so cooler air can be passed through them and out hundreds of small holes in the blade. In addition, some blades are coated with a ceramic thermal barrier. Turbofan A term used to refer to a jet engine with a * large fan at the front that produces most of the engine's thrust. Unscheduled Engine A measure of how often a particular engine * Removal Rate (UER) model must be removed from an airplane for repair or refurbishment before the normal maintenance interval or due to an unexpected engine anomaly preventing it from continued safe operation. Rates are quoted in terms of events per 1000 engine I flight hours.