INTERNATIONAL CONFERENCE ON ENGINEERING DESIGN ICED 01 GLASGOW, AUGUST 21-23, 2001

ASPECTS OF A BETTER UNDERSTANDING OF CHANGES

Claudia Eckert, Winfried Zanker and P John Clarkson

Keywords: Change processes, planning, workflow methodology.

1 Introduction

Change is one of the most powerful driving forces in design. Products are changed to improve them and change is crucial to reaching and maintaining product competitiveness. In some cases change is necessary to reach product performance standards which have not been met as a result of problems. Other changes are undertaken to adapt an existing product to meet new needs and requirements. We differentiate therefore between emergent changes, which occur due to problems during a design or change process, and initiated changes, which are driven by customer wishes. In all cases, a single change often triggers many other changes [1] and hence, independently of the source of the change, complex change propagation can occur.

Thus an increased understanding of the causes and sources of change, the nature of change and its consequence as well as of efficient ways of handling change processes is essential for achieving high quality products and efficient development processes. The main focus of this paper is to increase this awareness and understanding of the change process. The paper will describe, analyse and build up models of change propagation, and will provide advice about how to deal with changes in an efficient and effective way.

The work is based on observations of industrial development processes (section 2).. Three important aspects of change propagation are described: through a “macro-level view” of the entire process involved in executing one change and its consequences (section 3.1); through a closer look at the link between two connected/corresponding parts or systems (section 3.2); and through a “micro-level view” explaining the behaviour of an individual component in a given change situation (section 3.3). The paper analyses the relationship between these views to offer some suggestions what should be considered when dealing with changes and how to handle changes processes intelligently (sections 4 and 5).

2 Methodology and boundary conditions

The paper draws primarily on a case study from GKN Westland Helicopters, a UK aerospace company, comprising 23 interviews with senior engineers across a broad range of subject areas and several in depth studies of particular instances of change [2]. Questions and responses were classified and collated across a number of subjects including: change processes in the company; causes of change; effects of change; and issues concerning communication and representation, which provide a background for understanding the change processes. The product at the centre of the case study was a helicopter, a very complex product with about 20 major sub-systems and 10,000 single parts that are closely linked to each other. The product can be categorised as a typical ”mechatronic” system in which mechanical, electrical/electronic and software ”parts” are combined.

As a result of the complexity of the product, the complexity of the change process is also very high. About 1000 people from various disciplines – design, manufacturing, assembly, marketing, sales etc. – work very closely together in order to develop a new product release, in response to particular customer requirements, within a period of about 2 years from the signing the contract to the delivery of the first helicopter.

3 Change propagation

The behaviour of an engineering product is defined by the complex interactions between its parts and systems. Parts have to interact with each other, parts have to interact with systems and systems have to interact with other systems. Thus changes to parts or systems are highly likely to result in changes to others, which in turn can propagate to other systems. The interviewed designers commented that they typically expect up to four follow up changes arising from each initiated change. A change rarely occurs alone and multiple changes can have interacting effects on other systems. Therefore we have to be aware not only of individual change chains but of complex change networks [3]; an example of which is shown in Figure 1.

Part A System A:Engine Part F Part C ...... Engine auxiliaries Part E Part B Part G Engine casing ... Part D Fuselage Engine System B: Transmission

Part I ... Part K

System C: ... Gearing ...

Part H Part J ... Clutch

Legend: Correlation between parts Correlation between systems

Figure 1. Change networks through change propagation

3.1 The change process – a macro-level view

Looking at the whole process it is interesting to see how many parts and systems are involved in a change process that started with a change to a single component. From the data of the study different types of change processes were identified depending on the number of affected elements (parts or systems) within one change process. This view on changes presents the macro-level-view. Change processes can be divided into two categories depending whether the change process has ended within a certain change volume and time frame (see Figure 2):  ending change processes in which ripples – a small and quickly decreasing volume of changes, typically routine, such as changes to cabling and wiring – and blossoms – a high number of changes, which are brought to a close within expected limits, for example routine change to the fuselage – can be distinguished.

 unending changes processes in which avalanches – an increasing volume of changes that may or may not be brought to a conclusion after a given end point (within a certain time or number of changes) – are seen. Typically one major change causes several other equally major changes, for example a hook, that requires reinforcement of the entire fuselage.

defined border “time” defined border “number” Number of affected ? parts or avalanche systems

blossom

ripple ?

time

Figure 2. A macro view on change

The essential issue for planing, organising and controlling an entire development project, in which a lot of single change processes take place, is to know in advance where the “change- route” is going to go. For all occurring problems several solutions might be possible. Two or more alternative routes of the change process, involving different parts and systems, can achieve the same goal. Therefore the awareness of the behaviour of the single elements (parts or systems) and the kind of connection (linking parameters) between them is necessary to decide which of the alternative change processes should be followed.

3.2 Linking parameters

The crucial factor in whether a change will propagate is how the parts and systems are linked together. What do the arrows in Figure 1 represent? With reference to studies in systems engineering (see for example [4]) basically three types of ‘flows’ between system elements can be identified [5]: flow of material, flow of energy and flow of information.

A closer look reveals a set of ‘linking parameters’ between parts and systems [6] including geometry (length, breadth, depth), force, torque, temperature, heat transfer, mechanical or electromagnetic vibrations, material parameters (for example, the coefficient of expansion or the heat-transfer coefficient) etc. (see figure 3). These parameters are named direct parameters [7]. Further parameters Further parameters Element A Element B Element C

Gear box/ Engine Bearings Engine casing

Power Power Geometry Geometry Mech. vibrations Mech. vibrations

Figure 3. Examples of parameters that link elements

It is important to note that the linking parameters that can cause further changes are changing themselves during the change process as shown in the following fictitious simplified example. The installation of a more powerful engine (increasing power = torque x angular velocity as changing physical parameters) might require an increase in the bearing diameter (parameter geometry) and hence a new engine casing (parameter geometry and material parameters) (Figure 4). Hence, the change forcing parameter is changing itself.

Further parameters Material parameter Element A Element B Element C

Gear box/ Engine Bearings Engine casing

Power Power Geometry Geometry Mech. vibrations Mech. vibrations

Legend: Change relevant parameter Change irrelevant parameter

Figure 4. Change relevant parameter in a change process

It is an enormous task to build a model that tries to take all possible ‘linking parameters’ between all parts/systems within a complex product into account. The effort in building and maintaining such a model is unlikely to be commercially justified. Whilst Boeing have built models of over 10,000 design tasks, and their linking parameters, using design structure matrices [8] in order to assist design process planning, these models were not intended to be dynamically updated, as would be required to assist change prediction.

The case study that has informed this paper also led to the development of algorithms to predict the risk of change spreading from one component to another [9]. The entire product was represented in a 19 by 19 matrix capturing the major systems of the craft. A further model, developed by GKN Westlands, captured past change instances in a (initially sparsely populated) 400 by 400 matrix breaking the craft down one level further in the product hierarchy. These examples indicate the complexity of a potential detailed model consisting of 1000s of parts with could be linked by dozens of different parameters. Therefore a complete and detailed model of a product with parameter connection might be unrealistic. However, awareness of some essential relationships (parameters) between the most important parts and systems can be a big help in predicting which way changes to one part might have an impact on others. Hence provision can be made in the design to enable future changes and change routes can be directed.

3.3 The component – a micro-level view

As explained above each single element (part or system) can be influenced through various single parameters by other elements and can influence other elements by some (maybe other) parameters as well.

As Figure 5 illustrates, parameter propagation for single elements (parts/systems) falls into three categories:  the element is affected by more parameters than it effects which result in changes to other elements (A)

 the element is affected by the same number of parameters as it effects which result in changes to other elements (B)

 the element is affected by less parameters than it effects which result in changes to other elements (C)

Further parameters Further parameters

Element X Element X

Parameter a a Parameter a a c c b b c b b c

(A) (B)

Further parameters

Element X

Parameter a a c b b c Legend: Change relevant parameter

(C) Change irrelevant parameter

Figure 5. Change relevant parameters in a change process

In many instances it is a likely heuristic that the number of changes that affect a part or system and the number of changes that it passes on correlates to the number of influenced and affected parameters. The parameters are related to the functions the part or system performs, so that functional changes propagate through the parameters that are affected. In addition, systems or parts are affected by ‘unwanted flows’ such as vibration, heat or noise, which can also propagate through parts or systems. However, it is not possible to make predictions about the change behaviour of the part or system solely on its parameters. It is necessary to look at the properties of the part or system itself. Parts/systems fall into the following rough categories, illustrated in Figure 6:  Absorbers can absorb more changes than they themselves cause. A very small number of the systems are total absorbers, which take a large number of changes without causing further changes. Absorbers reduce the overall complexity of the change problem.

 Carriers take the same amount of changes as they cause themselves. They do not increase the complexity of the overall problem. Many simple geometric components fall in this category, for example a rod that remains the same length and passes on geometric change.

 Multipliers generate more changes to other systems than they require themselves. Change propagation becomes more complex through multipliers and change avalanches can arise from unexpected change multipliers. n f o o i

t e p r e r

o Absorber g s e b D A

Carrier

Multipliers Degree of Propagation

Figure 6. Change propagation behaviour of parts and systems

Change propagation behaviour is not a static property of the part or system, it depends of the state of the design. A change absorber can easily become a multiplier if the particular change is too big to absorb. For example, an engine might be able to support a certain increase in weight of the craft. However, if the weight-increase is too large the engine must be modified or a new engine selected.

Whether a change can be absorbed depends on two factors: The initial specification of the part or system and the margins that are included in the design of the part or system. The margins themselves are not static but may change over the history of the design. The designers observed typically added a 25 % safety margin to the specification of many components, which was gradually used as the design was put together. A part or component becomes a change multiplier when the safety margins are used up. Therefore seemingly small changes can have a huge impact on the rest of the changes, if they are responsible for a change absorber becoming a change multiplier. This behaviour is amplified when several changes are carried out simultaneously. While designers might be aware of the initial changes affecting a system, they might be less aware of other changes that will affect a system further down the change route. Therefore the key issue in predicting change propagation within complex engineering system is in understanding the tolerance margins of the key parameters relating to the major systems. In practice, tolerance margins are often not known. They are often not recorded when design decisions are made, or are at least not presented in an easily accessible form. In addition, the real margins of systems are often not known, because the initial design decisions were based on experience.

Tolerance limits are also rarely known, since parts or systems are often tested to see whether they fulfil a specification rather than to identify their performance limits. Testing systems to breaking point is expensive and while computer-modelling techniques have vastly improved, models can still not fully test interconnected properties of complex systems. For example, those interviewed commented, that while they try to anticipate vibration problems as much as possible with mathematical models, they also allow time in their design planning for eliminating vibration during prototype testing.

4 Strategies for handling margins

Knowledge of the tolerance margins on key systems is the first step to handling them sensibly. They should be recorded along with their associated parameter for each key system. It is crucial that values of the margins are updated, so that designers are aware of their current values when changes are planned or carried out. Information about past and intended changes must therefore be actively communicated to all relevant parties or recorded centrally. During the initial design process of the product, designers must find a medium between two extreme strategies:  Strategy 1: pushing the direct parameters to the limit. This leads to the optimal design under the present circumstances, but provides no reserve in case of later changes. While this is hard to set during initial design due to the interconnectedness of design systems, this is potentially the best option for the present design. For example, in the case of a helicopter it pays designers to produce a craft that is as light as possible, to optimise performance, by not including tolerance margins on heavy parts such as the fuselage.

 Strategy 2: reserve margins (for the direct parameters). This accepts that the design is not optimal right now, but flexible to include some changes that are likely to come. In this case potentially expensive redundancy is designed into the product, however, making future redesign much cheaper. For example, when the strength tolerance of the fuselage, which is directly related to its weight is exceeded, reinforcements to the fuselage become extremely expensive. The use of reserve margins can help to avoid this cost.

The most suitable strategy for a particular company depends on its circumstances. A company that develops a new product internally with the expectation that it will be updated later is advised to include reasonable tolerance margins on those parameters (and thus on parts/systems) that they expect to be changed later. For example, if a version with a stronger engine is foreseen, it might be wise to build the initial design with stronger than required engine mountings. In this case the management of margins is closely linked to the management of product innovation. However, the situation is very different when companies develop products for a particular customer, who is not willing to settle for a sub-optimal product and does not intend further extensions of the requirements. 5 Conclusions

The key to avoiding avalanches in a change process lies in the intelligent management of parameter margins for key components. Only sufficient margins can stop a system from turning from a change absorber into a change multiplier. This can be avoided through high redundancy in the product or, more economically, through an intelligent anticipation of later changes.

Acknowledgements

The authors are grateful to the UK Engineering and Physical Sciences Research Council (EPSRC) and GKN Westland Helicopters Ltd for their support of this project.

References

[1] Lindemann, U and Reichwald, R (1998), “Integriertes Änderungsmanagement,” Springer, Berlin. [2] Eckert, C M, Clarkson, P J and Zanker, W (1999), Customisation in complex engineering domains, University of Cambridge Technical Report, CUED/C-EDC/TR88. [3] Zanker, W and Lindemann, U (1998), “Weaknesses of todays development processes,” In: Proceedings of IV International Congress of Project Engineering, Cordoba, Universidad de Córdoba, Área de Proyectos de Ingeniería.

[4] Daenzer, W F and Huber, F (HRSG.), Systems Engineering – Leitfaden zur methodischen Durchführung umfangreicher Planungsvorhaben. 8. Aufl. [5] DIN-Fachbericht 12, (1987), Einteilungsschema für technische Systeme, 1. Aufl. Berlin: Beuth. [6] Steinmeier, E (1999), Realisierung eines systemtechnischen Produktmodells - Einsatz in der PKW-Entwicklung, Aachen: Shaker. (Konstruktionstechnik München, Band 28). [7] Andreasen, M M and Hein, L (1987), Integrated Product Development, IFS (Publications) Ltd. [8] Eppinger, S D , Whitney, D E, Smith, R P and Gebala, D A (1994), “A Model-Based Method for Organizing Tasks in Product Development,” Research in Engineering Design, 6: pp1-13. [9] Clarkson, P J, Simons, C S and Eckert, C M (2001) “Change Prediction for Product Redesign,” Proceedings of ICED 2001, Glasgow.

Dr Claudia Eckert Engineering Design Centre University of Cambridge Trumpington Street Cambridge CB2 1PZ United Kingdom Phone: +44 (0)1223 332 662 Fax: +44 (0)1223 332 662 E-mail: [email protected]