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 G
Part B
Engine casing
...
Fuselage
Part D
Engine
System B: Transmission
...
Part I
System C: ...
Part H
Gearing
Part K
...
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”
Number of
affected
parts or
systems
defined border “number”
?
avalanche
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 “changeroute” 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
Element B
Element A
Engine
Further parameters
Element C
Gear box/
Engine casing
Bearings
Power
Power
Geometry
Mech. vibrations
Geometry
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
Element B
Element A
Engine
Material paramet er
Element C
Gear box/
Engine casing
Bearings
Power
Geomet ry
Mech. vibrations
Legend:
Power
Geometry
Mech. vibrations
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
Parameter a
b
Element X
Parameter a
a
c
b
c
b
a
c
b
(A)
c
(B)
Further parameters
Element X
Parameter a
b
a
c
b
(C)
c
Legend:
Change relevant parameter
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.
Degree of
Absorption

Absorber
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.
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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: cme26@eng.cam.ac.uk