

MILHDBK470A: Designing and Developing Maintainable Products and Systems 
 

3.1  Understand the Customer's Maintainability Needs
Understanding the customer's maintainability needs is
the first and most obvious step in meeting the objective of a maintainable
product. It is important that the level of maintainability addressed here is
that which is measured by the user, the operational maintainability, not
necessarily that measured during design and development. Many factors can
affect operational maintainability, not just the design characteristics of the
product or the manufacturing processes used to make the product. This point
will be addressed later in Section
3.6 .
An important part of understanding customer needs is to collect and study lessons learned on prior products, preferably products similar to the one being acquired. By learning which problems have plagued products in the past, the maintainability engineer can adopt design approaches that reduce if not eliminate the problem in the new product.
Quantitative maintainability requirements should be derived using the same process used to derive other product design requirements. This process consists of performing a needs analysis and through the use of tools such as Quality Function Deployment (QFD). QFD is a tool for translating defined customer requirements into appropriate design requirements at each stage of design and development. The method uses a matrix known as the House of Quality, as depicted in Figure 3. Following are definitions of the terms used in the House of Quality.
Whats 
 The product characteristics, functions, or levels of performance wanted by the customer. These are the customer needs or requirements. The Whats are sometimes divided into Primary, Secondary, and Tertiary requirements. Examples of each for a fighter aircraft are, respectively, Operating Characteristics, Sorties, and 4 Sorties per Day. 
Hows 
The ways in which theWhatscan possibly be met. Also called design requirements. A Howfor the fighter sortie requirement might be a product availability of 0.92 
Importance 
The value or importance placed by the customer on eachWhat. Typically stated as Greatest, Average, or Least. 
HowstoWhats Relationships 
The relative strength of the relationship between a What (a requirement) and a specific How. Typically stated as Very Strong, Strong, or Weak or a corresponding numerical value. 
Weighted Importance 
The importance of eachHowbased on either itsHow to What relationship value and number of tertiary Whats (absolute weighting) or the relationship value, risk, and number of tertiary Whats (relative). 
Weighted Importance is calculated as follows:

How Correlation 
 The strength of the technical interrelationships between theHows. Typically stated as Very Strong, Strong, or Weak. 
Risk 
 The degree of technical and cost risk associated with eachHow. Typically stated as Greatest Average, or Least.

N 
 The total number of requirements (Whats). 
Figure 3 
House of Quality
Briefly, the following steps are used in the QFD approach (see Figure
4).
 Enter theWhatsalready determined. If necessary, further define theWhatsas Primary, Secondary, and Tertiary requirements.
 Determine theHows, the design requirements, based on technical experience and knowledge.
 DevelopWhatHowrelationships, assigning a numerical value to each (for example, a Very Strong relationship might be assigned a 5, a Strong relationship a 3, and a Weak relationship a 1). Determining relationships is based on experience and technical knowledge. To provide an easily understood graphical display, symbols, as shown in Figure 4, are used.
 Define and assign customer importance factor for each of the lowest level (primary, secondary, or tertiary) requirements and the degree of technical and cost risk associated with eachHow. Assign numerical values to the factors and
degrees of risk (e.g., Greatest =
5, Average = 3, Least = 1). < BR >
 Develop relationships among the Hows (not shown in Figure 4). Use the same definitions for the strength of the relationship and the corresponding numerical value that were used for the WhatHow relationships. Knowing the relationship among Hows will be important during trades.
 Calculate the relative and absolute weights for the Hows. For each How (DR1, DR2, and DR3), sum the relationship values in that column. The results are 39, 35, and 32, respectively. Ranked ordered, the Hows are given absolute weights of 1, 2, and 3. Now
multiply the relationship values in each column by the corresponding
importance and add the products yielding the following sums: 117, 67, and
100, respectively. Rank ordered, the relative weights are 1, 3, and 2,
respectively, for DR1, DR2, and DR3.
 Multiply the relative weights by the Risk factors of the Hows. The products of this multiplication indicate the attention merited by each How . DR2 rates the most attention, DR3 the next most, and DR1 the least.
The righthand side of the complete House of Quality (reference Figure 3) is used to project the relative level of effort, cost, required manufacturing capability, and the supplier's competitive position regarding each What. Projections are usually stated as Greatest, Average, and Least.
By using successive QFD "Houses of Quality", with the Hows from one used as the Whats of the next, increasingly more detailed (lower level) requirements can be derived.
Figure 4 
Example Excerpt of House of Quality




 
 