Different textbooks and other reference documents define maintainability in
slightly different ways. However, consolidating the ideas in these definitions
yields the following definition:
Maintainability. The relative ease and economy of time and resources with which an item can be retained in, or restored to, a specified condition when maintenance is performed by personnel having specified skill
levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair. In this context, it is a function of design.
In succeeding sections, this definition will be examined in more detail. For now, it is sufficient to note that maintainability, a design characteristic, concerns the relative ease and cost of preventing failures (retaining an item in a specified condition) or correcting failures (restoring an item to a specified condition) through maintenance actions3.
Maintainability is a design parameter. Although other factors, such as highly trained people and a responsive supply system, can help keep downtime to an absolute minimum, it is the inherent maintainability that
determines this minimum. Improving training or support cannot effectively compensate for the effect on availability of a poorly designed (in terms of maintainability) product. Minimizing the cost to support a product and maximizing the availability of that product are best done by designing the product to be reliable and maintainable.
Testability, an important subset of maintainability, is a design characteristic that allows the status (operable, inoperable or degraded) of an item to be determined, and faults within the item to be isolated in a timely and efficient manner. The ability to detect and isolate faults within a system, and to do so efficiently and cost effectively, is important not only in the field, but also during manufacturing. All products must be tested and verified prior to release to the customer. Paying attention to testability concerns up front will pay benefits during the testing phases of manufacturing. Therefore, a great deal of attention must be paid to ensuring that all designs incorporate features that allow testing to occur without a great deal of effort. Design guides and analysis tools must be used rigorously to ensure a testable design. Not doing so leads to greater costs in the development of manufacturing and field tests, as well as in the development of test equipment. Trade-offs must be made up front on the use of built-in-test (BIT) versus other means of fault detection and isolation. Further, the expected percentage of faults that can be detected and isolated to a specified or desired level of ambiguity must be determined as an important input to the logistics analysis process. The consequences of poor testability are higher manufacturing costs, higher support costs, and lower customer satisfaction.
No matter how they may define maintainability,
commercial and military users measure the performance of products in their own
ways, to suit their own needs. A car owner may be most concerned with low cost
of operation and few visits to the repair shop. An airline may be most
concerned with staying on schedule. These measures may or may not include
factors totally determined by the design. So, the way in which a customer
measures the maintainability of a product in use may not be meaningful to a
designer, and a translation from the user's measures to measures more
appropriate for design may be needed. Table V shows how operational (the user's) maintainability and design
maintainability differ. Also see Appendix
Table V - Operational and Design Maintainability
- Used to define, measure and evaluate supplier's program
- Derived from opertaional needs
- Selected such that achieving them allows projected satisfaction of
- Expressed in design parameters
- Includes only effects of design and manufacturing
- Typical terms
- MTTR (mean-time-to-repair)
- Ai (inherent availability)
- Used to describe performance when operated in planned environment
- Not normally appropriate for contract requirements
- Used to describe needed level of maintainability performance in
- Includes combined effects of item design, quality, installation
environment, maintenance policy, repair, delays, etc.
- Typical terms
- MDT (mean-downtime)
- Ao (operational
3In designing for maintainability, we want to develop a product that is serviceable (easily repaired) and supportable (can be cost-effectively kept in or restored to a usable condition).