Establishing Availability Requirements Using Characteristics Factors and Expert Opinion Page: 3 of 6
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Availability criteria for individual systems is often established using a "bottom-up" approach when a
detailed design is available. A "top-down" approach is intuitively more satisfying for less defined
facilities.? This method takes an overall facility requirement and divides it first among facility systems and
then among subsystems. The overall facility availability requirement is distributed proportionately among
facility systems, based on each system's capability for meeting a design objective. The approach imposes
(a) higher availability requirements on those systems in which an incremental increase in availability is
easier to achieve and (b) lower availability requirements when an increase is more difficult and costly.
Optimizations such as these result in lower facility costs.
2. DETERMINATIONS OF WEIGHTING FACTORS
Characteristics that influence equipment reliability and maintainability are described. Experts, using
engineering judgment, score each characteristic for each system whose availability design goal is to be
established. The Analytic Hierarchy Processis used to produce a set of weighted ranks for each
characteristic of each alternative system. Expert elicitation relies on a series of exhaustive judgements that
compare pairs of characteristics.
Characteristics chosen that influence reliability are system complexity, design immaturity, and stressful
operating environment. System complexity may be evaluated by considering the probable number of parts
or components that make up the system-the higher the number of parts the more likely there will be
failures. The lack of a proven design increases the probable number of failures. Components operating in
stressful conditions (e.g., high temperatures, vibration, and radiation fields) have more failures than those
operating in benign environments.
Characteristics that influence maintenance are the elements of repair time: fault detection and
diagnosis time, preparation time to conduct the repair, fault correction time itself, and time to restart the
system once the fault is corrected.
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Haire, M.J. & Schryver, J.C. Establishing Availability Requirements Using Characteristics Factors and Expert Opinion, article, June 18, 2000; (digital.library.unt.edu/ark:/67531/metadc706081/m1/3/: accessed February 21, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.