Reliability is the probability (likelihood) a system will perform its intended function with no failures during a specified period of time (mission time) under specified conditions (environment). Maintainability is the probability a failed item will be restored or repaired to a specified condition within a period of time. Availability, a function of reliability and maintainability (R&M), is the probability a repairable system will perform its intended function at a given point in time or over a specified period of time when operated and maintained in a prescribed manner.

Ideally and from an R&M plan, the R&M team performs three major functions, namely, Design for R&M, R&M Verification, and Analytical Physics. Specific R&M tasks are: (1) Establish quantitative R&M requirements as design goals or specifications, (2) Allocate or apportion the R&M requirement to system elements, (3) Apply reliability design methods (e.g., parts and material selection, derating, stress-strength analysis, identification of alternative technologies, simplification [e.g., reduce parts count], and redundancy), (4) Apply maintainability design methods (e.g., fault isolation, diagnostics, parts standardization and interchangeability, modularization, accessibility, and repair versus replace), (5) Perform reliability analyses (e.g., reliability block diagram analysis [RBD], failure-modes-effects and criticality analysis [FMECA], and fault tree analysis [FTA]), (6) Perform maintainability analyses (e.g., maintenance and spares provisioning, preventive maintenance intervals, predictive maintenance, and ergonomics), (7) Participate in concurrent engineering activities (e.g., FMECAs and design reviews), (8) Perform R&M predictions or demonstrations, (9) Establish R&M test procedures and analyze test data, and (10) Monitor R&M performance (e.g., analyze field data and identify failure-root cause).

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