Reliability Requirements
For any system, one of the first tasks of reliability engineering is to adequately specify the reliability and maintainability requirements as defined by the stakeholders in terms of their overall availability needs. Setting only availability targets is not appropriate. Reliability requirements address the system itself, including test and assessment requirements, and associated tasks and documentation. Reliability requirements are included in the appropriate system or subsystem requirements specifications, test plans and contract statements. Provision of quantitative minimum targets (e.g. MTBF / Failure rate) is not sufficient, reliability requirements should drive a (system or part) design to incorporate features that prevent failures from occurring or limit consequences from failure. A design requirement should be precise enough so that a designer can "design to" it. To derive these requirements in an effective manner, a systems engineering based risk assessment and mitigation logic should be used. The requirements may be part of the output from functional or other failure analysis.
The maintainability requirements address the costs of repairs as well as repair time. Testability requirements provide the link between reliability and maintainability and should address detectability of failure modes (on a particular system level), isolation levels and the creation of diagnostics (procedures).
As indicated above, reliability engineers should also address requirements for various reliability tasks and documentation during system development, test, production, and operation. These requirements are generally specified in the contract statement of work and depend on how much leeway the customer wishes to provide to the contractor. Reliability tasks include various analyses, planning, and failure reporting. Task selection depends on the criticality of the system as well as cost. A safety critical system may require a formal failure reporting and review process throughout development, whereas a non-critical system may rely on final test reports. The most common reliability program tasks are documented in reliability program standards, such as MIL-STD-785 and IEEE 1332. Failure reporting analysis and corrective action systems are a common approach for product/process reliability monitoring.
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