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In reliability theory and reliability engineering, the term availability has the following meanings: * The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, ''i.e.'' a random, time. Simply put, availability is the proportion of time a system is in a functioning condition. This is often described as a mission capable rate. Mathematically, this is expressed as 100% minus unavailability. * The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval. For example, a unit that is capable of being used 100 hours per week (168 hours) would have an availability of 100/168. However, typical availability values are specified in decimal (such as 0.9998). In high availability applications, a metric known as nines, corresponding to the number of nines following the decimal point, is used. With this convention, "five nines" equals 0.99999 (or 99.999%) availability. == Introduction == Availability of a system is typically measured as a factor of its reliability - as reliability increases, so does availability. Availability of a system may also be increased by the strategy of focusing on increasing testability, diagnostics and maintainability and not on reliability. Improving maintainability during the early design phase is generally easier than reliability (and Testability & diagnostics). Maintainability estimates (item Repair (replacement ) rates) are also generally more accurate. However, because the uncertainties in the reliability estimates (and also in diagnostic times) are in most cases very large, it is likely to dominate the availability (and the prediction uncertainty) problem, even while maintainability levels are very high. Furthermore, when reliability is not under control, then many and different sorts of issues may arise, for example: * The need for complex testability (built in test sensors, hardware and software) requirements, * The need for detailed diagnostic procedures, * Manpower (maintainers / customer service capability) availability, * Spare part availability, * Dead on Arrival (DOA's) issues (non-quality impact on system availability), * Logistic delays of spares or manpower due to any reason, * Lack of repair facilities and tools - also for software (e.g. situation DoD F22 Raptor Fighter program), * Lack of repair knowledge and expert-personnel * Extensive retro-fit and complex configuration management costs and others. The problem of unreliability may also become out of control due to the "domino effect" of maintenance induced failures after repairs and more and more increasing efforts of problem solving, re-engineering en service efforts. Only focusing on maintainability is therefore not enough! * If failures are prevented, none of the others are of any importance and therefore reliability is generally regarded as the most important part of availability! Reliability needs to be evaluated and improved related to both availability and the cost of ownership (due to cost of spare parts, maintenance man-hours, transport costs, storage cost, part obsolete risks etc.). Often a trade-off is needed between the two. There might be a maximum ratio between availability and cost of ownership. Testability of a system should also be addressed in the availability plan as this is the link between reliability and maintainability. The maintenance strategy can influence the reliability of a system (e.g. by preventive and/or predictive maintenance), although it can never bring it above the inherent reliability. So, Maintainability and Maintenance strategies influences the availability of a system. In theory this can be almost unlimited if one would be able to always repair any fault in an infinitely short time. This is in practice impossible. Repair-ability is always limited due to testability, manpower and logistic considerations. Reliability is not limited (Reliable items can be made that outlast the life of a machine with almost 100% certainty). For high levels of system availability (e.g. the availability of engine trust in an aircraft), the use of redundacy may be the only option. Refer to reliability engineering. An availability plan should clearly provide a strategy for availability control. Whether only Availability or also Cost of Ownership is more important depends on the use of the system. For example, a system that is a critical link in a production system - e.g. a big oil platform – is normally allowed to have a very high cost of ownership if this translates to even a minor increase in availability, as the unavailability of the platform results in a massive loss of revenue which can easily exceed the high cost of ownership. A proper reliability plan should always address RAMT analysis in its total context. RAMT stands in this case for Reliability, Availability, Maintainability/Maintenance and Testability in context to the customer needs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Availability」の詳細全文を読む スポンサード リンク
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