Reliability, Availability and Maintainability (RAM) Engineering
Introduction
In reliability theory and reliability engineering, the term availability has the following meanings:.
For example, a unit that can be 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 is used, which corresponds to the number of nines following the decimal point. With this convention, "five nines" equals 0.99999 (or 99.999%) availability.
The availability of a system is generally measured as a factor of its reliability; as reliability increases, so does availability.
The availability of a system can also be increased by the strategy of focusing on increasing testability, diagnostics, and maintainability rather than reliability. Improving maintainability during the initial design phase is generally easier than reliability (and testability and diagnostics). Maintainability estimates (item repair (replacement) items) are also generally more accurate. However, because uncertainties in reliability estimates (and also in diagnosis times) are, in most cases, very large, the availability problem (and prediction uncertainty) is likely to dominate, even when maintainability levels are very high. Furthermore, when reliability is not under control, many different types of problems can arise, for example:
The problem of unreliability can also get out of control due to the "domino effect" of maintenance-induced failures after repairs and increasing troubleshooting, reengineering, and service efforts.
Reliability must be evaluated and improved in relation to availability and cost of ownership (due to spare parts cost, maintenance hours, transportation costs, storage cost, partially obsolete risks, etc.). A trade-off between the two is often needed. There may be a maximum relationship between availability and cost of ownership. The testability of a system should also be addressed in the availability plan, as this is the link between reliability and maintainability. Maintenance strategy can influence the reliability of a system (for example, through preventive and/or predictive maintenance), although it can never put it above inherent reliability. Therefore, maintenance and maintenance strategies influence the availability of a system. In theory, this can be almost unlimited if one could always repair any fault in an infinitely short time. This is practically impossible. Repair capacity is always limited due to testing capacity, labor, and logistics considerations. Reliability is not limited (reliable items that outlast the life of a machine can be made with almost 100% certainty). For high levels of system availability (for example, engine reliability availability in an aircraft), the use of redundancy may be the only option.