Availability

as, Availability is the probability that an item will be in an operable and committable state at the start of a mission when the mission is called for at a random time, and is generally defined as uptime divided by total time (uptime plus downtime).

According to Marc Brooker, to take advantage of redundancy, ensure that:[4] Reliability Block Diagrams or Fault Tree Analysis are developed to calculate availability of a system or a functional failure condition within a system including many factors like: Furthermore, these methods are capable to identify the most critical items and failure modes or events that impact availability.

Availability, inherent (Ai) [5] The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment.

Availability, achieved (Aa) [6] The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment (i.e., that personnel, tools, spares, etc.

This measure extends the definition of availability to elements controlled by the logisticians and mission planners such as quantity and proximity of spares, tools and manpower to the hardware item.

Lie, Hwang, and Tillman [1977] developed a complete survey along with a systematic classification of availability.

Availability measures are classified by either the time interval of interest or the mechanisms for the system downtime.

The aforementioned definitions are developed in Barlow and Proschan [1975], Lie, Hwang, and Tillman [1977], and Nachlas [1998].

For example, the North American Electric Reliability Corporation implemented the Generating Availability Data System in 1982.