Availability

Availability is well established in the literature of stochastic modeling and optimal maintenance. Barlow and Proschan define availability of a repairable system as "the probability that the system is operating at a specified time t." While Blanchard gives a qualitative definition of availability as "a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called for at an unknown random point in time." This definition comes from the MIL-STD-721. Lie, Hwang, and Tillman 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. If the time interval of interest is the primary concern, we consider instantaneous, limiting, average, and limiting average availability. The aforementioned definitions are developed in Barlow and Proschan, Lie, Hwang, and Tillman, and Nachlas . The second primary classification for availilability is contingent on the various mechanisms for downtime such as the inherent availability, achieved availability, and operational availability. (Blanchard, Lie, Hwang, and Tillman ). Mi gives some comparison results of availability considering inherent availability.

Availability considered in maintenance modeling can be found in Barlow and Proschan for replacement models, Fawzi and Hawkes for an R-out-of-N system with spares and repairs, Fawzi and Hawkes for a series system with replacement and repair, Iyer for imperfect repair models, Murdock for age replacement preventive maintenance models, Nachlas for preventive maintenance models, and Wang and Pham for imperfect maintenance models.