The aforementioned definitions are developed in Barlow and Proschan [1975], Lie, Hwang, and Tillman [1977], and Nachlas [1998]. The second primary classification for availability is contingent on the various mechanisms for downtime such as the inherent availability, achieved availability, and operational availability. Mi [1998] gives some comparison results of availability considering inherent availability. A very comprehensive recent book is by Trivedi and Bobbio [2017]. Availability is well established in the literature of stochastic modeling and optimal maintenance.

Availability, operational (Ao) [4]

More meanings of availability

The probability that an item will operate satisfactorily at a given point in time when used in an actual or realistic operating and support environment. It includes logistics time, ready time, and waiting or administrative downtime, and both preventive and corrective maintenance downtime. This value is equal to the mean time between failure (MTBF) divided by the mean time between failure plus the mean downtime (MDT). 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. Availability, achieved (Aa) [3]

The probability that an item will operate satisfactorily at a given

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point in time when used under stated conditions in an ideal support environment (i.e., that personnel, tools, spares, etc. are instantaneously available). It excludes logistics time and waiting or administrative downtime.

These examples are programmatically compiled from various online sources to illustrate current usage of the word ‘availability.’ Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. This website is using a security service to protect itself from online attacks. There are several actions that could trigger this block including submitting a certain word or phrase, a SQL command or malformed data. Furthermore, these methods are capable to identify the most critical items and failure modes or events that impact availability.

Examples of availability in a Sentence

It includes active preventive and corrective maintenance downtime. 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.

• This value is equal to the mean time between failure (MTBF) divided by the mean time between failure plus the mean downtime (MDT).
• Availability, achieved (Aa) [3]
  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. are instantaneously available).
• Availability measures are classified by either the time interval of interest or the mechanisms for the system downtime.
• It excludes logistics time and waiting or administrative downtime.
• It includes logistics time, ready time, and waiting or administrative downtime, and both preventive and corrective maintenance downtime.
• These examples are programmatically compiled from various online sources to illustrate current usage of the word ‘availability.’ Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors.

Barlow and Proschan [1975] define availability of a repairable system as “the probability that the system is operating at a specified time t.” Blanchard [1998] 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 [1977] developed a complete survey along with a systematic classification of availability. 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).