Availability in this context is System Availability. System Availability is the probability the system will function as designed for a particular duration. The duration could be a fixed time period (i.e., 24 hours a day, 7 days a week, or 364.9 days a year) or it could be over a particular mission (i.e., a flying mission, a patient stay, or a growing season). The ability of an item to be in a state to perform a required function under given conditions at a given instant of time or over a time interval, assuming the required external resources are provided. 1)
It is important to remember that Availability is expressed as a probability expressed in terms of Five Nines, and therefore, the Multiplication Rule of Probability needs to be considered when thinking about a system comprised of parts. Each part has its own probability of success (or failure). The Multiplication Rule of Probability means that to find the probability of the intersection of two events, multiply the two probabilities. The intersection of the events occurs when the probability of two events occurring is known. The Multiplication Rule of Probability determines the intersection of two different sets of events, called independent and dependent events.
Mission Duration (Tm) is the time the system needs to be operational. Tm can be expressed as a fixed time period (i.e., 24 hours a day, 7 days a week, or 365 days a year) or it could be over a particular mission (i.e., a flying mission, a patient stay, peak energy demand, or a growing season)
Mean Time Between Failure (MTBF) is a calculation of the arithmetic mean (average) time between failures of a system.
Downtime (Td) is the Mission Duration times the sum of all of the different kinds of time required to transition from being down to the time to be fully operational, divided by the Mean Time Between Failure.