Introduction of Availability


Availability is a performance criterion for repairable systems that accounts for both the reliability and maintainability aspects of a system. It is defined as the probability that the system is operating properly when it is required for use. That is, availability is the probability that a system is not failed or undergoing a repair action when it needs to be used. The numerical value of availability is expressed as a probability from 0 to 1. Availability calculations take into accounts both the failures and repairs of the system. For example, if a lamp has 99.9% availability, there will be one time out of a thousand that someone needs to use the lamp but it is non-operational because the switch is broken, or it is waiting for the replacement of a light bulb etc.   


The definition of availability is somewhat flexible, depending on what types of down times are considered in the analysis. As a result, there are a number of different classifications of availability.
·         Point (Instantaneous) Availability
·          Average Up-Time Availability (Mean Availability)
·          Steady State Availability
·         Operational Availability

Point Availability
Point, or instantaneous availability is the probability that a system (or component) will be operational at any random time, t. This is very similar to the reliability function in that it gives a probability that a system will function at the given time, t. Unlike reliability, the instantaneous will be operational if the following conditions are met:

1. It functioned properly during time t with probability R (t), or,
2. It functioned properly since the last repair at time r, 0< r<1, with probability

Average Up-Time (Mean) Availability
The mean availability is the proportion of time during a mission or time-period when the system is available for use. It represents the mean value of the instantaneous availability function over the period (0, T)   

Steady State Availability 
The steady state availability of the system is the limit of the instantaneous availability function as time approaches infinity. The instantaneous availability function approaches the steady state value very closely at time approximate to four times the Mean Time Between Failure (MTBF).

Operational Availability
Operational availability is a measure of availability, which includes all experienced sources of downtime, such as administrative downtime, logistic downtime, etc. The equation for operational availability is:

Where the operating cycle is the overall time period of operation being investigated, and up time is the total time the system was functioning during the operating cycle. The operational availability is the availability that the customer actually experiences. The previous availability definitions are a prior estimations based on models of the system failure and downtime distributions. In many cases, operational availability cannot be other factors that are the sole province controlled by the manufacturer due to variation in location, resources, and of the end user of the product.


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