**AVAILABILITY CLASSIFICATION**

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:

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