Availability (system)
Availability is the probability that a system will work as required when required during the period of a mission. The mission could be the 18-hour span of an aircraft flight. The mission period could also be the 3 to 15 month span of a military deployment. Availability includes non-operational periods associated with reliability, maintenance, and logistics.
This is measured in terms of nines. Five-9's means less than 5 minutes when the system is not operating correctly over the span of one year.
Availability is only meaningful for supportable systems. As an example, availability of 99.9% means nothing after the only known source stops manufacturing a critical replacement part.
There are two kinds of availability.
Operational availability is presumed to be the same as predicted availability until after operational metrics become available.
Operational availability is based on observations after at least one system has been built. This usually begins with the brassboard system that is used to complete system development, and continues with the first of kind used for live fire test and evaluation (LFTE). Organizations responsible for maintenance use this to evaluate the effectiveness of the maintenance philosophy.
Predicted availability is based on a model of the system before it is built.
Downtime is the total of all of the different contributions that compromise operation. For modeling, these are different aspects of the model, such as human-system interface for MTTR and reliability modeling for MTBF. For observation, these reflect the different areas of the organization, such as maintenance personnel and documentation for MTTR, and manufacturers and shippers for MLDT.
Mean Time Between Failure (MTBF) depends upon the maintenance philosophy.
If a system is designed with both redundancy and automatic fault bypass, then MTBF is the anticipated lifespan of the system if these features cover all possible failure modes (infinity for all practical purposes). Such systems will continue without noticeable interruption when these conditions are satisfied unless there are secondary failures. This is called active redundancy, which requires no maintenance to prevent mission failure. Active redundancy is required for systems that cannot be maintained, such as satellites.
If a system has no redundancy, then MTBF is the inverse of failure rate, .
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