Availability is a measure of the degree to which an item is in an operable state and can be committed at the start of a mission when the mission is called for at an unknown (random) point in time. There are four (4) availability measurements:
- Inherent Availability: A system with respect only to operating time and corrective maintenance. It ignores standby and delay times associated with preventive maintenance as well as Mean Logistics Delay Time (MLDT) and may be calculated as the ratio of Mean Time Between Failure (MTBF) divided by the sum of MTBF and Mean Time To Repair (MTTR), that is AI = MTBF/(MTBF + MTTR).
- Achieved Availability: A system with respect to operating time and both corrective and preventive maintenance. It ignores Mean Logistics Delay Time (MLDT) and may be calculated as Mean Time Between Maintenance (MTBM) divided by the sum of MTBM and Mean Maintenance Time (MMT), that is, AA = MTBM/(MTBM + MMT).
- Operational Availability (AO): The degree (expressed as a decimal between 0 and 1, or the percentage equivalent) to which one can expect a piece of equipment or weapon system to work properly when it is required, that is, the percent of time the equipment or weapon system is available for use.
- Materiel Availability: The percentage of the total inventory of a system operationally capable (ready for tasking) of performing an assigned mission at a given time, based on materiel condition.
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