Time is of fundamental importance in the quantification of the basic terms which were defined in other definitions found in this blog, for it is this factor which permits the attributes to be measured rather than described in merely qualitative terms. The usual measures of time –  the year, the month, the day, and the hour  form the basis for the computation of reliability. Where appropriate, the time concepts may be replaced by distance, cycles, operations or other quantities.

In general, the interval of interest is the total calendar time during which an item or system is in use. Let’s walk down the figure and define the terms:

• Active time is that during which an item is in an operational inventory.

• Inactive time is that during which an item is in reserve.

• Active time may be further broken down into uptime (during which an item is in a condition to perform a required function) and downtime (during which an item is not in a condition to perform a required function).

• Downtime may be further subdivided into maintenance time (that downtime which excludes modification and delay time), modification time (that downtime necessary to introduce any specific change(s) to an item to improve its characteristics, or to add new ones), and delay time (that downtime during which no maintenance is being accomplished on the item because of either supply or adminstrative delay.

• Delay time may be further subdivided into supply delay time (that element of delay time during which a needed replacement item is being obtained) and adminstrative time (that element of delay time not included in supply delay time).

• Maintenance time can be broken down into corrective maintenance time (during which corrective maintenance is performed on an item), and preventive maintenance time (during which preventive maintenance is performed on an item).

• Uptime (left side of figure) can be further subdivided into:

• Not operating time (during which the item is not required to operate).

• Alert time (during which an item is assumed to be in specified operating condition, and is awaiting a command to perform its intended mission)

• Reaction time (that element of uptime needed to initiate a mission, measured from the time command is received).

• Mission time (during which an item is required to perform a stated mission profile).

Concepts of Time (MIL-STD-721)

References:

1. MIL-HDBK-338, Electronic Reliability Design Handbook, 15 Oct 84
2. MIL-STD-721, Definitions of Terms for Reliability and Maintainability, 23 Oct 91