The capability of a system to perform its intended function when called upon to do so is often referred to by either of two terms: “operational readiness” and “availability.”

It is the emphasis on the phrase “when called upon” that differentiates this concept from the more general one of system effectiveness. This emphasis restricts attention to probability” at a point in time” rather than “over an interval of time,” the latter being descriptive of system effectiveness. It should be noted that sometimes this interval can be extremely long. There is an additional major difference. System effectiveness includes the built-in capability of the system – its accuracy, power, and so on. Operational readiness excludes these native system characteristics; that is, it excludes the ability of the system to do the intended job and includes only its readiness to do it at a particular time. In order to differentiate between two separate and useful concepts, it is well to formalize a distinction between the terms “operational readiness” and “availability.” It has been apparent in past discussions of system effectiveness that the terms are used by some to represent different concepts but are used almost synonymously by others. Both concepts relate the operating time between failures to some longer time period; they differ in what is to be included in this longer time period. “Availability” is defined in terms of operating time and down time, where down time includes active repair time, administrative time, and logistic time. On the other hand, operational readiness is defined in terms of all of these times, and, in addition, includes both free time and storage time, that is, all calendar time. Availability and operational readiness are defined as follows:

• The availability of a system or equipment is a measure of the degree to which an item is in an operable and committable state at the start of a mission when the mission is called for at an unknown (random) time (Includes operating time, active repair time, administrative time, and logistic time, but excludes mission time).
• The operational readiness of a system or equipment is the ability of an item (military unit) to respond to its operation plan(s) upon receipt of an operations order. (Total calendar time is the basis for computation of operational readiness.)

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