OVERVIEW

Availability may be defined as:

A measure of the degree to which an item is in the operable and committable state at the start of a mission when the mission is called for at an unknown (random) time.
DoD 5000.2

Availability is the probability that a system is fully mission capable (operational) when called upon to perform its intended function. Availability is expressed as a probability from 0.0 to 1.0, or as a percentage. Calculated availability includes both system failures and repairs.

Mean Down Time (MDT) constitutes the total elapsed time required to repair and restore a system to full operating status. It includes Mean Maintenance Time (MMT), Administrative Delay Time (ADT), and Logistics Delay Time (LDT). Administrative Delay Time includes activities like assignment priorities, labor strikes, and unnecessarily wasted time. Logistics Delay Time is the time spent while waiting for parts, test equipment, tools, facilities, transportation, and paperwork, for example. MDT is the most difficult measure of down time to predict.

The following two charts (Figures 2.1 and 2.2) graphically illustrate the different periods of operation for a typical electrical or electronics system.

Figure 2.1: System operating chart I

Figure 2.2: System operating chart II

The total time that one or more items of Equipment exists can be broken down into many time categories. A typical breakdown of Equipment Time is shown in Figure 2.3 below.

Figure 2.3: Breakdown of Equipment Time

The following further defines the Equipment Time breakdown in...