Inherent Availability is the probability that a system will be ready for operational use when required, based on the design characteristics only. Unlike Operational Availability (which is not calculated within eXpress), Inherent Availability does not take Logistics Delay Time into account. In eXpress, Inherent Availability appears in the Isolation Cost and Time Statistics section of the Fault Isolation Report.
In order for this metric to be accurate as a prediction, test times and object replacement times must be entered in terms of clock/calendar time (representing system life/usage). It will not be accurate if test or object times are entered in terms of maintenance time (e.g., man-hours). For example, if it takes three maintainers 2.5 hours on the clock to replace a component, the replacement time for that component should be set to 2.5 hours (the number of hours on the clock) if the Inherent Availability is to be used as a prediction.
Note: Because the Maintenance Ratio metric is based on maintenance man-hours, these two metrics will not be accurate as predictions within the same eXpress report. Of course, both metrics can always be used as an aide to optimizing a diagnostic design (provided they are not both used as predictions of system behavior under maintenance), regardless of the terms in which time attributes have been defined.
