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    What are the differences between how the Navy and the Army define and calculate Operational Availability (Ao)?


    We use metrics such as Operational Availability (Ao) to clarify events and data but sometimes the result is more the opposite, due to variations in the way different organizations define and use different measures of effectiveness (MOE).  It is critical that all persons involved have a common understanding of the metric or equation definition, when and how the data is being gathered, etc., in order to make it a useful metric. 
    Both the Navy and the Army use Ao, and fortunately the usage is similar. The Navy's "Operational Availability Handbook" publication defines Operational Availability as the percentage of time that the system will be ready to perform satisfactorily in its intended operational environment.  The equation is:  Ao = Up time / (Up time Down time).  Uptime is defined as the element of active time during which an item is in condition perform its required functions. Down time is the element of active time during which the item is not in a condition to perform its required functions; in other words, it is "down", and that would be due to either maintenance requirements or other logistics delays.  While this metric is very common, and quite useful, as the OPNAVINST 3000.12A ( points out, such a measurement may be taken only for a system in the operational inventory, and is not really measurable for a system in development. Also, should the Ao so measured be less than the desired threshold, this calculation does not further required analysis as to cause or corrective action. Therefore the next steps need to include a more detailed breakdown of the components of uptime and downtime, and review and analysis of Reliability, Maintainability and Supportability, all of which influence Ao.  In some organizations, factoring in certain time components and not others results in variations to the basic equation shown above, so again it is important that however the metric is defined, alcon understand it and use it consistently. The USN handbook is an excellent source for explanation of several variations of the basic Ao equation.  The common USN definition (Ao = Up time/(Up time Down time), is mirrored in Army publications such as DA PAM 73-1 "Test and Evaluation in Support Of Systems Acquisition" ( and DA PAM 70-3 "Army Acquisition Procedures" (  Additionally, there is a AUG 2004 MOA between the USAF, USN, USA AND USMC called "MOA On Operational Suitability Terminology and Definitions To Be Used in OT&E" ( which stipulates the same definition.  The "DOD Guide For Achieving Reliability, Availability and Maintainability" of AUG 2005, ( also describes the same Uptime divided by Total Time equation, while also discussing the interplay between various measures of availability with primary drivers such as Reliability, Maintainability, etc. 
    While the basic definitions are consistent across the Services for the most part, variations do exist; a program venturing beyond the common standard should ensure consistent MOE definition and usage.

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