Editing Availability (section)

=== Definitions within systems engineering ===
'''Availability, inherent (A<sub>i</sub>)''' <ref>{{cite web|title=Inherent Availability (AI) |url=https://dap.dau.mil/glossary/Pages/2045.aspx |work=Glossary of Defense Acquisition Acronyms and Terms |publisher=Department of Defense |access-date=10 April 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140413164657/https://dap.dau.mil/glossary/Pages/2045.aspx |archive-date=13 April 2014 }}</ref>
The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment. It excludes logistics time, waiting or administrative downtime, and preventive maintenance downtime. It includes corrective maintenance downtime. 
Inherent availability is generally derived from analysis of an engineering design:
# The impact of a repairable-element (refurbishing/remanufacture isn't repair, but rather replacement) on the availability of the system, in which it operates, equals [[mean time between failures]] MTBF/(MTBF+ [[mean time to repair]] MTTR).
# The impact of a one-off/non-repairable element (could be refurbished/remanufactured) on the availability of the system, in which it operates, equals the [[mean time to failure]] (MTTF)/(MTTF + the [[mean time to repair]] MTTR).
 
It is based on quantities under control of the designer.

'''Availability, achieved (Aa)''' <ref>{{cite web|title=Achieved Availability (AI) |url=https://dap.dau.mil/glossary/Pages/1380.aspx |work=Glossary of Defense Acquisition Acronyms and Terms |publisher=Department of Defense |access-date=10 April 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140413164705/https://dap.dau.mil/glossary/Pages/1380.aspx |archive-date=13 April 2014 }}</ref> 
The probability that an item will operate satisfactorily at a given 
point in time when used under stated conditions in an ideal support environment (i.e., that personnel, tools, spares, etc. are instantaneously available). It excludes logistics time and waiting or administrative downtime. 
It includes active preventive and corrective maintenance downtime.

'''Availability, operational (Ao)''' <ref>{{cite web|title=Operational Availability (AI) |url=https://dap.dau.mil/glossary/Pages/Archived/1476.aspx |work=Glossary of Defense Acquisition Acronyms and Terms |publisher=Department of Defense |access-date=10 April 2014 |url-status=dead |archive-url=https://web.archive.org/web/20130312154509/https://dap.dau.mil/glossary/Pages/Archived/1476.aspx |archive-date=12 March 2013 }}</ref>
The probability that an item will operate satisfactorily at a given point in time when used in an actual or realistic operating and support environment. It includes logistics time, ready time, and waiting or administrative downtime, and both preventive and corrective maintenance downtime. This value is equal to the mean time between failure ([[Mean time between failures|MTBF]]) divided by the mean time between failure plus the mean downtime (MDT). This measure extends the definition of availability to elements controlled by the logisticians and mission planners such as quantity and proximity of spares, tools and manpower to the hardware item.

Refer to [[Systems engineering]] for more details