989:
1203:. If the time interval of interest is the primary concern, we consider instantaneous, limiting, average, and limiting average availability. The aforementioned definitions are developed in Barlow and Proschan , Lie, Hwang, and Tillman , and Nachlas . The second primary classification for availability is contingent on the various mechanisms for downtime such as the inherent availability, achieved availability, and operational availability. (Blanchard , Lie, Hwang, and Tillman ). Mi gives some comparison results of availability considering inherent availability.
1196:. Barlow and Proschan define availability of a repairable system as "the probability that the system is operating at a specified time t." Blanchard gives a qualitative definition of availability as "a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called for at an unknown random point in time." This definition comes from the MIL-STD-721. Lie, Hwang, and Tillman developed a complete survey along with a systematic classification of availability.
963:
1508:
1118:
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
1210:
and repairs, Fawzi and Hawkes for a series system with replacement and repair, Iyer for imperfect repair models, Murdock for age replacement preventive maintenance models, Nachlas for preventive maintenance models, and Wang and Pham for imperfect maintenance models. A very comprehensive recent
1077:
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.
1111:
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
948:
1002:
Using parallel components can exponentially increase the availability of overall system, so long as those components fail independently. For example if each of your hosts has only 50% availability, by using 10 of hosts in parallel, you can achieve 99.9023% availability.
1123:) 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.
506:
953:
Availability is the probability that an item will be in an operable and committable state at the start of a mission when the mission is called for at a random time, and is generally defined as uptime divided by total time (uptime plus downtime).
1512:
1477:
821:
391:
720:
778:
272:
431:
1466:
611:
1082:
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
640:
99:) is a ratio of the expected value of the uptime of a system to the aggregate of the expected values of up and down time (that results in the "total amount of time"
1227:
1055:
Logistical aspects like; spare part (stocking) levels at different depots, transport times, repair times at different repair lines, manpower availability and more.
423:
813:
1206:
Availability considered in maintenance modeling can be found in Barlow and
Proschan for replacement models, Fawzi and Hawkes for an R-out-of-N system with
1093:
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
985:
Availability of parallel components = 1 - (1 - availability of component A) X (1 - availability of component B) X (1 - availability of component C)
75:
The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment.
943:{\displaystyle A_{\infty }=\lim _{c\rightarrow \infty }A_{c}=\lim _{c\rightarrow \infty }{\frac {1}{c}}\int _{0}^{c}A(t)\,dt,\quad c>0.}
1231:
68:
or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown,
1360:
291:
979:
Therefore, combined availability of multiple components in a series is always lower than the availability of individual components.
1584:
1329:
1258:
992:
10 hosts, each having 50% availability. But if they are used in parallel and fail independently, they can provide high availability.
1436:
1018:
are developed to calculate availability of a system or a functional failure condition within a system including many factors like:
976:
Availability of series component = (availability of component A) x (availability of component B) x (availability of component C)
1062:
Furthermore, these methods are capable to identify the most critical items and failure modes or events that impact availability.
648:
1523:
1517:
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1376:
731:
31:
109:
1268:
1120:
1083:
501:{\displaystyle X(t)={\begin{cases}1,&{\text{sys functions at time }}t\\0,&{\text{maintenance}}\end{cases}}}
42:
1011:
1199:
Availability measures are classified by either the time interval of interest or the mechanisms for the system
1322:
System
Sustainment: Acquisition And Engineering Processes For The Sustainment Of Critical And Legacy Systems
1248:
49:
616:
Average availability must be defined on an interval of the real line. If we consider an arbitrary constant
35:
1139:
1094:
973:
Let's say a series component is composed of components A, B and C. Then following formula applies:
455:
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1143:
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1253:
962:
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80:
1569:
619:
988:
1273:
399:
1564:
285:) is a ratio of the Mean Time To Failure (MTTF) and Mean Time Between Failure (MTBF), or
1444:
1177:
Outage due to equipment in hours per year = 1/rate = 1/MTTF = 0.01235 hours per year.
786:
1578:
1243:
1223:
1527:
1078:
Inherent availability is generally derived from analysis of an engineering design:
996:
In corollary, if you have N parallel components each having X availability, then:
1533:
1414:
1384:
1476:. North American Electric Reliability Corporation. July 2011. pp. 7, 17.
1353:
Reliability and
Availability Engineering: Modeling, Analysis, and Applications
1207:
1542:
Reliability and
Availability Engineering: Modeling, Analysis and Applications
1154:(This is a reliability parameter and often has a high level of uncertainty!)
65:
17:
1200:
982:
On the other hand, following formula applies to parallel components:
61:
987:
1467:"Mandatory Reporting of Conventional Generation Performance Data"
1283:
1112:
includes active preventive and corrective maintenance downtime.
386:{\displaystyle A={\frac {MTTF}{MTTF+MTTR}}={\frac {MTTF}{MTBF}}}
783:
Limiting average availability is also defined on an interval
494:
83:
systems might be specified as 99.98%, 99.999% or 99.9996%.
725:
Limiting (or steady-state) availability is represented by
715:{\displaystyle A_{c}={\frac {1}{c}}\int _{0}^{c}A(t)\,dt.}
1105:
It is based on quantities under control of the designer.
1052:
Active operational times / missions / sub system states
1152:
81.5 Γ 365 Γ 24 = 713940
999:
Availability of parallel components = 1 - (1 - X)^ N
824:
789:
734:
651:
622:
532:
434:
402:
294:
112:
1161:= 713940 / (713940+1) = 713940 / 713941 = 99.999860%
773:{\displaystyle A=\lim _{c\rightarrow \infty }A_{c}.}
1565:
Availability and the
Different Ways to Calculate It
1441:Glossary of Defense Acquisition Acronyms and Terms
1411:Glossary of Defense Acquisition Acronyms and Terms
1381:Glossary of Defense Acquisition Acronyms and Terms
942:
807:
772:
714:
634:
605:
500:
417:
385:
266:
267:{\displaystyle A={\frac {E}{E+E}}={\frac {E}{C}}}
1570:How to track and improve Technical Availability?
868:
839:
742:
548:
41:"Available" redirects here. For other uses, see
1228:North American Electric Reliability Corporation
642:, then average availability is represented as
8:
1007:Methods and techniques to model availability
1119:is equal to the mean time between failure (
1188:is well established in the literature of
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961:
1443:. Department of Defense. Archived from
1413:. Department of Defense. Archived from
1383:. Department of Defense. Archived from
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1138:If we are using equipment which has a
1066:Definitions within systems engineering
7:
1347:
1345:
1343:
1341:
1316:
1314:
1312:
523: > 0 is represented by
1560:System Reliability and Availability
1555:Reliability and Availability Basics
1544:, Cambridge University Press, 2017.
1474:Generating Availability Data System
1232:Generating Availability Data System
1306:, Addison Wesley, Reading, MA,1996
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1259:List of system quality attributes
396:If we define the status function
1511: This article incorporates
1506:
1211:book is by Trivedi and Bobbio .
1524:General Services Administration
1483:from the original on 2022-10-09
1437:"Operational Availability (AI)"
930:
91:The simplest representation of
27:Term in reliability engineering
1116:Availability, operational (Ao)
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966:series vs parallel components
958:Series vs Parallel components
606:{\displaystyle A(t)=\Pr=E.\,}
32:Availability (thermodynamics)
1407:"Achieved Availability (AI)"
1377:"Inherent Availability (AI)"
511:therefore, the availability
56:has the following meanings:
1269:Condition-based maintenance
1159:Inherent availability (Ai)
1109:Availability, achieved (Aa)
468:sys functions at time
103:of the observation window)
1601:
1540:K. Trivedi and A. Bobbio,
1084:mean time between failures
1012:Reliability Block Diagrams
43:Available (disambiguation)
40:
29:
1142:(MTTF) of 81.5 years and
1071:Availability, inherent (A
1058:Uncertainty in parameters
1585:Telecommunication theory
1172:= 1 / 713940 = 0.000140%
1170:Inherent unavailability
30:Not to be confused with
1304:Reliability Engineering
1249:Reliability engineering
1224:power plant engineering
1222:is used extensively in
50:reliability engineering
1519:Federal Standard 1037C
1513:public domain material
1025:Maintainability models
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635:{\displaystyle c>0}
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60:The degree to which a
36:Availability heuristic
1532: (in support of
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277:Another equation for
269:
1140:mean time to failure
1095:mean time to failure
1034:Common cause failure
1028:Maintenance concepts
822:
787:
732:
649:
620:
530:
432:
418:{\displaystyle X(t)}
400:
292:
110:
1264:Spurious trip level
1226:. For example, the
1220:Availability factor
1194:optimal maintenance
1190:stochastic modeling
1144:mean time to repair
1128:Systems engineering
1099:mean time to repair
1097:(MTTF)/(MTTF + the
1088:mean time to repair
1016:Fault Tree Analysis
907:
689:
1254:Safety engineering
1146:(MTTR) of 1 hour:
1022:Reliability models
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1279:High availability
1130:for more details
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81:high availability
16:(Redirected from
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1526:. Archived from
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1471:
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1452:
1447:on 12 March 2013
1433:
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1417:on 13 April 2014
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1387:on 13 April 2014
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1230:implemented the
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1150:MTTF in hours =
1046:Dormant failures
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808:{\displaystyle }
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1134:Basic example
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1528:the original
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1485:. Retrieved
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1449:. Retrieved
1445:the original
1440:
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1419:. Retrieved
1415:the original
1410:
1401:
1389:. Retrieved
1385:the original
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1303:
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1215:Applications
1205:
1198:
1186:Availability
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1086:MTBF/(MTBF+
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516:
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1037:Diagnostics
488:maintenance
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1290:References
1181:Literature
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1126:Refer to
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753:∞
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677:∫
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66:subsystem
1579:Category
1487:13 March
1478:Archived
1451:10 April
1421:10 April
1391:10 April
1355:. 2017.
1324:. 2022.
1238:See also
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1500:Sources
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