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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Posted by: Dick Creamer ( )
Organization:NAVAIR Depot Jacksonville, FL
Date posted: Mon Feb 11 15:29:10 US/Eastern 2002
Subject: Availability
I'm trying to calculate the Ao for a fielded avionics system which may not be turned on for every mission. There is no elapsed time meter on the system. I understand total uptime would include operating time plus standby time. The conflict I'm involved in is what constitutes standby time? Is it the total flight hours in which the system is not turned on? Or, is it the rest of the year that the system is not operating? Or, does the concept of standby time even apply? It stands to reason that the system is not going to be available if you don't turn it on. Help!


Subject: Availability
Reply Posted by: Bruce Dudley ( )
Organization: RAC
Date Posted: Thu Feb 14 13:31:47 US/Eastern 2002
The concept of availability was originally developed for repairable systems, equipment or units that are required to operate continuously, that is, twenty four hours per day, and are at any random point in time required to operate as specified or be considered down. In this concept, a system is considered to be in only two possible states, operating or in repair, and availability is defined as the probability that a system is operating satisfactorily at any random point in time. For your situation the "operating" period is the mission interval and the unit should be considered up whether or not it is operating or in standby. Standby ready to operate scenario should be considered a "up" state. You must figure the down state as that interval when the system has a mission critical failure until the mission is over. So an equation you could consider is: Ao = MTBCF / MTFCF + MEAN MISSION DOWN TIME

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