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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: Vincent ( )
Date posted: Mon Sep 16 23:24:12 US/Eastern 2002
Subject: Field MTBF
Currently, I am collecting field failures for my computation of Field MTBF. The fomular used : (Total fleet * utilization hours)/ total field failure. Can someone advice is my compution on Field MTBF is correct if not what is the correct approach? As years passed more failures encountered. How many years of data must I collect in order reflect correct field MTBF? rgds


Subject: Field MTBF
Reply Posted by: Jean-Marie CLOAREC ( )
Organization: LIGERON SA
Date Posted: Mon Sep 23 9:06:56 US/Eastern 2002
Your formular is partially true if the time between each fleet is very very small compared to the time duration of one fleet. In fact, system, component, ... can failed anywhere and not only in flight !!! and the problem is that they are not used similarly in use or not !!! So, with your formula, you're not taking into account failure whose cause is due to non-utilisation. And if you considerer the whole time (in use or not) you can't distinguish the failure from failure in use, or failure not used. It is a difficult problem in fact. But this last assumption is pessimistic, and when working on reliability topic, it's always better to be pessimistic than not.

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