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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: Dave Fetters ( )
Organization:United Defense
Date posted: Thu Jan 13 12:00:16 US/Eastern 2000
Subject: MIL STD 471 - M. Demo
I'm planning a Maintainability Demonstration. There are MTTR and MaxTTR requirements at each of 3 levels of maintenance. I wanted to use Test Method 8 in MIL-STD-471A in hopes of avoiding 50 observations on 6 different specifications (which the other methods seem to require). However, the DS-level MTTR spec. is 2 hours (greater than the 100 minute limitation called out in the second paragraph of the method). I can't figure out how important that 100 minute limitation is. In other words, is it possible to "cheat" by 20 minutes, or is there a workaround for longer MTTRs? Anybody know?


Subject: Maintainability Demonstration
Reply Posted by: Bruce Dudley ( )
Organization: Reliability Analysis Center
Date Posted: Tue Jan 18 9:25:34 US/Eastern 2000
Mil-Std-471(Maintainability Verification and Demonstration), which has been replaced by Mil-Hdbk-470A, indicates that the mean time interval to be tested should be "greater than 10 and less than 100 units of time" for test method #8. This is not a specific requirement for "minutes" it could be considered to be "hours" or "minutes and hours". I do not believe that a test using 120 minutes as the mean and 240 minutes as a maximum will causes any problem with the sequential decision matrix.


Subject: M Demo Method 8
Reply Posted by: Ned Criscimagna ( )
Organization: IIT Research Institute
Date Posted: Thu Jan 27 14:06:42 US/Eastern 2000

This reply is a follow-on to the email messages I sent privately to Mr. Fetters.  In responding to his question on Method 8 for maintainability demonstration, I have the following comments.  First, MIL-STD-471 has been canceled.  It was first converted to (re-titled as) a military handbook.  Then it was included in a revision to MIL-HDBK-470.  MIL-HDBK-470A, dated 4 Aug 1997 replaced both MIL-HDBK-470 and MIL-HDBK-471.

 However, since the method calls for no more than 100 "time units," I believe you can define a time unit as 1.2 minutes.  Hence, you could use the tables and convert your requirements to  the 1.2 minute time unit.  If the item in question passes the tests using the converted units, it will have done so "legitimately," that is, you will not have passed without distorted the statistics.
 Otherwise, you can convert the measurements to their logs and use a normal distribution to determine the mean and percentiles (this is the basis of the test anyway).
 You could develop your own tables for specific values of MTTR and Mmax by deriving them using the lognormal distribution.	

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