SRC Forum - Message Replies

Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

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

Original Message:

Posted by: Dave Fetters (david_fetters@udlp.com )
Organization:United Defense
Date posted: Thu Jan 13 12:00:16 US/Eastern 2000
Subject: MIL STD 471 - M. Demo
Message:
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?

Previous reply

Reply:

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

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.