SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and AnswersTopic Posted by: Reliability & Maintainability Forum (firstname.lastname@example.org )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Dave Fetters
Subject: Maintainability Demonstration
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.