SRC Forum - Message Replies
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
Posted by: James Bauer
Date posted: Wed Feb 11 16:58:32 US/Eastern 2004
Subject: LRU undetected failure rates
In performing a system safety analysis for an aircraft installation I used the product manufacturer's Failure Modes and Criticality Analysis as a starting point for my system level analysis. The manufaturer maintains that the undetected failure rate (failures not detected by Built In Test) for LRUs interfaced with the system can be assumed at 5% for digital components and 10% for analog LRUs. Is there any data or documentation to substantiate this position?
Subject: Undetected Faults
Reply Posted by: BWD
Date Posted: Thu Feb 12 14:18:03 US/Eastern 2004
Approximate K factors for percentages of undetectable faults is not logically possible. The reasons for this conclusion are the design techniques used, the operating scenarios used and the types of components used vary from unit to unit. The most common parts of a design that are not detectable are protection circuits such as voltage cut off, current over and under protection, EMI protection circuits, and unused gates or logic circuits. The failure mode for these undetectable faults is an open circuit. Many designs do not use these protection circuits so would have a much better detection rate, hence the problem of using a K factor for a safety analysis.
I would recommend that you perform a FMECA analysis and determine the exact value for the design under consideration. If this is to time consuming, then consider the review of the schematics to determine the number of protection circuits and use this as an approximation index.