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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

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Original Message:

Posted by: John (kelsmit@aol.com )
Date posted: Fri Jan 25 15:36:43 US/Eastern 2002
Subject: Unconfirmed Field Failures
Message:
When testing returned units (i.e. circuit cards) that have failed at customer sites, occasionally the reported failure or any failure can not be duplicated. Is there any information or rule of thumb regarding these unconfirmed field failure rates? I would be interested in any opinions or experiences.


Reply:

Subject: Unconfirmed Field Failures
Reply Posted by: Allan Brown (allan@ramsconsultants.com )
Organization: RAMS Consultants Ltd.
Date Posted: Mon Jan 28 15:12:55 US/Eastern 2002
Message:
We usually consider these to be "No Fault Found" even if they do have a failure unrelated to the customer's complaint. The unrelated failures are fixed and the unit is returned to the customer. The rationale is: the customer reported a failure and returned the unit on that basis. It is probable that the unit would not have been returned otherwise. However, keep a log of the NFF customer complaints in case there happens to be a procedural fault. This will show up when performing a trend analysis.


Reply:

Subject: NFF Failures
Reply Posted by: Joe Dzekevich (joseph_a_dzekevich@raytheon.com )Reliability Engineering
Date Posted: Fri Feb 8 12:42:21 US/Eastern 2002
Message:
You may want to run some stress tests on your NFF returns. There may be an intermittent or laten defect. In some cases, a HALT test or the like can find a latent or intermittent problem with NFFs. Just a thought.


Reply:

Subject: Unconfirmed Field Failures
Reply Posted by: David A. Douthit (davidd@amoug.org )
Organization: LoCan LLC
Date Posted: Thu Mar 7 21:27:53 US/Eastern 2002
Message:
John, Unconfirmed failures - No-Fault-Found (NFF) must be addressed very seriously. The one thing you can guaranty is that it will happen again. You must be aware of the differences between the testing conditions (environmental, software, and hardware interfaces) and the field conditions. You must make every effort to recreate these field conditions or remain caught in reaccurring compatibility issues. D. A. Douthit


Reply:

Subject: Unconfirmed Field Failures
Reply Posted by: John Turnbull
Date Posted: Fri Mar 15 5:55:30 US/Eastern 2002
Message:
I would suggest looking at the other components in the system. Could something, other than the returned component, have failed but in replacing the returned component been fixed. i.e. resetting system parameters. In power generating systems you can find that the generator trips the control unit off-line but because its "easier" to replace the control unit thats what the technician does. So where the generator caused the fault the control unit is sent back to base faulty and when tested is found to be an unjustified removal (NFF). We find this happens a lot. The easier an LRU is to remove the more NFF returns arise. Again, just a thought.


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