AN INVESTIGATION OF CANNOT DUPLICATE FAILURES

Various terms such as 'cannot duplicate (CND)', 're-test OK (RTOK)', ' no fault indicated (NFI)', 'no fault found (NFF)', and 'no trouble fou nd (NTF)', are used to describe the inability to replicate field failu res during laboratory assessment. This paper uses CND to refer to all such failures. CND failures can make up more than 85% of all observed field failures in avionics and account for more than 90% of all mainte nance costs. These statistics can be attributed to a limited understan ding of root cause failure characteristics of complex systems, inappro priate means of diagnosing the condition of the system, and the inabil ity to duplicate the field conditions in the laboratory. This paper ad dresses CND issues with reference to research carried out on samples o f an electronics board used as the seat-back processor modules on boar d the Boeing 777. The boards were monitored continuously using existin g on-board comprehensive built-in test equipment. It was found that th e hot temperature operating limits of the board decreased by up to 70 degrees C during highly accelerated environmental stress. Furthermore, improperly seated connectors were found to result in spurious compone nt failure reports from the built-in test equipment. This paper sugges ts that the observed drift in operating limit and connector issues are two likely root causes of CND failures and makes recommendations for addressing them. (C) Crown Copyright 1998. Reproduced with the permiss ion of the Controller of Her Majesty's Stationary Office.