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 found (NTF)', are used to describe the inability to replicate field failures 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 maintenance costs. These statistics can be attributed to a limited understanding of root cause failure characteristics of complex systems, inappropriate means of diagnosing the condition of the system, and the inability to duplicate the field conditions in the laboratory. This paper addresses CND issues with reference to research carried out on samples of an electronics board used as the seat-back processor modules on board the Boeing 777. The boards were monitored continuously using existing on-board comprehensive built-in test equipment. It was found that the hot temperature operating limits of the board decreased by up to 70°C during highly accelerated environmental stress. Furthermore, improperly seated connectors were found to result in spurious component failure reports from the built-in test equipment. This paper suggests that the observed drift in operating limit and connector issues are two likely root causes of CND failures and makes recommendations for addressing them. © Crown Copyright 1998. Reproduced with the permission of the Controller of Her Majesty's Stationary Office.