No fault found
No fault found (NFF), no trouble found (NTF) or no defect found (NDF) are terms used in the field of maintenance, where a unit is removed from service following a complaint of a perceived fault by operators or an alarm from its BIT (built-in test) equipment. The unit is then checked, but no anomaly is detected by the maintainer. Consequently, the unit is returned to service with no repair performed.
If there is an underlying fault that has not been detected the unit may be returned for repair several times with no fault identified. Alternative descriptors include:
- No fault found (NFF)
- Cannot duplicate (CND)
- Fault not found (FNF)
- No trouble found (NTF)
- No defect found (NDF)
- Hidden failures
- False failures
The NFF problem
As the figure shows once a fault has been reported, investigated, and no fault found any future problems caused by the fault cause additional work which is a waste of maintainer time. Different causes have been suggested for this issue.
- Some can be attributed to the way a possible fault is perceived by the user.
- Some can be attributed to the diagnostic methods available to the maintainer.
- Söderholm, Peter (January 2007). "A system view of the No Fault Found (NFF) phenomenon". Reliability Engineering & System Safety. 92 (1): 1–14. doi:10.1016/j.ress.2005.11.004.
- James, I.; Lumbard, D.; Willis, I.; Goble, J. (1 January 2003). "Investigating no fault found in the aerospace industry". Reliability and Maintainability Symposium, 2003. Annual: 441–446. doi:10.1109/RAMS.2003.1182029. ISBN 978-0-7803-7717-2.
- Qi, Haiyu; Ganesan, Sanka; Pecht, Michael (May 2008). "No-fault-found and intermittent failures in electronic products". Microelectronics Reliability. 48 (5): 663–674. doi:10.1016/j.microrel.2008.02.003.
- Khan, Samir; Phillips, Paul; Jennions, Ian; Hockley, Chris (March 2014). "No Fault Found events in maintenance engineering Part 1: Current trends, implications and organizational practices". Reliability Engineering & System Safety. 123: 183. doi:10.1016/j.ress.2013.11.003. hdl:1826/9947.
- Werner, Debra (February 2015). "Aerospace America" (PDF) (2). AIAA. Archived from the original (PDF) on 2017-05-10. Retrieved 2020-01-01. CS1 maint: discouraged parameter (link)