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.[1][2][3]

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:[4]

  • 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
Depiction of the no fault found cycle. Each clockwise cycle after the initial is a waste of maintenance resource.

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.

The fact remains that no fault found causes a cost to industry. NFF is thought to cost the United States Department of Defense in excess of US$2 billion per year.[5]

See also

References
  1. 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.
  2. James, I.; Lumbard, D.; Willis, I.; Goble, J. (1 January 2003). "Investigating no fault found in the aerospace industry". Annual Reliability and Maintainability Symposium, 2003. pp. 441–446. doi:10.1109/RAMS.2003.1182029. ISBN 978-0-7803-7717-2. S2CID 109886043.
  3. 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.
  4. 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" (PDF). Reliability Engineering & System Safety. 123: 183. doi:10.1016/j.ress.2013.11.003. hdl:1826/9947.
  5. Werner, Debra (February 2015). "Aerospace America" (PDF). No. 2. AIAA. Archived from the original (PDF) on 2017-05-10. Retrieved 2020-01-01.
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