No fault found explained

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

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 per year.^[5]

See also

• Troubleshooting
• Unintended consequences

Notes and References

1. Söderholm. Peter. A system view of the No Fault Found (NFF) phenomenon. Reliability Engineering & System Safety. January 2007. 92. 1. 1–14. 10.1016/j.ress.2005.11.004.
2. Book: James. I.. Lumbard. D.. Willis. I.. Goble. J.. Annual Reliability and Maintainability Symposium, 2003 . Investigating no fault found in the aerospace industry . 1 January 2003. 441–446. 10.1109/RAMS.2003.1182029. 978-0-7803-7717-2. 109886043 .
3. Qi. Haiyu. Ganesan. Sanka. Pecht. Michael. No-fault-found and intermittent failures in electronic products. Microelectronics Reliability. May 2008. 48. 5. 663–674. 10.1016/j.microrel.2008.02.003.
4. Khan. Samir. Phillips. Paul. Jennions. Ian. Hockley. Chris. No Fault Found events in maintenance engineering Part 1: Current trends, implications and organizational practices. Reliability Engineering & System Safety. March 2014. 123. 183. 10.1016/j.ress.2013.11.003. 1826/9947. free.
5. News: Werner. Debra. Aerospace America. https://web.archive.org/web/20170510083521/http://www.aerospaceamerica.org/Documents/Aerospace%20America%20PDFs%202015/February2015/Feature1_AA_Feb2015.pdf. February 2015. 2017-05-10. 2020-01-01. 2. AIAA.