No Fault Found (NFF)
No Fault Found (NFF)
The maintenance of aircraft electronic Line Replaceable Assemblies (LRUs), Weapon Replaceable Assemblies (WRAs), and their corresponding Electrical Wiring Interconnect Systems (EWIS) / cable harnesses pose an ever increasing maintenance challenge. Much of the challenge is related to intermittent faults which occur due to aging wiring and interconnections devices that degrade over time. Current Automatic Test Equipment (ATE) serially scans one function or circuit at a time as per its design. This can lead us further away from the root-cause identification of intermittent issues.
These intermittent problems manifest themselves as “bad actor” LRUs, WRAs, or wiring and drive significant costs and degradation to aircraft availability and readiness. In the maintenance community, intermittent faults are the primary driver of test results known by many names or codes, including:
- No Fault Found (NFF)
- Can Not Duplicate (CND)
- No Trouble Found (NTF)
- Retest OK (RTOK)
- No Evidence of Failure (NEOF)
- A799
Intermittent faults are a growing problem in electronic equipment. As integrated circuits (ICs) and other electronic discrete parts become increasingly capable and reliable, an ever growing portion of the electronic equipment maintenance problems encountered are not component failures, but rather inter-connectivity problems between the electronic piece parts.
Intermittent faults can be caused by:
- loose or corroded wire wraps
- cracked solder joints
- corroded connector contacts
- loose crimp connections
- hairline cracks in a printed circuit trace
- vertical interconnect accesses (vias)
- other phenomena common in electronic equipment
An intermittent fault will often manifest itself during an initial troubleshooting test, such as in the electronic back shop of a commercial repair facility or military flying wing, but by the time the equipment item has been transported to a commercial or military depot for repair, the intermittent circuit is no longer manifesting. This results in NFF. LRUs and WRAs test NFF roughly 50% of the time when inducted into repair facilities.
Intermittent Fault Detection and Isolation System 2.0 (IFDIS 2.0), which augments traditional testing methods, is used to detect and isolate intermittent faults as well as any other wiring problems in complex LRU and WRA chassis.
Portable Intermittent Fault Detector (PIFD) is the portable variant used to detect and isolate intermittent faults in Electronic Wiring Interconnect Systems (EWIS) and cable harnesses and is applicable at all levels of maintenance.