No Fault Found

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Impact of ‘No Fault Found’ (NFF) and Intermittent Fault Arisings on Aircraft Avionic System Maintenance Operations


1.0 Introduction3
2.0 The NFF Phenomenon4
2.1 Reasons NFF present themselves4
3.0 Impact of NFF5
3.1 Cost6
3.2 Phantom Supply Chain6
4.0 Challenges Faced By Maintenance Personnel6
5.0 Industry Approaches7
5.1 Symptom Diagnostics7
5.2 Ship or Shelve Policy7
5.3 Neural Network Technology7
6.0 Conclusion8
7.0 Bibliography9

1.0 Introduction:
No Fault Found is a phenomenon in which a symptom fault is reported by an end user however maintenance technicians cannot find the root cause of the fault. NFF is known under a variety of names such as Can Not Duplicate (CND), Re-Test OK (RTOK), Trouble Not Identified (TNI), No Trouble Found (NTF) and No Fault Indicated (NFI). This type of fault is recurrent and is hard to detect since the symptoms which originally led to an indication of a fault presence cannot be recreated using conventional techniques until the fault becomes a hard fault which is when the root cause of the fault is easier to pin point. This phenomenon has played a major role and still does in speculative component replacement by technicians which in turn increased the Total Operating Costs (TOCs) of several industries including automotive, telecommunication, computers and avionics. This report will discuss the impact of No fault found scenarios on the aviation industry with respect to several factors such as the scale of NFF arisings, reasons NFF present themselves, costs involved, aircraft availability, challenges faced by maintenance personnel and the Industry approaches adopted to combat this phenomenon.

Figure 1 Hard failures and NFF failures on military aircraft

2.0 The NFF Phenomenon
The severity of the NFF phenomenon is felt when analyzing the regularity of its occurrence in all electronic based industries. In the aviation sector its occurrence is not very delightful as its frequency increases it affects the system availability and creates a safety hazard, as failure during flight may lead to mission abortion whether in the commercial or military sector resulting in down time.

Figure 2 NFF frequency progression

2.1 Reasons NFF present themselves
The major contributor to a no fault found diagnoses are ‘Intermittent faults’. Intermittent faults are randomly occurring faults and are very hard to predict, they lead to momentary loss of system functionality causing an outage of the system in the order of nano seconds which in turn results in the loss of valuable information. A subsequent testing of the system results in a NFF diagnosis. These faults manifest themselves in the most inaccessible areas of the aircraft such as the Electrical Wiring Interconnection Systems (EWIS) and with the increase in the complexity of the system the probability of intermittent faults also increases. The root causes of intermittency are attributed to mechanical stresses in the 3C’s: * Cables

* Connectors
* Chassis

Mechanical and Environmental stresses encountered:
* Thermal Cycling
* Creep Corrosion
* Fretting Corrosion
* Moisture
* Metallization
* Layer Delamination
* Aging
* Vibration
* Strain Relaxation
These factors lead to intermittent faults and over a period of time if the fault is not identified and stopped at its root cause, it will become a hard fault which is detectable using conventional diagnosis techniques and necessary measures can be taken to repair the fault.

3.0 Impact of NFF
How do NFF affect TOCs? A simple answer to this question can be concluded by examining the rate of ‘First-Time Fix’ in NFF related scenarios but since that data is not readily available, the rate of ‘Diagnostic Success’ is examined instead. Although diagnostic success...
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