No Fault Found


The utilization of micro-electronic systems in avionics is characterized by the potential for recurring failures that may have an adverse effect on the product’s functioning. While some of these failures may be easily corrected in the course of avionic system maintenance operations, there are certain categories of operational failure that present particularly serious technical challenges to pilots and engineers. In this assessment, the analysis of ‘No Fault Found’ (NFF) failures is undertaken, with a view to establishing the scale of the problem presented by NFF failures, reasons for which NFFs manifest themselves, challenges that maintenance personnel has to face while doing with them, and the industry approaches generally used to combat this category of failures.

NFF Definition

In most general terms, a NFF failure may be defined as a failure (fault) that occurred or was reported to have occurred in the course of application’s use but was not found or discovered on close examination (Qi, Ganesan & Pecht 2008, p.663). While problem isolation inability may be caused by generally unsatisfactory state of the system itself or the methods used in the course of testing, the NFF failures still present particularly difficult issue to tackle for manufacturers and engineers.

The NFF failure may occur at each level of the system tested, with hardware, software, or human interface equally affected by its manifestation. The detection of NFFs generally takes place when a faulty subsystem is identified by a top-level systemic test and is substituted with another, properly functioning subsystem. However, when a test is carried out on the subsystem removed (line replaceable unit, LRU), the detecting software is unable to find the exact reasons for a fault, and reports it as ‘no found’.  The principal detection problem is the short-term nature of the NFFs, which follow particular environmental stresses endured by an aircraft (DE&S 2012, p.138).

The 2012 Copernicus Technology survey conducted among avionic maintenance personnel found that intermittent faults were viewed as the core causal factor for NFF occurrences (Copernicus Technology 2012, p.18). Following this category, lack of technician experience and the troubleshooting manuals’ deficiencies were identified as most important secondary factors for the NFFs. Software problems were effectively relegated to the category of the least prominent NFF factors.

The discovery and determination of the NFF reasons are of utmost importance to avionic maintenance operators and aircraft engineers. As this category of failures presents numerous challenges and inflicts significant expenses on the industry, it is necessary to review exact forms and scale that the NFF failures take on in avionic maintenance facilities. 

Scale of the Problem

In avionics, the occurrences of NFF failures are reported to exceed the respective levels for automotive, telecommunications, and other industries; according to data cited by Sorensen (2003), the NFF failure rate for military aircraft appeared to be as high as 50%. Similarly, the 1990 avionics field failure study results demonstrated that 21-70% of total electronic facilities’ failures in avionic applications were conditioned by the NFF factor (Pecht & Ramappan 1992). The commercial airlines and military aircraft seem to be particularly affected by the NFF, as 50-60% of total failure observations in this field are related to the NFF category (Maher 2004).

In particular, the Copernicus Technology surveys report that the NFF costs in military aircraft are specifically significant. For instance, in the course of a NcompassTM IFD testing for 381 radar line replaceable units’ chassis used in F-16 aircraft, it was reported that their maintenance costs would have exceeded $11 million, with $39 million-worth assets previously considered unrepairable (Huby 2012, p.139). This example may show the specific costs that are due to NFF occurrences in avionic maintenance. As to the industry-wide estimations of the NFF economic impact, the 1995 Air Transport Association (ATA) survey reported that 4,500 NFF events’ cumulative costs approached $100 million for the ATA members (Ambler, Bassat & Ungar, 1997, p.435). Lorel, Lowell, Kennedy, and Levaux (2000) estimate that, in 2000, more than $300 million were spent by the aircraft industry to provide for avionic maintenance operations necessitated by the NFFs occurrences. While the situation apparently improved with widespread utilisation of the Built-in Test (BIT) technology, the NFF financial costs still remain unacceptable to the majority of commercial operators and military institutions.

With respect to down-time costs of the NFFs, the latter’s impact on Mean Times to Repair (MTR) and Mean Time Between Unscheduled Removals (MTUBR) is far from negligible. In particular, Block, Tyrberg, and Söderholm (2009) place emphasis on the NFFs’ adverse effect in this regard. Similarly, the 2012 Copernicus Technology survey testified to the concerns expressed by the avionic maintenance personnel community on the issues of the NFFs impact in these fields.

Finally, additional maintenance services conducted due to the NFF presence may pose significant financial burden for the companies and institutions involved. As shown by Söderholm (2007), these maintenance actions’ costs and the delays inflicted by them may appear punishing for commercial airlines’ profits.

Reasons for NFFs occurrences

As reported by Prakash, Izquierdo, and Ceglarek (2009), the NFF root causes include early design errors, such as fallacious characterization of Key Product Characteristics (KPCs), Key Control Characteristics (KCCs), Customer Attributes, and similar product’s modular characteristics. In addition, the in-tolerance product failure potential cannot be precisely specified at the final product testing due to the presence of in-tolerance failure regions, having an adverse impact on the Taguchi’s quality loss function for the respective product (Prakash, Izquierdo & Ceglarek 2009, p.37). Thus, the KPCs and KCCs interaction failures provide the majority of the NFFs that are connected with design deficiencies.

Design problems facilitate the occurrence of intermittencies, which may be generally defined as temporary deviations from normal operating conditions of a circuit or a drive (Copernicus Technology 2011, p.15). It is the intermittent failures that are most often responsible for the NFF arising, and they present the hardest problem for the maintenance personnel in this respect.

Other factors influencing the NFF presence encompass both man-made errors in dealing with the respective equipment and random environmental stresses endured by the aircraft’s hardware. In particular, the ‘Producers’ subgroup of the Copernicus Technology survey interviewees considered ‘Environmental (heat, vibration, etc.)’ issues to be the second most important factor influencing the NFF occurrences (Copernicus Technology 2012, p.20). At the same time, component integrity problems were ranked higher by the airworthiness/QCI/regulatory, as well as the maintenance personnel participating in the survey (2012, p.21).

Challenges faced by maintenance personnel

The maintenance personnel perform a particularly important set of functions in dealing with the NFFs issues. According to Peti, Obermaisser, Ademaj, and Kopetz (2005), the majority of existing onboard failure diagnosing systems rely on imprecise information, which might lead to erroneous working component isolation, while the faulty item remains intact (2005, p.128b). The ‘try-and-error’ model to which the maintenance personnel were subjected until the last time led to gross inefficiencies in dealing with the problem.

In addition, the often opaque and nonspecific troubleshooting instructions found in many respective maintenance manuals have generally increased the chances for further processing errors. This has been demonstrated by 2012 Copernicus Technology findings as the maintenance personnel ranked troubleshooting guidance in maintenance manuals as the 6th in effectiveness, in comparison with the 2nd rank accorded to it by production/design/R&D personnel sub-group (Copernicus Technology 2012, p.21).

Therefore, the challenges that maintenance personnel have to tackle are primarily caused by the insufficiency of technical options that were previously implemented in the field, as well as by the instructions’ presentation deficiencies that might confound both engineers and technicians in dealing with specific NFF problems. Thus, it is necessary to review and analyze recent initiatives offered by the OEMs and maintenance organizations in order to resolve this issue.

Industry approaches to the NFF problems

The industry-specific applications for dealing with the NFFs encompass such areas of expertise as innovative diagnosing architecture development, hardware and software test equipment research and development, design failure prevention, etc. In this section, several of these selected approaches are reviewed for the purposes of establishing their comparative efficiency.

The IFD and integrity testing facilities, such as the NcompassTM complex presented by Copernicus Technology, are generally based on the neural-network principle, enabling them to provide for swifter and less cumbersome amelioration of the NFF problems than that previously offered by traditional diagnosing and repair systems. The NcompassTM model of response to NFF occurrences is built on continuous IFD detection. The high sensitivity of this system allows eschewing the usual scanning procedure in favour of constant event measuring (Copernicus Technology 2011, p.25). The NcompassTM IDF technology was awarded the U.S. DoD Maintenance Great Ideas Competition prize in 2010. With its ability to detect intermittency in <50 ns, as well as the detection probability exceeding that of conventional models by >106, the NcompassTM is poised to present a formidable instrument of the NFF combating.

Prognostic and systems health management systems constitute another important form of the NFF prevention facilities. As presented by Byington, Kalgren, Johns, and Beers (2003), this family of systems is based on the principle of enhanced onboard fault isolation, with specific software architecture structures used to delineate them from each other. In particular, the Integrated Systems Health Management (ISHM) system developed by the NASA Ames Research Centre is oriented towards combining fault detection, fault diagnosis/isolation, and fault prognosis, with sensor values used as inputs for detecting possible intermittencies (Schwabacher 2005, p.1). Such module-based system may prove fruitful for the future aerospace industry applications.

Finally, the maintenance-oriented fault models, such as the one proposed by Peti, Obermaisser, Ademaj, and Kopetz (2005), may be utilised in the NFF detection software coupled with the distributed component-based hardware systems. The real-time detection of faults and intermittencies would be based on more precise Field Replaceable Units classification, with several classification schemes proposed. This would greatly enhance the system’s diagnostic and maintenance capacities.


The NFF detection and prevention comprises one of the most serious challenges for avionic maintenance industry. In order to increase the systemic capacities for withstanding the NFF shocks, major innovations in the field of maintenance are required. Nevertheless, the efforts by Copernicus Technology and other maintenance equipment developers, as well as of individual researchers, may lead to the greater breakthroughs in this direction, significantly ameliorating the NFF problem.

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