No Fault Found, Retest-OK, Cannot Duplicate or Fault
No Fault Found, Retest-OK, Cannot Duplicate or Fault Not Found? Towards a standardised Taxonomy 5th November 2012 Dr Samir Khan Dr Paul Philips Mr Chris Hockley Prof Ian Jennions Agenda Overview of the NFF problem Cause and impact of NFF Tackling NFF The Multitude of Terms Impact of inconsistent terminology Maintenance culture Concluding Remarks The NFF problem System Clas
sica l vie w view No fault n Moder Fault Diagnostic success Strategy No fault Diagnostic failure s g in e? h
t er e m gh o s on wr The NFF problem NFF Definition Removals of equipment from service for reasons that cannot be verified by the maintenance process (shop or elsewhere). (Source ARINC Report 672 (2008) Guidelines for the reduction of No Fault Found (NFF). Avionics Maintenance Conference, Aeronautical Radio Inc) The NFF problem To Achieve Diagnostic Success? Implies identification of the root cause. That enables the correct maintenance activity to be performed. It suggests a closed-loop system that can relate the symptom to the fault to the correct
maintenance solution. The NFF problem Some NFF history 1993, BA concern at cost of removals where - nothing found wrong - same fault re-occurred 13.8% of all unscheduled removals = NFF Cost 17.6M per year 80.4% of all NFF were avionics 26.6% of all avionics removals were NFF BA Presentation ERA Conference 1996 BA estimate costs at 20M per year Blishke WR & Murphy (2003) pub -John Wiley & Sons 1996, Boeing state 40% rate of incorrect parts removal from the airframe R Knotts MPhil Thesis Exeter Avionics constitutes 75% of NFF occurrences in aerospace Aviation Week, Feb 9 2007
Common Stages of NFF y c n e u q fre Stage 1 consequ enc e rs o t a r pe o / r
e Us Stage 2 Contractors manu factu rers AC; Avionics 74%, pneumatic 19% In aerospace 50% of LRU are classified NFF Satellite industry - 50% increase Surveys in electronic equipment NFF 19->53% F16 radar 67% 85% of all operational faults in aircraft electronics Train control system 50% of faults AMC 2004 voted NFF the most imp issue
Some NFF events seem to be independent of life cycle stage Common Stages of NFF Results from Past Experiences and Studies NFF costs can be generated through the following activities: Removals for the wrong reasons Workshops failing to find and then repair the reported fault Inability to simulate the conditions in which the fault occurred Cost of removals due to No-Fault-Found can be huge where: Nothing has been found at fault The same fault re-occurs in the next or a subsequent mission, risking safety and/or mission success Reasons for NFF
Not enough emphasis on diagnostics training Pressure on quantity in workshops not quality No emphasis on history of components No tracking of rogue units Loose conn./ intermittent corrosion Reliability issues Causes of NFF Designer team integration System design Poor understanding Poor maintenance
Organisational setup Maintenance team Management team complexity BIT complexity BIT Why is Tackling NFF difficult? The solutions to NFF problems may not be possible because: Inconsistent fault Organizational culture Procedures and rules Technical inefficiencies
Workforce behaviour reporting Design related hard/software def. Insufficient time available for maintenance Poor on board maint. system Insufficient training of line and shop personnel Inadequate design for testability Performance Poor test measure coverage NFF Technical Causes of NFF
Undefined or limited performance measures BIT levels set too low BITE ability to detect a fault BITE inability to apply reliable fault diagnostics Lack of information on operating environment Inability to reproduce operating environment during test or diagnosis Intermittent failure caused by stress not replicated on test Inadequate design suitable for robust testing for all faults Inadequate fault models and fault trees for determining root causes Lack of understanding of interactions between different integrated systems and software Reluctance to adopt new technologies (health monitoring) due to the
need to alter system designs or because the data handling /decision making infrastructures are not available Organisational Time pressures on maintenance operations Organisational cultures with no cross-functionality, employee empowerment and encouragement to identify root causes Inadequate training or lack of training tools No commitment to sharing information and knowledge between designers, manufacturers, service providers and operators
Solutions likely to be disruptive to normal working practices= reluctance to change Behavioural and Procedural Discrepancies and faults in test procedures Incorrect fault reporting Wrong processes applied Incomplete documentation Lack of communication between maintenance personnel and other experts. Management policy; Coherent teams Tests should be certified by the supplier Diagnostic tests should be added before commissioning Modelling of intermittent faults; e.g. Detect probability of fault
Transfer of information Mitigation of NFF Recognise Design of fault fault alarms tolerant systems; add redundancy Design methodology eg. Include in-built health monitoring Address root cause of BIT deficiencies The Problem Within this field there are several identified problems: A lack of common understanding on what constitutes the phenomena resulting in Diagnostic Failure Wide concept and lack of commonality in processes Several existing standards deal with aspects of the
phenomena but may be overly specific Existing standards are incomplete and fail to fully define the problem in a way which has been accepted by industry. A lack of interaction between the aspects, processes, activities and stakeholders who suffer the impacts A Multitude of Terms Missing published academic literature Early calls made into testability attributes of electronic equipment, specifically to mitigate NFF but this has not yet been achieved across all test/maintenance levels. Cultural impacts Copernicus Technology Ltd survey Q1) How can a true gauge of the problem be investigated if there is no standardised term used in the maintenance history? Q2) Are all of these terms accurate do they actually describe the same event, or are there
subtle differences which need to be recognised? Do we need standards? Adopting standards help industries (and research) overcome technical barriers by promoting organisational success through better workflow paradigms and maintenance strategies Most of the existing descriptions of these phenomena do not provide any consideration for where exactly do these diagnostic failures occur within the maintenance process No Fault Found is a reported failure that The inability to replicate field failure during repair shop test/diagnosis (Kirkland, 2011) A failure that may have occurred but cannot be verified, or replicated at will, or attributed to a specific root cause, failure site, or failure mode (Qi et al, 2008). cannot be confirmed, recognised or localised during diagnosis and therefore cannot be repaired (Roke, 2009) Removals of equipment from service for
reasons that cannot be verified by the maintenance process (shop or elsewhere) (ARINC Report 672, 2008) Maintenance culture Is a single term enough? The majority of definitions lead to believe that a failure during operation (such as an intermittent fault) is the actual No Fault Found event, and hence leads to the majority of academic literature to classify NFF into three distinct categories: intermittent failures, integration faults Built In Test Equipment (BITE) failures However, such a practice may be incorrect as there are primarily the root causes which begin a sequence of events through various levels of maintenance, which blend with other factors such as organisational/ behavioural/cultural and technical abilities to result in the final outcome of NFF (drivers towards diagnostic failure). Maintenance culture Root causes Cables
Chassis (LRU) sources Influencing factors that lead to a diagnostic failure Lack of communication Test procedures Leads to a maintenance action Test coverage Poor design False alarm Design defects drivers Connectors Components
Location of the fault Wrong process BIT Incorrect reporting Insufficient training Intermittency integration Operator error faults Maintenance culture 1) There is no evidence of faults during testing so the unit can be certified as serviceable as there probably never was a fault 2) There has been no evidence of faults during testing but the test coverage may be inadequate so it is best to replace the unit just to be on the safe side 3) Repeating NFF issue > not a single event but a sequence 4) Fault Not Found instead of No Fault Found? A reported fault for which the root
cause cannot be found in other words a diagnostic failure (Cockram and Huby, 2009) Concluding Remarks Research Scope Lack of standards with different terms describing largely the same problem Human Factors is one of the core driver towards diagnostic failure To identify procedural, process and behavioural issues that need to be changed, learning from best practice in each industry. To devise strategies, methodologies and system design rules to mitigate the occurrence of intermittent failure mechanisms and to demonstrate their
effectiveness in reducing the likelihood of NFF occurrences. To develop a multi-disciplinary approach at the System level for the effective analysis of the root causes of NFF in order to assist design activity across domains. Concluding Remarks Key Challenges Providing solutions which do not have a negative impact on current operations i.e. increased downtime means reduced availability Specific training Development and adoption of appropriate Standards
Diagnostic procedures which encourages identification of root causes Cultural changes Working with design to identify the necessary improvements to mitigate NFF Through-life Contact info Samir: [email protected] Paul: [email protected]
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