No fault found (NFF) errors, i.e. errors which origin has not been established, irregularly occur in electronic devices. The actual cause of such errors varies but one, possibly more prominent, source for these soft errors is atmospheric radiation. The overarching aim of this thesis is to demonstrate: 1) the importance of keeping the atmospheric radiation environment in mind when designing robust airborne systems, 2) how to take this environment into consideration when applying mitigation techniques which may drastically reduce the risk of SEEs (Single Event Effects) which can cause NFF errors. To achieve these goals, Part 1 of this thesis describes how cosmic rays affect electronics (i.e. what kind of errors may be induced), which types of devices are susceptible to radiation, and why this subject is of extra importance for airborne systems. In addition, soft error mitigation techniques, which may be applied at different design levels to reduce the soft error rate (SER) or the impact of soft errors, are also presented. The aim is further corroborated by Part 2. Five subsystems of a modern aircraft are studied and real examples of failures potentially induced by atmospheric radiation are presented. For each of the five systems, all errors that have been reported for these (in the past few years) have been studied, and the number of errors found to be potentially induced by cosmic radiation has been listed and compared to number of expected soft errors based on calculations and previous experimental tests.

This research used human factors methods to investigate the relationship between no fault found (NFF) incident frequency and device usability. NFF reporting occurs when a medical device sent for repair is found to be operating normally. NFF incidents are one of the most recurrent failure modes, and therefore have considerable impact on cost, dependability and safety. An analysis of medical equipment maintenance data was conducted and six devices with a high NFF reporting frequency were identified. Semi-structured interviews and heuristics evaluations revealed that usability issues likely caused many of the NFF incidents. Other factors suspected to contribute to increased NFF reporting include accessory issues, intermittent faults and environmental issues. Finally, in order to validate the results, usability testing was conducted on three of the devices. 23 usability-related design flaws were identified. Therefore devices containing latent usability-related design flaws can be identified through analysis of medical equipment maintenance data.

This research used human factors methods to investigate the relationship between no fault found (NFF) incident frequency and device usability. NFF reporting occurs when a medical device sent for repair is found to be operating normally. NFF incidents are one of the most recurrent failure modes, and therefore have considerable impact on cost, dependability and safety. An analysis of medical equipment maintenance data was conducted and six devices with a high NFF reporting frequency were identified. Semi-structured interviews and heuristics evaluations revealed that usability issues likely caused many of the NFF incidents. Other factors suspected to contribute to increased NFF reporting include accessory issues, intermittent faults and environmental issues. Finally, in order to validate the results, usability testing was conducted on three of the devices. 23 usability-related design flaws were identified. Therefore devices containing latent usability-related design flaws can be identified through analysis of medical equipment maintenance data.

<p>No Fault Founds (NFFs) are an expensive problem faced by the airline industry. The underlying cause of NFFs are a major focus of research work in the field, but the dearth of consistent data is a roadblock faced by many decision makers. An important risk factor identified is the occurrence rate of NFFs.</p><p>This research work aims to help decision makers in the Airline Maintenance, Repair and Overhaul teams, when faced with recurring NFFs, to make a choice based on value derived from the system and risk preference of the decision maker under uncertainty. The value of the aircraft fleet is laid out using Net Present Value at every decision point along the system life cycle while accounting for the uncertainty in the failure rate information. Two extreme decisions are considered for the decision maker to choose between: rebooting the system every time a failure occurs and results in an NFF which allows for it to recur while reducing uncertainty of the failure rate; or eliminating the failure mode which assumes that the failure does not recur and therefore completely removes the uncertainty. Both decisions have their associated uncertain costs that affect the NPV calculated. We use a Monte Carlo approach to estimate the expected profit from deciding to eliminate the failure mode. We make use of Expected Utility Theory to account for the risk preference of a decision maker under uncertainty and build an Expected Utility Maximizing decision framework.</p>To conclude we give some guidance to interpret the results and understand what factors influence the optimal decision. We conclude that not accounting for uncertainty in estimating a failure rate for the future along with uncertainty in NFF costs can lead to an undesirable decision. If the decision maker waits too long to gather more information and reduce uncertainty, then rebooting the system for the remaining life could be more worthwhile than spending the large amount of money to Eliminate a failure mode. Finally, we conclude that, despite uncertainties in information of occurrence rates and costs of NFFs, an Expected Utility maximizing decision between the two options considered – Reboot and Eliminate – is possible given the available information.

This study was conducted in collaboration with BAE Systems Hägglunds in Örnsköldsvik. They wanted help with their objective of researching the necessary components of a testability analysis. The primary goal was to enhance the efficiency of troubleshooting and diagnostics for their combat vehicle, Cv90. As their vehicles become increasingly advanced, troubleshooting can become challenging and time-consuming. An example of a problem that can arise is when an error code is displayed to the crew, but the technicians at the workshop are unable to identify the root cause of the error. To gather information, scientific articles and other relevant documents were extensively reviewed. Additionally, interviews were conducted with employees at BAE. The scope of the study was limited to troubleshooting within a workshop setting, rather than in the field. It was assumed that the troubleshooting equipment and software were functional, and the focus was solely on identifying faulty components. The research was conducted both in Umeå and on-site in Örnsköldsvik. A methodology for implementing a fault detection system with effective testability was developed. This encompassed considerations for the construction and content of the troubleshooting system. Determining the system's requirements and devising methods for testing their fulfillment were crucial aspects. Prioritization of different functions based on their criticality was recommended. Critical functions should be addressed first and may require more costly and intricate solutions. Various approaches to enhance troubleshooting at a more granular level were identified. These included establishing better threshold values, accounting for measurement uncertainties in the test equipment, and emphasizing the importance of a robust test design that considers deviations from system equilibrium. Additionally, worn components were recognized as a potential cause for false indications that are challenging to diagnose. It is important to note that these improved fault detection methods have not yet been implemented in the vehicles. / Studien genomfördes i samarbete med BAE Systems Hägglunds i Örnsköldsvik. De ville få hjälp med sitt mål att undersöka de nödvändiga komponenterna i en testabilityanalys. Det primära målet var att effektivisera felsökning och diagnostisering av deras stridsfordon, Cv90. När deras fordon blir alltmer avancerade kan felsökningen bli utmanande och tidskrävande. Ett exempel på ett problem som kan uppstå är att en felkod visas för besättningen, men teknikerna på verkstaden kan inte identifiera orsaken till felet. För att samla information gjordes en omfattande granskning av vetenskapliga artiklar och andra relevanta dokument. Dessutom genomfördes intervjuer med anställda på BAE. Studiens omfattning var begränsad till felsökning på verkstaden, snarare än i fält. Det antogs att felsökningsutrustningen och programvaran var funktionella, och fokus låg enbart på att identifiera felaktiga komponenter. Arbetet utfördes både i Umeå och på plats i Örnsköldsvik. En metodik för att implementera ett felsökningssystem med god testbarhet utvecklades. Detta omfattade överväganden för felsökningssystemets konstruktion och innehåll. Att bestämma systemets krav och utforma metoder för att testa deras uppfyllnad var avgörande aspekter. Prioritering av olika funktioner baserat på deras kritikalitet rekommenderades. Kritiska funktioner bör åtgärdas först och kan kräva mer kostsamma och invecklade lösningar. Olika metoder för att förbättra felsökning på en mer detaljerad nivå identifierades. Dessa inkluderade att fastställa bättre tröskelvärden, ta hänsyn till mätosäkerheter i testutrustningen och betona vikten av en robust testdesign som tar hänsyn till avvikelser från systemjämvikt. Dessutom identifierades slitna komponenter som en potentiell orsak till felaktiga indikationer som är svåra att diagnostisera. Det är viktigt att notera att dessa förbättrade felsökningsmetoder ännu inte har implementerats i fordonen.

This work is intended to investigate the Aircraft Technical Diagnostics Systems by focusing on Central Maintenance Systems and the Fault Detection and Isolation (FDI) process among the avionic components. A review about the Computer Control Systems and background about the Avionic Architecture is presented prior to introducing to most popular FDI method; model-based diagnosis. The discussions about the onboard FDI practices are followed by a maintenance hangar FDI perspective which was concluded as a result of the field research. The outcomes of the field research and pointing the “real” fault isolation are analyzed to point the practical needs of a hangar FDI tools. Subsequently, a proposal technique, which can improve fault isolation by preventing No-Fault-Found (NFF), is given by discussing the methods to implement it. Finally, the results of investigation and conclusions of analysis are presented. / Baigiamajame darbe lietuviškos anotacijos nepateikta.