The study of human error has been considered a relevant topic for ergonomics and industrial safety discipline during the past several decades, particularly in connection with so-called safety-critical domains (petrochemical, nuclear, aerospace). In the manufacturing sector, the physical aspects of work seem to have attracted most of the attention of ergonomics research. However, in aerospace manufacturing, assembly errors can lead to significant economic losses (quality problems, reduced productivity) or even compromise the safety of the aircraft. The assembly of aircraft engines, the object of study in this thesis, is a job primarily done manually and involves significant cognitive requirements due to the complexity of the tasks performed. While some Industry 4.0 technologies may, in the future, support human performance in complex manual assembly, it is still vital to study the conditions under which this job is performed. Furthermore, the design or improvement of the assembly system, with or without the support of modern technologies, must consider factors that influence human performance to minimize the risk of errors.
In this thesis, we aim to study human error in aircraft engine production. We are interested in identifying the main types of errors that can occur during complex manual assembly, evaluate their risk and the influence of several contributing factors. We also focus on the evaluation of fatigue and workload and finally we illustrate the organizational influences on the assembly errors. In this way, our study focuses not only on workers and their immediate environment but also on the production system as a whole. The study was carried out in collaboration with an industrial partner and included a significant field component. The methodology used as a framework of reference in this thesis is a mixed-method, which explains the choice of a wide variety of methods and techniques: qualitative empirical, quantitative empirical and analytical. We followed a bottom-up approach, starting with the analysis of the assemblers' work and their work environment and extending the analysis to elements of the organizational system.
Our results show that some tasks are particularly vulnerable to errors, such as bracket installation, cushioned loop clamp installation, and caps-cover installation. Statistical analysis of the quality records indicated that these three tasks accounted for two thirds of the quality problems during the 36-month period selected. The human reliability analysis suggests that the geometry of some assembly parts (brackets and cushioned loop clamps) accompanied by suboptimal design and display of work instructions (static 2D drawings, inconsistencies in the order of steps presented in the drawings, small screen size, screen away from the assembly position), can have a negative impact on assembler performance (increased risk of error). In other cases, the lack of immediate feedback from the system or obvious means to monitor the progress of the task can also affect performance. This is a result of the high number and variety of assembly parts that make it difficult to keep track of the entire work activity. In addition, distractions and interruptions are also factors that influence the probability of error in the context studied. Some of the more prolonged interruptions are related to the discontinuity of tasks due to lack of parts (problems with suppliers), while others are related to the back and forth between the work bench (PC monitor, parts bins) and the assembly position.
On the other hand, the analysis of the results of the study of fatigue and workload shows that perception of the overall workload, working the evening shift, and fatigue felt at the beginning of the shift are predictors of potentially harmful levels of fatigue to performance (fatigue level ≥ 5 on the Samn Perelli scale). Participants also reported getting less than seven hours of sleep on 60% of the nights assessed. In addition, workers perceived the cognitive component of workload as the most important of the six components of workload assessed. This appears to be a result of the complex nature of aircraft assembly. Finally, systems-oriented analysis methods provided a holistic view of human error within the assembly system. We identified, for example, that instability in the supply of assembly parts causes discontinuity in the production flow, which in turn generates periods of low and high production. While high production leads to overtime and atypical working hours, low production can lead to more distractions and interruptions. In addition, the policy of overtime payment agreed upon through collective bargaining may induce workers to accumulate a significant number of hours of work that may exceed the generally established limit of 40 hours per week. Similarly, in conjunction with budgetary aspects, the company's technological strategy influences the computerized manufacturing support system currently in place. The design of work instructions that do not consider the needs of end users is another example of organizational factors that influence the performance of assembly workers. Thus, we can see that assembly errors are also associated with macro-level factors of the work system.
In conclusion, our results suggest that different factors related to the design and management of the assembly system have a potential impact on workers’ performance. These factors may increase the risk of assembly errors. Particular attention should be paid to cognitive workload because of its possible effects on fatigue in complex assembly. This should result in an intervention strategy that considers the interaction between workers and work instructions (content, display mode, realism of images) and the means to ensure immediate feedback and monitoring of task progress. In this sense, the adoption and integration of certain human support technologies in a digital industry context represent a solution to explore (assembly guidance system). It should be noted that an overload of work hours accompanied by atypical work schedules could also contribute to fatigue and a possible deterioration of performance. Such working time management practices should be avoided or minimized (e.g., by ensuring stable production levels). Otherwise, clear guidance on certain limits would be necessary. Worker awareness of sleep hygiene is also essential. In all cases, solutions must be focused on a systems perspective, i.e., take into account influencing factors at multiple levels in the organizational structure. Although some 4.0 technologies such as augmented reality offer an interesting potential, other "classic" solutions are also important. In both cases, the analysis with the systemic approach is an essential element to successfully implement solutions to increase performance and decrease the risk of errors.
| Date | 4 Oct 2022 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Sylvie Nadeau (Supervisor) & Kurt Landau (Co-supervisor) |
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