Ergonomics is broadly classified into physical, cognitive, and organizational aspects. The characterization of manufacturing tasks as predominantly labor-intensive, reliant on physical strength, often leads to an emphasis on analyzing only the physical aspects of ergonomics, such as working postures, handled weight, and so on. Some instances include the assessment of organizational aspects such as resource management, break hours as well. However, the cognitive aspects in manufacturing context are very rarely explored. Hence, this study aims to understand the sources of cognitive load in a manufacturing scenario. 17 participants performed assembly and disassembly of an aircraft wing-box. A self-reported cognitive load assessment including 5-point Likert scale rating for perceived cognitive load along with qualitative feedback regarding reasons and associated activity during the task performance was collected. Cognitive Load Theory was applied to pinpoint the root causes of cognitive difficulties encountered in the use case. Inter-relation in different types of cognitive load and task performance parameters were explored using correlation analysis. The results reveal potential sources of cognitive load associated with the involved manual operations. Subsequently, several ergonomic interventions to control these sources were proposed with implications for workplace design and task design. Future research scope involves a quantitative assessment of the effectiveness of the suggested interventions for cognitive load reduction. The findings are intended to support decisions regarding workplace design, task planning, resource allocation, job rotation, etc., in manufacturing units with consideration of cognitive load in order to ensure overall well-being and productivity of the workers.