Abstract

Aero-engine assembly activities type and volume limit the manufacturers’ ability to automate all processes; thus, the assembly of engines and many supporting processes heavily rely on the operators’ skills. Human operators show significant variability in their performance, which is usually referred to as human errors. Such errors are identified as the Escape to perform an action within the safe operating limits and often lead to quality defects. With a particular focus on one of the engine families (Group A engines) of a typical aero-engine plant, the first objective of our analysis revolves around identifying the interruptions and distractions incurred during the engine assembly process that can potentially lead to quality defects induced by human errors. Based on the analysis of the quality reports, the group A engines produced in this facility have a higher percentage of quality defects as compared with the average quality defect rates of other engine groups within the company.
It is noteworthy that quality reports suffer from a lack of information regarding the relationship between reported quality defects and human errors. In other words, the data extracted from these reports cannot be directly used to identify sources of such errors. Therefore, two main data collection methods, namely observations through site visits and interviews, are alternatively used. Afterward, a combination of various Lean Manufacturing and quality engineering approaches, namely Value Stream Mapping (VSM), cost of quality escapes, cause-and-effect diagram, Escape Mode and Effect Analysis (FMEA), and Analytic Hierarchy Process (AHP) are explored to provide a prioritized list of external root causes that can potentially lead to human errors at the assembly facility.
The current Value Stream Map analysis results, along with our observations from the assembly facility and the conducted interviews, are presented as twenty-five sources of interruptions and distractions that could be the root causes of quality defects (due to human errors). The results are summarized in a fishbone diagram with six major categories; material, communication, manpower, methods, environment, and movement. Afterward, by using FMEA and AHP methods, those twenty-five sources of interruptions and distractions are prioritized. Results revealed that the leading root causes include: Reworks/QNs/Andons; insufficient root cause analysis of frequent problems/breakdowns; frequent changes in production priorities; informal information flow and inaccuracy of transferred information; distractive people or activities in assembly area; slow and complicated procedure of technical support; lack of inspection or quality control between sub-assemblies; and inefficient order of executing assembly flow sequences.
The second goal of this study is to propose Lean manufacturing and quality engineering solutions in order to eliminate the root causes identified in the first objective. The proposed solutions rely on: i) improving the quality control system via enhancing the documentation methods for recording quality Escape; in addition to ii) developing a Split Mixed-Model VSM for the engine assembly facility. The first solution, in particular, aim to use statistical control tools and root cause analysis to reduce assembly Escapes caused by human errors. Implementing a Split Mixed-Model VSM, on the other hand, contributes to a smooth flow of products while systematically eliminating sources of interruptive and distractive issues during the assembly process. More precisely, it mainly targets the elimination of quality defects caused by stoppages due to shortage of components/sub-assemblies, late delivery of materials, delivered materials with quality defects, and insufficient assemblers or support teams. In other words, the proposed split mixed-model value stream is expected to favourably affect the quality and performance of the process in this plant. When developing this VSM, the order of doing assembly instructions is rearranged; the new TAKT time for each engine model in the Group A engine family is calculated, and a new layout for the assembly facility is accordingly provided.