*3.2. Approach*

This work is of a theory-building nature. First, we reviewed the literature for candidate approaches to develop and construct a Bowtie diagram. Further, we reviewed di fferent existing systems in the safety and broader risk managemen<sup>t</sup> field that help identifying and categorising risks, hazards, threats, and root causes. These include general risk approaches, human factors models, and cause and effect classifications. From these we selected one approach, namely 6M, as the basis for the new Bowtie framework.

Next, we resolved the ambiguity in the hazard–top event coupling, and applied the 6M to the Bowtie process. In doing so, we contextualised it to the area under examination. We also addressed the structure of the escalation factors. We identified that strings of escalation factors were often common to di fferent places in the Bowtie, and we proposed making these into modules for better representation.

The method was then applied to the specific case of visual borescope inspection of gas turbine components in an MRO environment. A total of 15 aircraft maintenance inspectors from the industry partner participated. The experience profiles were: five inspectors with more than twenty years of experience in visual inspection, seven inspectors who had worked for over ten years in the field, and three operators with up to ten years of experience. Their certifications included borescope operation and non-destructive testing (NDT). Each was observed independently for approximately 30 min during a real inspection process, and they were asked to articulate the risks and threats of the process, and what barriers were or could be in place to prevent negative outcomes. The specific instructions were:


All the operators were familiar with Failure Mode and E ffects Analysis (FMEA), and some were also familiar with the Bowtie methodology. Their verbal comments and insights were noted, and subsequently used by the primary author to construct the Bowtie diagrams shown in this paper. Ethics approval was obtained from the University of Canterbury (HEC 2020/08/LR-PS) and permission from the industry partner. The results and limitations of the new approach were then validated by discussion with the two highest certified borescope inspectors (who both held a level 2 certification in borescope inspection, which is the highest certification achievable in the industry), and a human factors and risk analyst for the organisation. They were presented with the Bowtie results and asked to comment. They confirmed the accuracy of the results, commented on the method (they were generally in favour), and made suggestions for improvements (primarily in the precise wording of threats and barriers). For the visualisation of the Bowtie diagrams, the software 'BowTieXP' revision 9.2.13 was used [68].
