*1.1. Study Rationale*

The most recent in-depth study into the nature of aircraft maintenance error, by the UK CAA [2], was published in 2015, ye<sup>t</sup> only made use of data up to 2011. This shows that there is an underlying need to provide an up-to-date analysis of maintenance error types in order to understand the trends as well as emerging issues. Additionally, a majority of the most recent available studies are only from the 1990s, warranting the scope of this study to look at maintenance accidents and serious incidents from the early millennium onwards.

There is also suggestion that the most popular aircraft maintenance taxonomies presently in use can be difficult to apply to retrospective analyses. Therefore, exploration of a new taxonomy to further aid the process may assist in the categorisation of aircraft maintenance-related occurrences. Further to this, it may be of benefit to review how the discerned maintenance-specific issues correlate with key risks identified for CAT as a whole, as discussed by the UK CAA [2,3] and EASA [4].

**Problem statement:** since 2015, the European Aviation Safety Agency consistently identified aircraft maintenance as one of the safety issues and included it in the safety risk portfolios for "Commercial Air Transport (CAT)–Large Aeroplanes" [4–7]. However, there has been no further analysis conducted to enable the stakeholders to develop any mitigation strategies to address risks associated with aircraft maintenance and continuing airworthiness. Therefore, further analysis of accidents, serious incidents, and occurrence data was essential to better understand the causal and contributory factors in this area.

While the analysis of occurrence data extracted from European Central Repository was subject to another study [8], this paper focuses on the accidents and serious incidents where maintenance actions or continuing airworthiness processes played a causal or contributory role. The ICAO Annex 13 definitions of 'accident', and 'serious incident' apply, which can be found in Appendix B. Also, the details of the nature of flights are shown in Table 1 below and the aircraft types can be found in Appendix A.


**Table 1.** Breakdown of Events by Nature of the Flights (the full list of categories for the 'nature of flights' used by the Aviation Safety Network can be accessed @ https://aviation-safety.net/about/ASNstandards.doc).
