*5.3. Aircraft Maintenance Fraction*

The Australian Transport Safety Bureau's report into HF in aircraft maintenance [31] states that "precise statistics are unavailable" for the proportion of aviation accidents and incidents that are the result of improper maintenance. This is the inspiration for RQ1. While it would also be interesting to know the proportion of aviation incidents that have a maintenance contribution or causation, the ICAO official accident data is exactly that, only accident data. We have a database that contains n number of accidents each year, and we have a dataset that contains m number of accidents with maintenance contributions each year. Figure 13 shows this annual percentage of ICAO official accidents which have maintenance contributions.

**Figure 13.** Percentage of all accidents in the ICAO official accident data set that have been identified with a maintenance contribution, 2008 to 2019. The shaded band illustrates the confidence interval of the average, (2.8 ± 0.9%).

Annually, the average percentage of accidents with maintenance contributions is given as (2.8 ± 0.9%). However, the result of the regression test for this was statistically significant, in terms of a reduction by 0.14% per year. That is, on average, this percentage is reducing by 1 every 7 years. The peak in 2016, and then the lack of an accident in 2019 suggests this reducing trend is an artifact of the small sample size. To conclude, approximately 3% of accidents are the result of aircraft maintenance.
