*5.2. Logistic Regression Results*

Research question 4 (RQ4) was posed to test the hypothesis that older aircraft are 'more dangerous' than newer aircraft. Clearly the context of this statement is only with regards to those accidents with a maintenance contribution. The two aspects here (fatalness and aircraft damage) and their relationship to age can be assessed with logistic regression. Figure 12a shows the resultant logit fitted to the fatalness data (0 = non-fatal, 1 = fatal), and the logit predicts that older aircraft are statistically significantly

more likely to be associated with a fatality (McFadden's pseudo R<sup>2</sup> = 0.11, χ2 = 3.97, *p* = 0.046); to be clear, this is just statistically significant, and the R<sup>2</sup> suggests only 11% in the variation of fatalness is due to the age of the aircraft, and the odds are even for an aircraft that is 34 years old. The limitation here is the small sample size, giving only a few fatal outcomes. Looking at the outcome for the aircraft (Figure 12b), the logit now much more clearly increases with age as hypothesized (McFadden's pseudo R<sup>2</sup> = 0.30, χ2 = 14.6, *p* <0.01). That is, about 30% of the variation in the aircraft damage outcome is predicted by the aircraft's age, and the odds are even for an aircraft that is 18 years old.

**Figure 12.** Results for the logistic regression investigating the effect of aircraft age on: (**a**) fatalness of maintenance accidents (0 = no fatalities, 1 = fatalities); (**b**) aircraft damage from maintenance accidents (0 = aircraft repaired, 1 = aircraft damaged beyond repair = hull loss).
