**5. Conclusions**

This study showed an association of 11 September 2001, with pilot AASs. The copyca<sup>t</sup> effect was present for one year after 11 September 2001. The causal factors behind this statistical association remain unclear in context to the theoretical approaches on suicidality. The use of aircraft as the means to commit the terrorist attacks in the U.S. on 11 September 2001, may have had a negative effect on a few acutely vulnerable pilots. This vulnerability warrants further investigation, particularly with reference to the copyca<sup>t</sup> phenomenon and it needs to be taken into account in aviation medical safety risk assessments.

**Author Contributions:** A.V. contributed to the study design, data procurement, results interpretation and drafted the article. T.L. drafted first version of the ms. A.V., T.L., I.J., E.P. were in charge of data analysis. A.V., T.L., I.J. contributed to results interpretation. All authors A.V., B.B., A.S., T.L., I.J., E.P., P.N., R.B. critically revised the article and gave their final approval for publication.

**Funding:** The Academy of Finland, Finnish Medical Foundation, Sigrid Juselius Foundation, and the Tampere Tuberculosis Foundation.

**Conflicts of Interest:** The authors declare no conflicts of interest.
