On my first reading I thought that the system being analyzed here was the place, but on closer inspection that wouldn't work: referring to other planes would be referring to the environment of the system. It seems the implicit system here is actually that of all planes flying; traffic control.

Pan, A., Liu, L., Wang, C., Guo, H., Hao, X., Wang, Q., Huang, J., He, N., Yu, H., Lin, X., Wei, S., & Wu, T. (2020). Association of Public Health Interventions With the Epidemiology of the COVID-19 Outbreak in Wuhan, China. JAMA. https://doi.org/10.1001/jama.2020.6130