How air transport networks respond to long-lasting disruptive events like COVID-19: The first step toward long-term resilience

Resilience is critical for the air transport network (ATN) to cope with disruptive events like the COVID-19 pandemic, while the first step in understanding resilience is to reveal how the ATN responds to disruptions. This paper presents a method for investigating how the ATN responds to the COVID-19 pandemic from its outbreak through deterioration, adaptation, and restoration. The concept of system state is introduced to investigate when the ATN has substantially changed. By comparing the structure of the temporal ATN in different weeks based on network similarity assessment, the changing points of the ATN are detected, where the network system has switched from one state to another. To quantify what are the changes to the ATN's structure, several performance metrics, including global and local connectivity, are evaluated around the changing points. Moreover, to study the response time of the ATN to the pandemic, the time lag between the changing points of the ATN and that of the confirmed cases is examined. Finally, the proposed method is applied to the ATNs of Mainland China and the U.S. during the COVID-19 pandemic, resulting in some interesting findings and policy implications. It is found that in a pandemic outbreak, China is more likely to suspend routes directly, while the U.S. is reducing flight frequency to maintain essential airport connections. Further, the state transition of ATNs caused by policies is temporary, while the state change caused by the pandemic is permanent.