  The U.S. air transportation network (ATN) is critical to the mobility and the
functioning of the United States. It is thus necessary to ensure that it is
well-connected, efficient, and robust. Despite extensive research on its
topology, the temporal evolution of the network's robustness and tolerance
remains largely unexplored. In the present paper, a temporal study of the
domestic U.S. ATN was performed based on annual flight data from 1996 to 2016
and network analytics were used to examine the effects of restructuring that
followed the 9/11 events along with the current state of the system. Centrality
measures were computed to assess the system's topology and its global
robustness. A node deletion method was applied to assess the network's
tolerance by simulating a targeted attack scenario. The study showed that the
9/11 terrorist attacks triggered vast restructuring of the network, in terms of
efficiency and security. Air traffic expanded, as new airports and air routes
were introduced. Airlines reconsidered their strategy and optimized their
operations, thus allowing the network to recover rapidly and become even more
efficient. Security concerns resulted in significant improvement of the
network's robustness. Since 2001, the global traffic and topological properties
of the U.S. ATN have displayed continuous growth, due to the network's
expansion. On the other hand, the robustness of the system has not shown an
improving tendency. Findings suggest that although the system's ability to
sustain its operational level under extreme circumstances has lately improved,
its tolerance to targeted attacks has deteriorated. The presented methodology
may be applied on different network levels or different transportation
networks, to provide a general perspective of the system's vulnerabilities.
