  Urban air mobility (UAM), as envisioned by aviation professionals, will
transport passengers and cargo at low altitudes within urban and suburban
areas. To operate in urban environments, precise air traffic management, in
particular the management of traffic overflows due to physical and operational
disruptions will be critical to ensuring system safety and efficiency. To this
end, we propose UAM network design with reserve capacity, i.e., a design where
alternative landing options and flight corridors are explicitly considered as a
means of improving contingency management. Similar redundancy considerations
are incorporated in the design of many critical infrastructures, yet remain
unexploited in the air transportation literature. In our methodology, we first
model how disruptions to a given UAM network might impact on the nominal
traffic flow and how this flow might be re-accommodated on an extended network
with reserve capacity. Then, through an optimization problem, we select the
locations and capacities for the backup vertiports with the maximal expected
throughput of the extended network over all possible disruption scenarios,
while the throughput is the maximal amount of flights that the network can
accommodate per unit of time. We show that we can obtain the solution for the
corresponding bi-level and bi-linear optimization problem by solving a
mixed-integer linear program. We demonstrate our methodology in the case study
using networks from Milwaukee, Atlanta, and Dallas--Fort Worth metropolitan
areas and show how the throughput and flexibility of the UAM networks with
reserve capacity can outcompete those without.
