  Technologically driven transport systems are characterized by a networked
structure connecting operation centers and by a dynamics ruled by
pre-established schedules. Schedules impose serious constraints on the timing
of the operations, condition the allocation of resources and define a baseline
to assess system performance. Here we study the performance of an air
transportation system in terms of delays. Technical, operational or
meteorological issues affecting some flights give rise to primary delays. When
operations continue, such delays can propagate, magnify and eventually involve
a significant part of the network. We define metrics able to quantify the level
of network congestion and introduce a model that reproduces the delay
propagation patterns observed in the U.S. performance data. Our results
indicate that there is a non-negligible risk of systemic instability even under
normal operating conditions. We also identify passenger and crew connectivity
as the most relevant internal factor contributing to delay spreading.
