  Functional networks, i.e. networks representing dynamic relationships between
the components of a complex system, have been instrumental for our
understanding of, among others, the human brain. Due to limited data
availability, the multi-layer nature of numerous functional networks has
hitherto been neglected, and nodes are endowed with a single type of links even
when multiple relationships coexist at different physical levels. A relevant
problem is the assessment of the benefits yielded by studying a multi-layer
functional network, against the simplicity guaranteed by the reconstruction and
use of the corresponding single layer projection. Here, I tackle this issue by
using as a test case, the functional network representing the dynamics of delay
propagation through European airports. Neglecting the multi-layer structure of
a functional network has dramatic consequences on our understanding of the
underlying system, a fact to be taken into account when a projection is the
only available information.
