  Understanding the relation between planned and realized flight trajectories
and the determinants of flight deviations is of great importance in air traffic
management. In this paper we perform an in depth investigation of the
statistical properties of planned and realized air traffic on the German
airspace during a 28 day periods, corresponding to an AIRAC cycle. We find that
realized trajectories are on average shorter than planned ones and this effect
is stronger during night-time than daytime. Flights are more frequently
deviated close to the departure airport and at a relatively large angle to
destination. Moreover, the probability of a deviation is higher in low traffic
phases. All these evidences indicate that deviations are mostly used by
controllers to give directs to flights when traffic conditions allow it.
Finally we introduce a new metric, termed difork, which is able to characterize
navigation points according to the likelihood that a deviation occurs there.
Difork allows to identify in a statistically rigorous way navigation point
pairs where deviations are more (less) frequent than expected under a null
hypothesis of randomness that takes into account the heterogeneity of the
navigation points. Such pairs can therefore be seen as sources of flexibility
(stability) of controllers traffic management while conjugating safety and
efficiency.
