The variables with the greatest influence on ATM safety barriers

•The most influential factors in the progression of a possible loss of separation between en-route aircraft are analysed.•Air Traffic Controllers (ATCo) play a fundamental role in ensuring the safety of aircraft.•Bayesian networks provide high predictive power for estimating low probability events and allow the integration of knowledge modelling with data inference.•A predictive methodology is developed with the objective of characterising airspace safety between en-route aircraft in terms of Separation Minima Infringements (SMI).