In a method of detecting a collision risk and preventing air collisions, it is proposed that probabilities should be calculated for the likely presence of one's own aircraft in predetermined sectors at a number of selected times (probabilities of presence) and these probabilities for one's own aircraft and those for other objects should be used to calculate the probabilities of one's own aircraft and at least one of the other objects being present simultaneously in a given sector (probabilities of collision) for the predetermined sectors and selected times.