capture phase or during the capture phase itself thus this first condition makes it possible to restrict the engagement of the slowdown process to the aforesaid phases in the course of which it is preferable to reduce the vertical speed of the airplane ac because said speed could potentially generate ta or ra alerts and a second condition relating to air traffic in a predetermined zone surrounding said airplane ac this second condition makes it possible to restrict the engagement of the process for slowing down said airplane ac solely when the proximity with an intruder aircraft al justifies a vertical speed reduction it may involve various items of information provided by the tcas anticollision system ta alert and ra alert data relating to the intruder aircraft al thus in a first exemplary embodiment the tcas anticollision system determines the following information the presence or otherwise of an intruder aircraft al in a predetermined detection zone for example a rectangular zone centered on the airplane ac and defined by a vertical side of 3600 m 18000 feet and a horizontal side of 55 km 30 nautical miles and should an intruder aircraft al be detected in said detection zone parameters associated with said intruder aircraft al relative altitude vertical speed etc the analysis of the aforesaid information provided by the anticollision system 2 makes it.