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Since its introduction in the 90’s, TCAS II, presented as a straightforward and very reliable technological tool, has significantly reduced the risk of collision. Paradoxically, the introduction of this system has been accompanied with numerous incidents and one major accident in 2002, mainly due to unclear rules, poor air-ground cooperation and poor human decision. In order to investigate these potential human factors issues, a part-task air-ground simulation was conducted: 10 pilots and 10 controllers were involved in the simulations of 4 scenarios containing TCAS occurrences. Data collected included video camera recordings for behavioral analysis, Heart Rate (HR) for stress evaluation, questionnaires and debriefings for perceived risk levels and situational awareness assessment. The observations and errors were analyzed through the CREAM methodology. The debriefings were led through a self-confrontation technique, together with pilots and controllers. Results show that the simulations of TCAS situations were able to produce a significant physiological stress response with significant increase of HR when a resolution happens. Questionnaires and debriefings show that, in most of the observed cases, aircrew, and controllers are not sharing the same mental picture of the involved traffic and the risk of collision. This raises important issues in terms of cooperation between controllers and aircrews in such demanding occurrences. This should allow identifying risky situations and the related generic causes. The results will be discussed, aiming at a potential improvement of the system, in terms of Human Machine Interface, training and consistency of procedures.