In view of the SESAR and NextGEN objectives of increasing both the capacity and the safety of the Air Traffic Management (ATM) system, there is a need to conduct safety risk analysis of current or new operations, covering the joint effect of airborne and ground-based safety nets in ATM. The subject of the research presented in this paper is Airborne Collision Avoidance System (ACAS) which presents an airborne safety net within an ATM context, for current practices as well as advanced concepts. The aim of the research described in this paper is fivefold: (a) to verify existing ACAS models regarding their coverage of evaluation needs of ACAS operations; (b) to develop a stochastic and dynamical model of ACAS II including interactions with pilots and air traffic control; (c) to develop a systematic validation process that allows building model confidence; (d) to initially apply this validation process to the newly-developed ACAS model; and (e) to use the ACAS model to assess the potential collision risk reduction by ACAS II for a historical en-route mid-air collision event. The specific modelling formalism used for this is Stochastically and Dynamically Coloured Petri Nets (SDCPN). The developed SDCPN-based ACAS model contains the technical, human and procedural elements of ACAS operations and fully supports mathematical analysis as well as rare event Monte Carlo simulation of aircraft encounters. In order to build confidence into the developed model and to judge model credibility, a systematic multilevel validation process is defined and is successfully applied. The SDCPN-based ACAS model is demonstrated to work well for a historical en-route mid-air collision event and is very powerful in determining the most critical elements contributing to the non-zero collision risk of ACAS operation.