ACAS II is an airborne avionics system designed to reduce the risk of mid-air collision. Its carriage by a majority of aircraft within Europe is mandatory. ACASA is a set of studies that has investigated several areas related to ACAS II operations in Europe. Work Package 1 of the ACASA project investigated safety. The safety of ACAS was assessed purely in terms of the degree to which it achieved its stated aim of preventing mid-air collisions.