The Airborne Collision Avoidance System (ACAS) II is mandated for carriage by certain fixed-wing aircraft in European airspace. The mandate does not extend to helicopters. Operational experience and safety studies have demonstrated the benefit to be derived by fixed-wing aircraft that equip with ACAS II but the same benefit would not necessarily be enjoyed by helicopters due to limitations in helicopter performance, and the design of the hardware and software components of ACAS II.