Interview with Mie Utsunomiya. The objective of ACAS is to advice pilots in near real-time how to avoid potential collisions. Modern demands for more operational flexibility with increased safety call for a new type of ACAS. This new type of ACAS has been dubbed ACAS X. Increasing levels of traffic have lead to an increase in nuisance alerts. ACAS-X will minimise these, while maintaining existing levels of safety. ACAS X will take us into the future by accommodating reduced separation minimal, the introduction of new aircraft types (i.e. RPA) and the new operational paradigm being considered, ie: 4D trajectory, flight-centric control and other new procedures.