Reducing mid-air collision risk in controlled airspace: Lessons from hazardous incidents

The ATM system can be (over-) simplified to consist of three structural system layers acting as the system controls: Planning (pre-operational), Operation (the flight in progress), and Alert (the ground and air protection enabled by conflict alert systems, on which the controller/pilot will act). A hazardous event is one in which a high degree of conflict between aircraft is observed plus a low confidence that the remaining system layers would generally provide the necessary corrective action.