Regulations and Requirements
In civil aviation, several mechanisms are present to minimize the probability of collision with other aircraft, objects or terrain. Generally speaking, they are categorized as separation assurance and collision avoidance.
The first category aims to keep aircraft separated according to minimum separation distances both in the lateral and vertical planes. These minimum values depend on several factors, such as the airspace class, the flight rules, the flight phase, the air traffic control (ATC) surveillance means (if any), the performance of the onboard navigation systems, etc. Roughly speaking, lateral minimum separation between aircraft can range from 3 nautical miles in terminal areas with ATC radar separation service to up to 60 nautical miles for two aircraft at the same altitude in a North Atlantic track. Yet, in non-controlled airspaces minimum separation does not involve precise separation minima and aircraft must remain well clear from each other. Well clear is a qualitative rather than a quantitative term used in current regulations when referring to the minimum miss distance between two aircraft that are avoiding a collision.
On the other hand, collision avoidance is considered as a last resort manoeuvre to prevent a collision in case of a loss of separation. In some cases, collision avoidance between aircraft is performed cooperatively, meaning ...