An Airprox is a situation in which, in the opinion of a pilot or air traffic services personnel, the distance between aircraft as well as their relative positions and speed have been such that the safety of the aircraft involved may have been compromised.
How close do aircraft have to come for the pilot or air traffic controller to file an Airprox report?
As the definition of an Airprox makes clear, if the pilot or controller feels that the distance between aircraft as well as their relative positions and speed have been such that the safety of the aircraft involved was or may have been compromised then a report can be filed. In some cases, the aircraft are miles apart but an Airprox is filed because the required separation standard has not been met. The judgement as to whether or not to file an Airprox is for the pilot and/or controller to make.
The term 'near miss' is in common usage but has no relevance to Airprox. If a pilot or controller is of the opinion that the distance between aircraft as well as their relative positions and speed was such that the safety of the aircraft involved was or may have been compromised then he or she may report an Airprox. In Airprox 016/2002 for example, the separation recorded on radar between the two aircraft was 400ft vertically and 3 miles horizontally: this is hardly a 'near miss' in the way people generally use these words. In the judgement of the air traffic controllers who reported the event it was an Airprox and was therefore fully investigated and assessed by the Airprox Board.
In controlled airspace, an air traffic control service is provided. In uncontrolled airspace - more correctly, all airspace other than controlled airspace - the air traffic service is limited to advice and/or information to pilots.
The definition of an Airprox includes the words "...in the opinion of a pilot or a controller...". People on the ground or passengers in an aircraft sometimes feel that they have witnessed a 'close proximity' event but it is the opinion of those most directly involved which decides whether or not such an event is an Airprox.