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Category: Loss of Separation Loss of Separation
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AIRPROX (Aircraft Proximity)


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. (ICAO Doc 4444: PANS-ATM).

ICAO defines a series of classifications for AIRPROX events which have been reported and subsequently investigated by an appropriate body. It is required that this classification should be assigned on the basis only of actual risk, not potential risk. This means that only the residual risk after any avoiding action is considered. The available classification categories are:

  • A - Risk of collision. The risk classification of an aircraft proximity in which serious risk of collision has existed. An A AIRPROX may or may not be deemed to be a Serious Incident as defined by ICAO Annex 13.
  • B - Safety not assured. The risk classification of an aircraft proximity in which the safety of the aircraft may have been compromised.
  • C - No risk of collision. The risk classification of an aircraft proximity in which no risk of collision has existed.
  • D - Risk not determined. The risk classification of an aircraft proximity in which insufficient information was available to determine the risk involved, or inconclusive or conflicting evidence precluded such determination.

The definition and classification of an AIRPROX given above was agreed prior to the introduction of ground radar and airborne systems (ACAS) capable of measuring accurately the actual separation of the aircraft involved.

An AIRPROX may occur as a result of a Level Bust or Airspace Infringement. Safety nets such as ACAS and STCA mitigate the resultant risk of collision.

An AIRPROX is commonly referred to as a Near Midair Collision (NMAC) in the USA; however, they are not precisely the same as a comparison of their definitions demonstrates. Whereas an NMAC is always an AIRPROX, an AIRPROX is not necessarily an NMAC.

Reporting and Investigation of an AIRPROX

ICAO require the establishment of AIRPROX reporting and investigation procedures and these are specified in national procedures.

Typically, national authorities establish a special committee to investigate AIRPROX reports which allocates the actual risk classification and to recommend further action. Some States use their Annex 13 Accident Investigation Agency to also investigate all AIRPROX, not just those which are considered to be a Serious Incident. Very few states freely publish their AIRPROX investigations - Switzerland and the UK are notable exceptions.

An AIRPROX should be reported as soon as possible to facilitate investigation of the incident. If circumstances allow, the pilot should report the incident immediately to ATC using RTF, the details will then be reported by ATC to the appropriate body. If it is not possible to report an AIRPROX in flight (e.g. because the frequency in use is too busy) the pilot should report the incident as soon as possible after landing. ATS units to whom AIRPROX incidents are reported should also report the circumstances of which they are aware to the appropriate body. In all cases, initial verbal reports should be followed up by full written reports using any prescribed form which may be in use for that purpose.

Depending on circumstances, an AIRPROX may qualify as a Serious Incident which will then require that it be reported to and investigated by the National Accident Investigation Agency under the terms of ICAO Annex 13, Chapters 4 and 5. Non-Annex 13 AIRPROX investigations are usually a special case of State Mandatory Occurrence Reporting and most are followed up by or in association with the applicable Regulatory Authority under the terms of ICAO Annex 13 Chapter 8, in contrast to one which is deemed to have also been a Serious Incident to whereas an Investigation carried out in accordance with the principles of ICAO Annex 13 must be independent of any Regulatory or other influence.

Further Reading