The combination of the two words, air and proximity. An occurrence in which aircraft come into such close proximity that a threat to the safety of the aircraft exists or may exist, in airspace where the aircraft are not subject to an air traffic separation standard, or where separation is a pilot responsibility.

Standard pressure

The pressure of 1013.2 HPa which, if set up on the pressure sub-scale of a sensitive altimeter, will cause the latter to read zero when at mean sea level in a standard atmosphere.

No-transgression zone (NTZ)

A corridor of airspace of defined dimensions located centrally between the two extended runway centre lines where controller intervention is required to manoeuvre aircraft when this airspace is penetrated by an aircraft conducting a simultaneous approach to a parallel instrument runway.


Notice to airmen: a notice distributed by means of telecommunication containing information concerning the establishment, condition or change in any aeronautical facility, service, procedure or hazard, the timely knowledge of which is essential to personnel concerned with flight operations.

Non-directional beacon (NDB)

A special radio station, the emissions of which are intended to enable
a mobile station to determine its radio bearing or direction with reference to that special radio station.


That period of time between the end of evening civil twilight and the beginning of morning civil twilight.

Navigation specification1***sub***RNAV specification

A navigation specification based on area navigation that does not include the requirement for performance monitoring and alerting, designated by the prefix RNAV, for example RNAV 5, RNAV 1.