Composite separation is a combination of vertical separation and one of the other forms of horizontal separation (lateral or longitudinal). It allows using minima for each which may be lower than, but not less than half of, those used for each of the combined elements when applied individually. Composite separation is applied on the basis of regional air navigation agreements. It should be noted that composite separation is not meant to be used in surveillance environment, i.e. the combination of 500 ft vertical and 2.5 NM horizontal separation is out of scope.
Composite separation is a means of improving airspace utilization and safety in route systems where conventional lateral separation is 90 NM or greater, due to the dispersion of some flights to other routes and altitudes.
Composite separation may be considered where navigation is dependent on long-distance station-referenced aids and/or on self-contained systems and the frequency of flights crossing the axis of the track system is not significant.
There are two types of composite separation provided for in ICAO Annex 11: composite lateral/vertical separation and composite longitudinal/vertical separation. An example of the former is shown on the pictures below.
An example of airspace where composite separation is not applied. The minima are 2000 ft vertical, 100 NM lateral and 15 min longitudal separation.
An example of airspace where composite vertical/lateral separation is applied. This allows flights FTNAS and KRA42 to be separated by 50 NM and 1000 ft from aircraft on adjacent tracks.
Prior to implementing composite separation, existing traffic flows are examined and aircraft navigation performance and expected dispersion are evaluated. After the introduction of composite separation, navigation performance is monitored so that the safety levels can be maintained.
Composite separation requires that aircraft are properly established at the correct level and on the correct track on entry into the system. Aircraft position at the end points of the route structure where composite separation is applied needs to be accurately determined either by means of surveillance coverage or establishment of appropriate procedures.
At present, composite separation may be applied between aircraft operating at or above FL 290 within the flexible Pacific Organized Track Systems (PACOTS), North Pacific (NOPAC) route system between the United States and Japan and the route system between Hawaii and the west coast of the United States, within the Fukuoka, Oakland Oceanic and Anchorage Oceanic FIRs. It consists of a combination of at least 50 NM lateral and 1000 ft vertical separation. Composite separation is only applied between aircraft flying on parallel tracks within the composite route systems. If an aircraft crosses a composite route system then non-composite separation must be provided.
- ICAO Annex 11
- ICAO Doc 9426 - Air Traffic Services Planning Manual
- ICAO Doc 7030 - Regional Supplementary Procedures