National authorities lay down vertical and horizontal separation standards to facilitate the safe navigation of aircraft in controlled airspace. Observance of these standards ensures safe separation from the ground, from other aircraft and from protected airspace (see Loss of Separation). Separation standards may sometimes serve to reduce exposure to Wake Vortex Turbulence although there are many occurences of significant wake vortex encounter at separations much greater than prevailing minimum separation.
National separation standards are based on the provisions of ICAO Doc 4444 (Procedures for Air Traffic Management), especially Chapter 5. Differences from these standards (if any) are published in national AIPs.
The methods used to achieve separation are varied and complex, depending on the phase of flight and the relative trajectories of the aircraft involved. The examples quoted below illustrate the more simple methods used for aircraft in the cruise.
Vertical separation is achieved by requiring aircraft to use a prescribed altimeter pressure setting within designated airspace, and to operate at different levels expressed in terms of altitude or flight level.
ICAO specify minimum vertical separation for IFR flight as 1000 ft (300 m) below FL290 and 2000 ft (600 m) above FL290, except where Reduced Vertical Separation Minima (RVSM) apply. Most national authorities follow a similar rule, but may specify a different level at which the rule changes.
Lateral separation is achieved by various means, which include the following:
- By position reports which positively indicate the aircraft are over different geographic locations (see Fig. 1).
- By requiring aircraft to fly on specified tracks which are separated by a minimum angle. Both aircraft must be established on radials or tracks which diverge by a specified amount depending on the type of navigation aid in use, and at least one aircraft must be at a distance of 15 NM or more from the facility (see Fig. 2).
Longitudinal separation is applied so that the spacing between aircraft is never less than a specified amount. For aircraft following the same or diverging tracks, longitudinal separation may be achieved by requiring aircraft to make position reports and comparing the time of their reports (see Fig. 3) and by speed control, ensuring that the speed of the following aircraft does not exceed the speed of the leading aircraft. Reduced separation may apply if the leading aircraft is maintaining a higher speed than the following aircraft.
Separation minima based on ATS surveillance systems
When surveillance systems are used (based on radar, ADS-B or MLAT) the minimum separation prescribed by ICAO Doc 4444 is 5 nm (unless otherwise stated by the appropriate ATS authority). This minimum may be reduced by the appropriate ATS authority but not below:
- 3 nm when the surveillance systems' capabilities at a given location permit this;
- 2.5 nm between succeeding aircraft which are established on the same final approach track within 10 nm of the runway threshold. A number of additional criteria must be met in order to apply this separation minimum (described in detail in ICAO Doc 4444, 220.127.116.11 b)).
The separation minimum or minima based on radar and/or ADS-B and/or MLAT systems to be applied are to be prescribed by the appropriate ATS authority based on the equipment used. Therefore, higher than 5 nm horizontal separation minima may be applied in certain parts of the airspace and/or at certain times.
Wake Vortex Turbulence
In addition to traffic separation standards applied to aircraft on approach to or taking off from the same or closely spaced runways, more restrictive separation minima are defined (see Mitigation of Wake Turbulence Hazard) to ensure that following aircraft, especially smaller ones, are not hazarded by the effects of wake vortex turbulence generated by a preceding aircraft.
Aircraft are categorised according to their MTOM (maximum take-off mass) and minimum separation times or distances are applied to aircraft in sequence so that aircraft following a higher MTOM aircraft are given greater spacing than those following a same or lower MTOM category. Details of the separation applied depends on circumstances, for example whether aircraft are using the same, parallel or crossing runways, and whether using the same or different take-off commencement or touchdown positions.
If, during an emergency situation, it is not possible to ensure that the applicable horizontal separation can be maintained, emergency separation of half the applicable vertical separation minimum may be used. This means that a 1000 ft vertical separation minimum may be reduced to 500 ft and 2000 ft vertical separation minimum may be reduced to 1000 ft. All flight crews concerned must be advised if emergency separation is used.
The use of emergency separation is described in ICAO Doc 4444, 15.7.1.
- Doc 4444 (Procedures for Air Traffic Management),
- Annex 2 (Rules of the Air),
- Annex 11 (Air Traffic Services).