Separation is the product the air traffic control service exists to deliver, and this lesson is Doc 4444's chapter on how it is manufactured. First the general provisions: who is separated from whom, and what a controller may never clear. Then the three dimensions the job is done in: vertical separation with its 1,000 and 2,000 ft minima, lateral separation built from routes and radials, and longitudinal separation built from time, distance and the Mach number technique, closing with the surveillance minimum that replaces most of it wherever radar reaches.
Who is separated from whom
The airspace class table of the air traffic services module compressed this rule; Doc 4444 states it as a list. Vertical or horizontal separation shall be provided:
Between all flights in Class A and B airspace.
Between IFR flights in Class C, D and E.
Between IFR and VFR flights in Class C.
Between IFR flights and special VFR flights.
Between special VFR flights when so prescribed by the appropriate ATS authority.
So the type of separation delivered depends on where you are and what you are: an IFR flight is separated from other IFR everywhere in controlled airspace, but from VFR traffic only in classes A through C; in classes D and E, VFR traffic is something you are told about, not separated from.
One exception is built into the list itself, and it relieves specifically the IFR-versus-IFR requirement in classes D and E: during the hours of daylight, a flight may be cleared to climb or descend subject to maintaining its own separation and remaining in visual meteorological conditions. The moment such a clearance is accepted, the responsibility for avoiding collision with the traffic concerned moves to the flight deck. That is the general answer to who avoids collisions in VMC: the pilot-in-command: every VFR flight separates itself by see-and-avoid except where the class table says otherwise, and an IFR flight that accepts an own-separation clearance takes the same duty on for the manoeuvre.
Three provisions frame the controller's side. No clearance shall be given to execute any manoeuvre that would reduce the spacing between two aircraft to less than the applicable separation minimum: the minima are floors for planning, not targets to shave. Larger separations than the minima should be applied in exceptional circumstances such as unlawful interference or navigational difficulties, with care not to strangle the traffic flow. And when the separation in use is failing, the repair must come first: where a type of separation or minimum cannot be maintained, another shall be established before the time the current minimum would be infringed. The same logic obliges a crew whose navigation, communications, altimetry, flight control or other systems degrade below the airspace requirement to advise ATC without delay, so the controller can rebuild the separation on what still works.
The kinds of separation
The family tree the syllabus asks for has two main branches. Vertical separation stacks aircraft at different levels. Horizontal separation splits into the two main methods:
Lateral separation — different routes or places, side by side.
Longitudinal separation — spacing along the same or crossing tracks, managed in time or distance.
Around the core stand the named variants:
Composite separation — a prescribed combination of partial vertical and partial horizontal components used on some oceanic route systems.
Separation based on ATS surveillance systems — the picture on the display replaces the procedural geometry.
Visual separation — the own-separation clearances above, where the pilot's eyes do the work.
Vertical separation
Vertical separation is obtained by requiring aircraft using prescribed altimeter setting procedures to operate at different levels, expressed as flight levels or altitudes: the whole transition machinery of the previous lesson exists so that two aircraft told to fly different numbers really are apart. The levels assigned are selected from the tables of cruising levels in Appendix 3 of Annex 2 (the semicircular system taught with the rules of the air), though the track-to-level correlation may be waived by clearance or by the ATS authority in the AIP. The minimum itself:
A nominal 300 m (1,000 ft) below FL 290, and a nominal 600 m (2,000 ft) at or above FL 290 — the basic rule.
Within designated RVSM airspace, under a regional air navigation agreement: 300 m (1,000 ft) below FL 410, and 600 m (2,000 ft) at or above FL 410 — the reduced vertical separation minimum that doubled the capacity of the level bands airliners actually cruise in, bought with stricter altimetry requirements on the aircraft.
Three assignment rules complete the vertical picture. Cruising levels of aircraft flying to the same destination shall, in so far as practicable, be assigned in a manner that will be correct for an approach sequence at destination: the aircraft that will approach first gets the level that lets it descend first, so the arrival does not have to unstack itself in the terminal area. Priority follows occupancy: an aircraft at a cruising level normally has priority over aircraft requesting that level, and between aircraft at the same level, the preceding aircraft normally has priority. And a level freshly vacated is not instantly free: an aircraft may be cleared to a level previously occupied by another after the latter has reported vacating it, except when severe turbulence is known, the higher aircraft is in a cruise climb, or the performance difference between the two could erode the minimum, in which case the clearance waits until the vacating aircraft has reported at or passing a level the required minimum away.
Finally, the paired-climb case the syllabus singles out: two aircraft may be cleared to maintain a specified vertical separation between them during climb or descent on exactly two conditions: the pilots are in direct communication with each other, and the clearance is issued with their concurrence. The controller delegates the vertical spacing to the two crews, and only crews who can hear each other, and have agreed to it, can hold it.
Lateral separation
Lateral separation is obtained by requiring operation on different routes or in different geographical locations, as determined by visual observation, by navigation aids, or by RNAV equipment, with the specified distance between the protected routes never less than an established value that includes a buffer for navigational inaccuracy. The simplest form is geographical separation: position reports which positively indicate the aircraft to be over different geographic locations, as determined visually or by reference to a navigation aid: one aircraft reporting over one point, the other over a different one, and the geometry of the points does the separating.
On intersecting tracks the separation is angular, and the minima depend on the aid the tracks are built on. Lateral separation exists when: