Procedural Control
General
The general principles of air traffic control are the same regardless of whether procedural or surveillance methods are used (i.e. the controller monitors the traffic situations, detects and solves conflicts by providing separation, and ensures orderly flow of the air traffic). The difference lies in the way situational awareness is built and updated (by pilot reports, estimates and visual observation), the separation minima themselves (as described in ICAO Doc 4444, Chapter 5) and the support tools (flight strips instead of a situation display).
Where procedural control is employed, separation is ensured by dividing the airspace either vertically or laterally. In a Control Zone where reliable surveillance is not available, it may be classified as procedural, meaning separation is maintained using specified time or distance criteria. There are four types of procedural separation: lateral, longitudinal, vertical, and visual.
Typical Applications
Typical applications of procedural control include:
- In airspace where surveillance cover is not available (e.g. oceanic airspace or sparsely populated areas)
- In terminal movement areas (TMAs) if the traffic levels are such that they do not warrant the installation and maintenance of a surveillance system
- In aerodrome control zones (CTRs), especially if the traffic density is relatively low and the aerodrome layout is not complex (e.g. only one runway, one apron and a few taxiways)
- Backup solution in case of complete failure of all surveillance-based systems
Separation
Longitudinal Separation
Longitudinal separation is only applied between aircraft flying on identical or reciprocal tracks.
To maintain longitudinal time separation, pilot-provided estimates must be used to ensure that the time interval between two aircraft meets or exceeds the required minimum.
Time
Same Track:
Aircraft following the same track, including situations where one is climbing or descending through another’s level, must maintain a separation of at least 10 minutes.
Reciprocal Track:
Aircraft on reciprocal tracks must be separated using vertical separation. A vertical separation minimum of 10 minutes must be in place before the estimated passing time.
After the estimated time of passing, no aircraft may climb or descend through the other’s level until at least 10 minutes have passed. If positive identification of passing is confirmed, this requirement may be reduced, which will be discussed later in this section.
Distance
Distance Reports:
The lead aircraft must be navigating directly to or from the designated navigational aid. The distance report from the lead aircraft must always be obtained before requesting it from the following aircraft.
Distance checks must be performed as outlined in the table below:
| Condition | Minimum Interval for Distance Checks |
|---|---|
| No speed control applied | 15 minutes |
| Speed control applied to limit closure to ≤ 35 knots or Mach 0.06 | 15 minutes |
| Closure rate exceeds 35 knots or Mach 0.06 | Standard is not valid |
| Speed control applied to maintain a stable or increasing separation | 30 minutes |
Methods of Ensuring Distance-based Separation:
| Method | Conditions | |
|---|---|---|
| 20nm Exists Between | Aircraft are on the same track. | |
| 5nm Definite Passing | Reports indicate that the aircraft have passed and the distance is opening. | |
| Sight and Pass | - Both aircraft report sighting and passing the other by day, and in Oceanic by night. - Both aircraft are above A100. - There is no possibility of incorrect identification by other aircraft. |
|
| Opposite Side of Visual Fix | - Both aircraft report passing the same visual fix. - The visual fix must be a prominent topographical feature within 10,000 FT of the levels of each aircraft. |
Lateral Separation
Time-based Crossing Track
Aircraft on crossing tracks, including cases where one is climbing or descending through another’s level, must maintain a separation of 15 minutes at the intersection point.
If a 15-minute separation cannot be ensured at the crossing point, vertical separation must be applied. This vertical separation remains in effect from the time the second aircraft is 15 minutes from the intersection until the first aircraft has passed it by 15 minutes.
Lat Sep Table
Lateral separation is determined based on the concept of a Lateral Separation Point (Lat Sep point)—the distance at which procedural lateral separation is considered to exist, depending on the angle of intersection between two tracks. The required separation distances are outlined in the table below:
| Intersection Angle | Required Separation |
|---|---|
| 0° - 15° | No lateral separation |
| 16° - 44° | 11nm |
| 45° - 135° | 8nm |
| 136° - 180° | No lateral separation |
Visually, an aircraft at point ABC is considered laterally separated from aircraft in the green zones, whereas it is not laterally separated from aircraft in the red zones. These criteria only apply when both aircraft are tracking directly to or from the crossing point.
This method is particularly useful for planning restrictions when surveillance coverage is not guaranteed.
Vertical Separation
Vertical separation is the most commonly used method in non-radar environments, allowing controllers to maintain aircraft at different flight levels without the risk of loss of separation—unless one aircraft needs to climb or descend.
The primary reference for aircraft altitudes and assigned levels is pilot-reported data.
| Altitude / Flight Level | Separation Standard |
|---|---|
| SFC - FL290 | 1000ft |
| FL290 - FL410 | RVSM: 1000ft, Non-RVSM: 2000ft |
| FL410 - FL600 | 2000ft |
| Any Supersonic Aircraft, regardless of FL | 3000ft |
RVSM airspace is only applicable between FL290 and FL410.
An aircraft cannot be cleared to climb or descend into a level that was previously occupied by another aircraft until that aircraft has confirmed it is clear of the relevant levels.