Procedural Control - Techniques

Techniques

The central rule of procedural control is that each aircraft is cleared onto a predetermined route (airway), and no aircraft traveling on the same or intersecting routes at the same level shall come within 10 minutes' flying time of another (or sometimes 15 minutes depending on the accuracy of the available radio navigation beacons).

Using procedural control, the controller must maintain a mental picture of the location of aircraft based on each aircraft's flight progress strip, which contains its route, altitude and estimated times over reporting points. That information is compared against all other aircraft in the sector to determine if there are any conflicts. For aircraft that conflict, the controller issues an altitude, speed or routing change that both separates the conflicting aircraft from each other, while still remaining separated from all others. After all conflicts have been resolved in this way, the sector is considered "separated" and the controller only needs to check again for conflicts when new aircraft are added, if an aircraft needs to change its altitude, or if the aircraft reaches a reporting point significantly earlier or later than previously estimated.

There are two main techniques controllers use to organize flight progress strips in order to best detect conflicts: grouped together by altitude, or grouped together by route intersection points (fix posting area). Altitude grouping is the easiest and most common method when most aircraft in the sector tend to be level at a cruising altitude, such as in trans-oceanic sectors. Each aircraft's strip is placed in a "bay" (labeled section of the strip rack) depending on its altitude. Because aircraft cruising at different altitudes will never conflict regardless of route of flight or estimated times, they don't need to be checked against each other. Only aircraft at the same altitude (i.e. in the same bay) need to be further evaluated. If an aircraft needs to transition to a new altitude (whether changing cruising level, or descending to or climbing from an airport), it only needs to be checked against others in the bays between the current altitude and destination altitude.

The fix posting area method is easier however when there are a lot of altitude changes because of numerous departures and arrivals within the sector. This is a more obscure method today because most airports around the world with significant enough traffic to warrant this technique have at least a radar covering the climb out and arrival area, and only need procedural control for points between (in which case altitude grouping is likely more efficient).

Nevertheless, the fix posting area method was the main technique used in U.S. en route system prior to the 1960s, for example, when en route radar became more widespread. This method evolved from similar techniques used in railroad dispatch. It simplifies conflict detection because aircraft on different routes can only possibly conflict at intersection points with other routes, or with aircraft on the same route. Each intersection point ("fix posting area") in the sector is made into a separate bay on the strip rack, and multiple strips are printed for each aircraft, one per bay. The controller then sorts strips within each bay by the estimated time the aircraft is expected over that point. Only aircraft estimated to cross within 10 minutes of each other are considered possible conflicts, at which point altitudes are then compared to determine if there actually is a conflict. Aircraft on the same route however (in either direction) must be checked for conflict regardless of time over the intersection.

As new altitudes are issued, all of the strips for that aircraft must be annotated with the new data and separation checked again at all of the intersections it passes through. This is more laborious than the altitude method of grouping where only one strip is needed, but possible conflicts between multiple aircraft climbing and descending are easier to discern or discard this way since all intersection points and times are pre-calculated and laid out separately. With the altitude grouping method, if two aircraft are determined to conflict based on altitude, the controller must then manually determine the intersection points and times to further determine or rule out the conflict.