Aircraft which are depending on air traffic control for instructions, i.e. those which are flying and relying primarily on their navigation instruments will generally be flying IFR, i.e. INSTRUMENT FLIGHT RULES, and must be separated from each other by the Air Traffic Controller using one of (2) two main methods.
1. Procedural Control or
2. Radar Control
In our discussion of Procedural separation, there will be the assumption that Radar is not available, since, in order to be as expeditious as possible, the controller will always use radar for separation purposes when and where radar is available. (Radar allows the controller to fit more aircraft into the same airspace without compromising international safety standards.)
Procedural Control is divided into three categories.
1. Vertical Separation
2. Longitudinal Separation
3. Lateral Separation
VERTICAL SEPARATION
Vertical separation may be considered as the vertical spacing required between two aircraft as they proceed, either along the same route or to cross the routes of each other if they are in close proximity.
Vertical separation is usually expressed in Feet
To meet international standards, there must never be less than a minimum of 1000 feet vertically between aircraft, if they are flying below 29,000 feet. Above 29,000 feet there must usually be a minimum of 2000 feet vertically between aircraft. However within recent times, due to the accuracy of state of the art technology in aircraft radar and other height-finding devices on aircraft, the international aviation body, (ICAO) has recommended 1000 feet separation above 29,000 feet in some areas of the world.
Since aircraft may move in any of three dimensions, it is possible to provide some kind of vertical spacing between them after they take off. When aircraft are desirous of proceeding to a destination along the same route, and they are leaving the same airport around the same time, the most reasonable form of separation on that occasion may be vertical separation, since there probably will not be enough of a difference between their departure times to separate them in any other way.
For instance if two of BWIA's Boeing 737's are expected to depart from Piarco around the same time, and are both requesting to proceed to a height or altitude of 23,000 feet (more accurately referred to as Flight Level 230) to Barbados, the most appropriate separation at that time will most likely be vertical separation. In other words, the controller will instruct the first aircraft to climb to Flight Level
230 and the second one to Flight Level 210, (making sure that the second one does not climb faster that the first), thus providing a vertical separation of 2000 feet between them. This leaves the level in between (FL 220) for any aircraft proceeding in the opposite direction from Barbados to Trinidad. (Refer Diagram above.)
LONGITUDINAL SEPARATION.
Longitudinal Separation of aircraft may be defined as the longitudinal distance between two aircraft, moving in the same direction, one behind the other, along the same line or route and flying at the same level.
Such Longitudinal Separation may be expressed and applied in minutes or in nautical miles (NM).
EXAMPLE. Many drivers on the road do not ensure that they have enough longitudinal separation between their vehicles. The result is usually a tailgating accident. As an example of this form of separation, consider the following. If you have four seconds of longitudinal separation between you and the car ahead, this means that if the car ahead suddenly stops dead (an impossibility, due to inertia), you will not hit this vehicle until four seconds later, assuming that you make no manoeuvre to avoid the accident.
In a similar situation, when two aircraft of the same speed are instructed to, or allowed to climb to the same altitude or flight level to proceed along the same route, the controller must ensure that they have the required time or distance separation between them.
Such time separation as expressed in (minutes), and may be any of the following:-
3 minutes
5 minutes.
10 minutes
15 minutes or
20 minutes; depending on the circumstances.
Ten (10) minutes longitudinal separation has now become the normal longitudinal separation in use by Piarco controllers, but within and over the Eastern Caribbean islands where Radar Separation cannot be used.
The circumstances which determine which one of those figures above the controller will use, can be the subject of extensive discussion, particularly where the faster aircraft is behind. Here is where Time Separation gets really interesting.
As applied and expressed in NM, longitudinal separation may be 10 to 20 NM, based upon a navigation facility known as a DME, (Distance Measuring Equipment) and depending upon a number of circumstances. (Refer Diagram above.) This is not Radar Separation.
LATERAL SEPARATION
Lateral separation may be defined as the minimum distance required between aircraft which are traveling at the same altitude and along parallel or near parallel routes, possibly to the same destination; or crossing the altitudes of each other in climbs or descents in the vicinity of airports.
This lateral separation distance is expressed in NM.
As aircraft approach or leave airports, they must be separated as they climb or descend though the altitude or levels of each other. Since the destination or departure airport may be considered as a single point on the surface towards, or away from which the airplanes are moving, the controller must ensure that a form of separation is applied whether aircraft are both departing, both arriving or passing each other in opposite directions. The most appropriate form of separation in these circumstances is lateral separation and must be effected at some minimum distance from the airport to ensure safety.
If this is not possible, vertical separation must be applied until the incoming aircraft is either visible to the controller or reports over the airport electronic navigation beacon. Such a procedure may cause some justifiable delay to arriving aircraft.
This lateral separation distance is usually expressed in nautical miles, but there is more to this, as described in the diagram below.
As the diagram suggests, in the vicinity of the airport, Lateral Separation may be achieved using the required pilot-reported distance from the common electronic airport homing beacon plus a required angular difference between the aircraft being separated at the time.