Collision Detection - Line of Constant Bearing


Some examples of human factor issues existing simultaneously within an air traffic control environment are decision making, cognitive processing, human interaction (communication), workload management, workstation ergonomics and situational awareness.

Controllers are working in an ever changing three-dimensional environment where every decision they make has an impact on the present and future traffic situation.

At the same time they are being inundated with a variety of visual and audio inputs from various sources which they need to filter, assess and prioritise accordingly. They have to maintain communications with aircraft and other controllers using standard phraseology, ensuring that all clearances and instructions are read-back correctly, while avoiding read-back or hear-back errors.

One of the biggest challenges within this environment, for trainee controllers in particular, is to correctly identify conflicting traffic, accurately determine the level of risk presented by each conflict, and then prioritise their control instructions to ensure that the required separation standard is established.

They must then ensure that their instructions are being complied with, while continuing to maintain situational awareness of every aircraft and each potential conflict within their area of responsibility.

Conflict Detection

The primary objective of air traffic control is to prevent collisions between aircraft. Air traffic controllers shall issue clearances, instructions and information to aircraft under their control, to achieve a safe, orderly and expeditious flow of air traffic. This requires air traffic controllers to detect potential conflicts between aircraft and develop positive control actions to provide the required longitudinal, vertical, or lateral separation between them.

Because air traffic controllers continually process a multitude of information, from a variety of sources, cognitive processing can be severely impaired at times due to information overload. Using a line of constant bearing between converging traffic is one of the simplest tools available to a radar controller to detect potential collisions between aircraft.

This method acts as an initial filter, being a quick and easy assessment tool that controllers can use to determine which aircraft will pass safely, which ones may require further attention, and which ones definitely pose a collision hazard, requiring control action to be taken.


Within the air traffic control community the following definitions are used to indicate the relationship between aircraft tracks:

Reciprocal Track

  • A term used in the application of separation, indicating tracks that converge or diverge at an angle of 136 degrees to 180 degrees inclusive.

Crossing Track

  • A term used in the application of separation, indicating tracks that converge or diverge at an angle of 45 degrees to 135 degrees inclusive.

Same Track

  • A term used in the application of separation, indicating identical tracks or tracks that converge or diverge at an angle of 1 degree to 44 degrees inclusive.

While each of these definitions talk about tracks that “converge” and “diverge”, the biggest threat to aircraft is when they are converging on a common point in space and separation between them is reducing. Once their tracks diverge the distance between them will quickly increase, therefore this technique is only applicable to those tracks that converge towards a common point.

As aircraft performance varies with altitude and phase of flight (climbing, descending or cruising) this technique only works successfully in the cruise phase of flight when the aircraft have levelled off, are established on course, and have reached cruising speed.

The following table shows a sequence of actual radar traffic, with 2 aircraft converging on the same point in space. Note that they are vertically separated (Flight Level 350 and Flight Level 360) and are flying at considerably different speeds (510 kts and 400 kts).

Despite the variance in speeds, the bearing between them remains steady at 0490, and they eventually merge at the crossing point.

69 Nautical Miles
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26 Nautical Miles
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0 Nautical Miles
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Radar images by Neil Kirkwood.
Images embedded from Flicker on 13 August 2012

While a radar screen provides a two dimension image or “plan” view, this technique can be used just as effectively in the real world by making simple visual observations relative to a single reference point, such as identifying a conflicting aircraft from the cockpit window and mentally marking its position on the windscreen.

If it remains in exactly the same spot without moving, then you are maintaining a constant bearing relative to it, and will eventually collide with that aircraft, regardless of your relative speeds.

If the relative bearing is changing, even slightly, you will miss each other.


Some Air Navigation Service (ANS) providers have developed sophisticated software programmes which include a crossing-point calculator built into the controller workstations.

These are called PIV’s or predict-intercept vector lines. A line can be "hooked" onto any two conflicting aircraft and a calculation can be automatically made based on their relative bearings, current tracks and relative speeds.

The principle behind PIV follows the same as that described above for a line of constant bearing, with two additional pieces of information being made available to the controller.

First, the calculation of how much lateral distance will be between the aircraft at the crossing point, followed by the actual time that the aircraft will meet. In the example below the aircraft will meet with no lateral separation and with a crossing time of 00:30 (0/00:30).

This is a very useful tool to assist controllers make quick decisions regarding possible conflicts, although, as with most computer programmes, there are times when it is not available or the system may fail. In this situation controllers would need to revert back to using a more basic tool, such as that described above.

Predict Intercept Vector (PIV)
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Radar image by Neil Kirkwood.
Images embedded from Flicker on 13 August 2012

Wind Effect

Wind direction also affects each aircraft individually, depending on their altitude and geographical location relative to the wind at that altitude. That is, if two aircraft are operating in different geographical locations (e.g. 200nm apart) is it reasonable to expect that they will be affected by different winds in each of their respective areas.

As the aircraft converge and enter different wind zones, their tracks may begin to change. If this occurs then the line of constant bearing between them will also change and they will no longer meet at the same point in space.

What started out as a potential collision hazard between aircraft may end up being a non-event once they are both operating in the same area and being affected by the same winds. Lateral separation may not be assured at that point and positive control action may still be needed to ensure that they pass each other with the minimum radar separation; this may involve a slight heading change for one or both aircraft.

1. Maurino, D. E., Reason, J., Johnston, N., & Lee, R. B. (1995). Beyond Aviation Human Factors. Hants, England: Ashgate.
2. Manual of Air Traffic Control RAC 4-1
3. ATC Manual of Operations (MANOPS) - Definitions

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