What does “constant bearing, constant danger” mean?
And why does it sometimes lead to a very dangerous problem?
This expression is a neat abbreviation of a simple principle:
If two objects continue in a straight line and are on collision course, then they will have a constant angle between them. Take a look at this example:
Two cars are approaching a junction. They continue in a straight line. Both drivers ignore the road markings. Their speed will determine whether they will hit each other.
In the example below, the red car is going very slowly, the yellow car is moving quickly. The yellow car passes in front of the yellow car. Seen from the perspective of the driver of the red car, the yellow car’s bearing changes. First she sees it through her right window, then in front on the right, then dead ahead, then to her left. The purple arrow shows the angle that the driver would have to look to see the yellow car.
In the example below, the red car is moving very quickly and the yellow car is moving slowly. The red car passes just in front of the yellow car. (Obviously crazily dangerous driving, but that’s not the point here!). Seen from the perspective of the driver of the red car, the yellow car’s bearing changes again, only this time moving from in front and then sliding to the right – from the windscreen to the right window.
However dangerous these two scenarios were, a collision was narrowly averted. There wasn’t a “constant bearing”.
However, in the example below, there was a constant bearing… And the result…
The two cars smashed into each other.
This is not actually a very useful technique for drivers on the roads, but it can be interesting at times for passengers.
However this technique is used commonly at sea, even in the age of electronics that can do the calculations for us, partly because it works, but also because it makes long watches more interesting.
In aircraft it has practical applications, but also leads to one very interesting and dangerous problem.
A Dangerous Problem
“It’s the fly on the windscreen that will kill you!”
Imagine you’re in a cockpit and an aircraft is approaching from the right at exactly the same altitude, just like the car scenario above.
If you will pass safely in front or behind each other then you will see the bearing change – the other aircraft appears to move. Motion is a lot easier to spot than stillness.
BUT, if you are on a collision course, then there will be “constant bearing, constant danger” situation – and that means the other aircraft doesn’t appear to move relative to you. It can appear like a fly stuck on the windscreen, until, a few seconds later the fly suddenly grows in size and by then it might be too late!
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