The Navigation Computer - Triangle of Velocities — Page 130, Lesson 120

I want to walk you through the core idea behind the navigation computer and the triangle of velocities, starting with the effect of wind on an aircraft. Aircraft do not always travel in the direction in which they are pointed. If there is any cross-wind, the track—the actual path over the ground—will be different from the heading, which is the direction the nose is pointing. To help you understand this, I'm going to start with a simple analogy: a boat crossing a river. Imagine a river that is 10 nautical miles wide, with a current of 5 knots. If you stand at Point A and throw a stick into the river, after one hour that stick will have been carried 5 nautical miles downstream. That's the effect of the moving water. Now imagine that the river has been dammed and has become a lake with no current—still water. You are at Point A with a boat that has an outboard motor and a speed through the water of 10 knots. If you set off from Point A to cross directly to the opposite bank, one hour later you will be at Point B, directly across from where you started. Now let's combine these two ideas. The dam is removed, the lake becomes a river again, and we have our 5-knot current back. We set off in our boat from Point A heading towards Point B. However, the effect of the current is to take us downstream, just like the stick. This time we finish up at Point C—10 miles across the river and 5 miles downstream. Notice that we pointed the boat—that's our heading—towards B, but we have actually travelled—that's our track—to C. If we want to finish up at Point B, we need to aim off to the left towards Point D. This is a continuous process. If we head towards Point B, after 12 minutes—which is one-fifth of an hour—we will be 2 miles across the river and one mile displaced to the right. After 24 minutes, we will be 4 miles across the river and 2 miles downstream, and so on. The boat tracks along the vector AC. Exactly the same process takes place with aircraft travelling through the air. On a day when the air is completely calm—analogous to the lake—the aircraft tracks where it is pointed. We get completely calm days about six times a year. However, we still have to be able to navigate on the other 359 days, so we need to be able to take account of wind, or moving air—analogous to the river. The term 'wind' tends to suggest something transient, and many newcomers to aviation find it hard to imagine that something so unsubstantial could have such an effect on such a dense, massive aircraft as a Boeing 747, or a dense, fast-moving aircraft like a Tornado. That's why this chapter uses this rather elaborate river analogy. It's easy to imagine a boat or a twig being carried downstream by a 10-mile expanse of river moving at 5 knots. But wind is not simply little puffy, transient eddies of air. Wind is the continuous movement of an air mass. An air mass is a large, homogeneous body of air travelling over the ground. Normally, we do not regard it as an air mass unless it is at least, say, 60 nautical miles by 60 nautical miles in size. This whole air mass travels over the ground, just like the river. Your aircraft is supported by that air mass, and as the air mass moves, it carries the aircraft with it relative to the ground. That's the fundamental idea we'll build on as we move into the triangle of velocities.