The Navigation Computer - Multi-drift Winds and Wind Components — Page 173, Lesson 155

Let’s start with the idea of wind components, because this is the heart of what we’re doing. When we fly, the wind almost never blows exactly along our track or exactly along the runway centre line. There is almost always an angle between the direction we want to go — the track or the runway direction — and the direction the wind is coming from. That angle is what we call the ‘angle-off’. Now, if the wind direction and the runway direction were exactly the same, then the full force of the wind would act straight along the runway. There would be no sideways effect at all. But that’s rare. In the example we have here, the wind is blowing at an angle-off of 30° from the right. So only part of the wind’s force is directed along the runway, and the other part is directed across the runway, from right to left. We take the wind velocity — in this case 300°(M)/30 knots — and we resolve it into two components. One acts along the runway, and we call that the headwind component. The other acts across the runway, and we call that the crosswind component. Now, we could solve this mathematically using trigonometry. The headwind component would be 30 knots × cos 30°, which is 26 knots. The crosswind component would be 30 knots × sin 30°, which is 15 knots. That’s the precise way to do it. But for the ATPL General Navigation exam, we don’t want to rely on mental arithmetic or rule-of-thumb methods — those aren’t precise enough. The exam questions have to be resolved using the Navigation Computer to get the required level of accuracy. So we use the computer to find these components quickly and accurately. Let me just make sure you’ve got the key terms: headwind component is the part of the wind acting along the runway in the direction of travel, and crosswind component is the part acting across the runway. The angle-off is the angle between the wind direction and the runway or track direction. And the wind velocity is given as a direction in degrees magnetic, like 300°(M), and a speed in knots, like 30 knots. So the whole point here is: we take a single wind velocity, and we split it into two perpendicular components — one along the runway, one across it — and we use the Navigation Computer to do that resolution accurately. That’s the foundation for everything that follows in this chapter.