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

Let’s start with the core idea: wind almost never blows exactly along your track or along the runway centre line. There is almost always an angle between the direction you want to go and the direction the wind is coming from. That angle is what we call the ‘angle-off’. So when the wind is at an angle-off, we split the wind into two parts. One part acts along the runway — that’s the headwind component. The other part acts across the runway — that’s the crosswind component. In the example we have, the wind is blowing at 300°(M) at 30 knots, and the angle-off is 30° from the right. So part of that 30-knot wind pushes along the runway, and part pushes across it from right to left. Now, the precise way to resolve this is with trigonometry. The headwind component equals the wind speed times the cosine of the angle-off. So here it’s 30 knots × cos 30°, which is 26 knots. The crosswind component equals the wind speed times the sine of the angle-off. So 30 knots × sin 30°, which is 15 knots. But for the ATPL General Navigation exam, we don’t want to do that mentally every time — the mental arithmetic and rule-of-thumb methods aren’t precise enough. So we use the Navigation Computer to get the required level of accuracy. The computer does the same sine and cosine resolution for us, just faster and more reliably. Why does this matter? Because wind components are important both en route — especially in Flight Planning — and in take-off and landing. If the wind direction and runway direction were exactly the same, the full force of the wind would act along the runway and there would be no crosswind effect at all. But that’s rare. In practice, you always have some crosswind component, and you need to know it accurately for performance and limits. So remember the two components: headwind component along the runway, crosswind component across it. The angle-off is the key input, and the Navigation Computer resolves it for you.