The Navigation Computer - Multi-drift Winds and Wind Components — Page 189, Lesson 164

Let’s pick up right where we left off. We had just rotated the plastic disc around to the wind direction, and now you read off the value at the intersection. In our example, that value is 16 knots. That 16 knots is the crosswind component you’d get with that wind on that runway. Now, I want to give you a practical fallback. If you find the whole multi-drift procedure too complicated, you can always treat the problem as a headwind problem on the reciprocal runway. Let me show you what that means. Take the same wind we used before — 320/10G30 — but instead of landing on the runway we were considering, land on runway 27. Runway 27 is the reciprocal of runway 09, so it faces the opposite direction. When you work it as a headwind problem on runway 27, you get the same answer: 16 knots. So the crosswind component is identical, and you’ve simplified the calculation by flipping to the reciprocal. Now let’s move on to a new type of problem: the allowable wind angle for a given wind speed. This is another situation that the navigation computer solves very quickly. The question it answers is this: between what directions can a wind of a given speed blow without exceeding a stated crosswind component? In other words, you know the wind speed, you know your crosswind limit, and you want to find the range of wind directions that keep you within that limit. Here’s a concrete example. Suppose you need to find the directions between which a wind of 50 knots may blow without creating a crosswind component of more than 30 knots on a runway of direction 250° magnetic. So the runway is 250°(M), the wind speed is 50 knots, and the crosswind limit is 30 knots. Step one: with the runway direction set against the true index, mark a cross on the centre vertical line to represent the wind speed of 50 knots. So you place a mark on the vertical centre line at the 50-knot position. Step two: turn the plastic disc clockwise until the cross indicates a crosswind of 30 knots. Note the direction that now appears against the true index. In this example, that direction is 214°(M). So when you’ve rotated the disc so the cross sits at the 30-knot crosswind line, the true index reads 214° magnetic. Step three: now turn the plastic disc anticlockwise and repeat the procedure, reading off the direction that appears against the true index. This time you get 286°(M). So you have two directions: 214° and 286°. Step four: the same crosswind component will be present if the wind direction is between 034°(M) and 106°(M) — those are the reciprocals of the directions you found in steps two and three. But note this important qualifier: if the wind is blowing from between 034° and 106°, you’ll have the same crosswind component, but you’ll also have a tailwind component on this runway. So the range of directions that keep you within the crosswind limit without a tailwind is 214° to 286°; the reciprocal range 034° to 106° gives the same crosswind but adds a tailwind, which you’d need to consider separately. Let me make sure the logic is clear. The crosswind component depends on the angle between the wind direction and the runway direction. For a given wind speed, the crosswind component is maximum when the wind is perpendicular to the runway, and it decreases as the wind becomes more aligned with the runway. So there are two directions — one on each side of the runway — where the crosswind component equals your limit. Those are the 214° and 286° directions. The reciprocal directions, 034° and 106°, give the same crosswind magnitude because the angle between the wind and the runway is the same, but the wind is now blowing from the opposite side, which means it has a tailwind component instead of a headwind component. So the complete answer to the problem is: a wind of 50 knots will not exceed a 30-knot crosswind component if it blows from between 214°(M) and 286°(M), or from between 034°(M) and 106°(M) — but in that second range you’ll have a tailwind component on the runway. Now, the excerpt ends with a set of practice questions. These are the book’s practice questions — let’s try them one at a time.