Let's take a specific example — Page 182, Lesson 162

I want to walk you through a technique for using the navigation computer to check landing limits when you have both a minimum headwind requirement and a crosswind limit. This builds directly on the crosswind limit lines we drew earlier, but now we add a headwind limit line as well. Let's take a specific example. The runway is R/W 35, and your aircraft has two limits: a minimum headwind of 10 knots — you need at least that much headwind component — and a crosswind limit of 20 knots, meaning you cannot land if the crosswind component exceeds that value. First, you draw in the 20 knot crosswind parallel markers, just as before. But this time, in addition, you draw in a 10 knot headwind limit line. That gives you two sets of lines on the wind slide: the parallel crosswind limit lines and a horizontal headwind limit line. shows the relationship between wind direction and the runway, and shows the wind components. Now, you rotate the wind disc so that the direction the wind will be coming from is at the top — in this case, 030° magnetic. and show this process. Once rotated, look at where your two limit lines intersect the centre line of the computer. In this example, the headwind limit line intersects at 11 knots, and the crosswind limit line intersects at 31 knots. shows how you read off these values. Here's what those intersection points tell you. If the wind from 030° magnetic is less than 11 knots, you have less than the required 10 knots of headwind down the runway. You cannot land. If the wind is greater than 31 knots from that direction, you have more than 20 knots of crosswind component. You cannot land. But if the wind is between 11 and 31 knots from 030° magnetic, you have 10 knots or more of headwind component AND you have 25 knots or less of crosswind component. You can land. Notice the crosswind limit line gave us 20 knots, but the intersection at 31 knots means that up to that wind speed, the crosswind component stays within the 20 knot limit. Now, let's talk about tailwind components. In general, you should avoid landing with a tailwind. If you're given a runway with a tailwind component, ask Air Traffic Control if you can use the other end. It's usually poor airmanship to accept a tailwind, and you won't have a leg to stand on afterwards if you run off the end — even if your brakes fail. However, you may occasionally have questions that involve tailwind components, so you need to know how to deal with them. The first thing to notice is that you cannot directly plot the horizontal line representing a tailwind component in the normal way, because it's above the horizontal scale line. You're in the curved part of the wind slide. Let's take a worked example. Your runway is 09, variation is 10 degrees east, and the ATIS wind is 320/10G30 — that's direction 320, speed 10 knots gusting to 30 knots. Your maximum tailwind component for this particular landing is 10 knots. The question is: what is the maximum wind speed from the given direction that you can accept? First, note that ATIS gives wind in magnetic direction, so the value of variation — 10 degrees east — is unnecessary information. Ignore it. Watch out for this trap in exam questions. Bring the runway direction, 09, round to the top of the computer. Now move the speed slide to bring 10 knots under the centre dot. Draw in the tailwind component limit line at that position. shows this step. Then — and this is critical — do not forget to move the speed slide back to put zero under the centre dot after you've drawn the line. shows the final position of the tailwind limit line. Once you've done that, you can rotate to the wind direction and read off the maximum acceptable wind speed from the intersection of the tailwind limit line with the centre line. That gives you your answer.