Crosswind limit — Page 182, Lesson 161

Let’s pick this up right where the wind-component work leaves off, because this is a genuinely different kind of problem. Up to now, you had a single wind value and you resolved it into headwind and crosswind. Here, the situation is reversed: you have a limit — a maximum crosswind you’re allowed to land in — and you need to find out how strong the wind can get before you hit that limit. Let me set the scene. Your aircraft has a crosswind limit of 25 knots. That’s a hard operational limit: you may not land if the crosswind component exceeds 25 knots. Now, the wind isn’t a single fixed value — it’s forecast, it’s variable, it’s going to change. So you can’t just plot one wind on your Navigation Computer and read off the answer. You need a different technique. Here’s the key idea: instead of putting the wind on the computer, you put the runway and the crosswind limit on the computer, and then you rotate the wind disc to see where the wind direction intersects your limit line. Let me walk you through the example. Say the runway is R/W 23. First, you put the runway QDM — that’s the magnetic direction you’re heading to land, 230° — up at the 12 o’clock index at the top of the computer. Then you draw in the 25-knot crosswind limit as parallel lines, either side of the centre line. Those two lines represent the maximum crosswind you’re allowed — one for a crosswind from the left, one from the right. Now you rotate the wind disc to the direction the wind is coming from. And here’s a trap I want you to watch carefully: don’t forget to apply variation if necessary. In this example, the wind comes from a TAF — a Terminal Aerodrome Forecast — and TAF winds are given in true direction. So if the true wind direction is, say, 195°T, you must convert it to magnetic. The magnetic direction here is 195°(M) — in this particular case the variation happens to be zero, so the numbers match, but don’t assume that’s always true. Always check whether your wind is true or magnetic before you rotate the disc. So you rotate the wind disc so that 195°(M) is at the top. Now, note where the applicable crosswind limit line crosses the centre line. In this example, it crosses at 41 knots. What does that 41 knots mean? It means this: when the wind from 195°(M) reaches 41 knots, it will produce exactly 25 knots of crosswind on R/W 23. So, provided the wind from 195°(M) is less than 41 knots, you may land. The moment it reaches or exceeds 41 knots, your crosswind component hits or exceeds your 25-knot limit, and you cannot land. Now, let’s take this one step further, because real operations often have two limits at once — headwind and crosswind. Think about landing a heavy aircraft on a single-runway airport where the runway is short. You might be on your Landing Distance Available — your LDA — performance limits. That means you need a certain amount of headwind to help you stop. Say you need at least 10 knots down the runway to land — that’s your minimum headwind. At the same time, there’s a crosswind blowing, and you have your crosswind limit to respect. So now you have two constraints: you need at least 10 knots of headwind, and you can’t exceed your crosswind limit. The example sets up R/W 35 with an ATC W/V — that’s the wind direction and speed given by Air Traffic Control. The technique is the same idea, but now you’re plotting both limits on the computer — the headwind limit line and the crosswind limit lines — and you rotate the wind disc to the wind direction. Where the wind direction line crosses your limits tells you the maximum wind speed you can accept while still satisfying both conditions. Let me make sure the core concept is crystal clear, because this is the heart of it. The crosswind limit line is drawn parallel to the runway centre line, at a distance that represents 25 knots of crosswind. When you rotate the wind disc to the wind direction, the point where that wind direction crosses the limit line gives you the wind speed at which the crosswind component equals your limit. Below that speed, you’re legal; at or above it, you’re not. And when you add a headwind limit, you’re finding the wind speed that satisfies both the minimum headwind and the maximum crosswind simultaneously. One more thing to keep straight: the wind direction is always the direction the wind is coming from, not blowing towards. That’s why you rotate the wind disc to put the wind’s from direction at the top. And remember the variation step — TAF winds are true, runway QDM is magnetic, so you must convert before you rotate. So, to summarise the whole procedure: put the runway QDM at the top, draw your crosswind limit lines parallel to the centre line, rotate the wind disc to the wind’s magnetic direction, and read where the limit line crosses the centre line — that’s your maximum allowable wind speed. Then, if you also have a headwind requirement, plot that limit too and find the wind speed that satisfies both. That’s how you manage a crosswind limit when the wind isn’t a single fixed value.