Crosswind limit — Page 182, Lesson 161

I want to walk you through a very practical calculation that every pilot has to do before landing — working out whether the wind is within the aircraft's crosswind limit. This is a different kind of problem from the ones we've looked at so far, because here you don't start with a single known wind value. Instead, you know the runway and the maximum crosswind the aircraft is certified for, and you need to find the maximum wind strength you can accept from a given direction. Let's take the specific example from the book. The crosswind limit is 25 knots. That means the aircraft is approved to land only when the crosswind component is 25 knots or less. The runway in use is R/W 23, so the runway QDM — the magnetic direction you fly to align with the runway centreline — is 230°. The technique is different from a standard wind-component calculation because you don't have a single wind speed and direction to plug in. Instead, you put the runway and the crosswind limit onto your Navigation Computer, and then you rotate the wind disc to see what wind speed would produce that limit. Here's the step-by-step procedure. First, put the R/W QDM of 230° up at the top of the computer — that's the 12 o'clock position. Then, draw in 25-knot crosswind limit lines — these are parallel lines either side of the centre line of the computer. So you have two lines, one to the left and one to the right of the centre line, each at a distance that represents 25 knots of crosswind. Now, rotate the wind disc to the direction that the wind is coming from. In this example, the wind comes from a TAF — a Terminal Aerodrome Forecast — which gives wind direction in true degrees. So you must apply variation to convert it to magnetic. The true wind direction is given, and after applying variation, the magnetic direction becomes 195°(M). So you rotate the wind disc so that 195° is at the top. Now, look at where the applicable crosswind limit line crosses the centre line of the computer. In this case, that crossing point is at 41 knots. What does that tell you? It tells you that when the wind from 195°(M) reaches 41 knots, it will produce exactly 25 knots of crosswind on R/W 23. So, provided the actual wind from 195°(M) is less than 41 knots, you are within the crosswind limit and may land. If it's 41 knots or more, you exceed the limit and must not land. Now, the book also introduces a more complex scenario: Headwind and Crosswind Limits combined. Sometimes you are limited by both. For example, you might have to land a heavy aircraft on a single-runway airport, and that runway might be short. In that case, you are on your Landing Distance Available (LDA) performance limits — that's the length of runway declared available for landing. You may need, say, at least 10 knots of headwind down the runway to achieve the required landing distance. At the same time, there is a crosswind limit to respect. The example given is: R/W 35, and an ATC wind velocity — that's the wind direction and speed as reported by Air Traffic Control. The book doesn't complete the example in this excerpt, but the principle is that you would plot both the headwind requirement and the crosswind limit on your computer, and find the envelope of acceptable winds. Let me show you the diagrams that illustrate this. shows the headwind and crosswind components being read off, and the concept of maximum wind strength for a given crosswind component. and show the process of rotating the wind disc to the runway QDM. And and show the basic wind direction and runway relationship and the wind components. So, to summarise the key points: the crosswind limit is a fixed value — here 25 knots. You set the runway QDM at the top, draw the limit lines, rotate to the wind direction (applying variation if needed), and read where the limit line crosses the centre line. That gives you the maximum wind speed from that direction that keeps you within the crosswind limit. And when you also have a headwind requirement, you must satisfy both constraints simultaneously.