MAGNETIC — Page 182, Lesson 159

Let’s start with the wind components on the Navigation Computer, because that’s the core skill here. Take this example: runway 27, and the ATIS gives you a wind of 310/30. That means the wind is coming from 310 degrees true, at 30 knots. First, on the graph-paper part of the Navigation Computer — the scale at the bottom of the slide — you plot the wind velocity in the normal way. So you mark the point that represents 310/30 on that grid. Now rotate the wind disc so that the runway QDM of 270 is at the 12 o’clock index. QDM is the magnetic heading you’d fly to the runway — here, 270. So you’re aligning the disc so that the runway direction is straight up. Once that’s set, read vertically down the y-axis. That gives you the headwind component. In this example, it’s 23 knots headwind. Then read across from the centre line — the x-axis. That gives you the crosswind component. Here it’s 19 knots crosswind, from right to left. So the wind is 23 knots against you, and 19 knots pushing you sideways. That’s the whole point of the wind component calculation — you split the wind into the part that slows you down and the part that pushes you off the runway. Now, the second part: maximum wind strength for a given crosswind component. Sometimes the wind is blowing from a known direction, but it’s gusting. A gust is defined as an increase in wind speed lasting less than one minute. And gusts are only forecast or reported if they’re 10 knots or more above the mean wind speed. So if the mean is 20 and it gusts to 28, that’s reported. If it only gusts to 25, it isn’t. You might want to land between gusts. In that case, you continue the approach down to the point where you must commit — either land or go around. For a visual approach, that committal point is far lower than an instrument decision height. It may be 50 feet or lower, depending on how your aircraft handles. At that last moment, you request a spot wind readout from the tower. If the wind strength from the actual direction gives a crosswind component at or below your crosswind limit, you can land. Otherwise, you must go around. But here’s the key point: the moment of visual committal is no time to start calculating crosswind limits. You should have thought about it beforehand. Before you start the approach, you calculate what wind strength from the forecast direction corresponds to your crosswind limit on the runway in use. You do those calculations at the top of descent, so you can make an instant decision at the visual committal point. Now the case at the end: TAF wind 180/10G50, variation 15W, runway — and that’s where the excerpt cuts off. But the idea is: the forecast wind is from 180, 10 knots gusting to 50, with 15 degrees west variation. You’d apply that variation to convert from true to magnetic, then work out what wind strength from that direction gives you your crosswind limit. That’s the calculation you do at the top of descent. So the whole lesson is: plot the wind, align the runway, read off headwind and crosswind — and plan your crosswind limit before you get to the committal point, not at it.