Let me set the scene — Page 173, Lesson 147

We’re starting a new topic now: the navigation computer, and specifically multi-drift winds and wind components. This is the part of your general navigation syllabus where we take the wind face of the flight computer and use it to solve real problems — not just one drift reading, but several, and then we move into breaking the wind down into headwind, crosswind, and tailwind components. Let me set the scene. The chapter is called "The Navigation Computer – Multi-drift Winds and Wind Components." The wind face is the side of your mechanical flight computer with the circular slide rule and the drift lines. Up to now you’ve probably used it for a single wind velocity and a single track. Multi-drift winds are the technique for when you don’t know the wind at all — you fly several headings, note the drift on each, and from those readings you reconstruct the wind. That’s the "multi-drift" idea: multiple headings, multiple drift readings, one wind solution. The chapter is laid out in a clear order. First we have "Further Uses of the Wind Face," then "Multi-drift Winds," then "Two-drift Winds." After that come questions and answers. Then we shift to "Wind Components" — the requirement for wind component calculations, whether wind directions are true or magnetic, the calculation itself, maximum wind strength for a given crosswind component, headwind and crosswind limits, tailwind components, and allowable wind angle for a given wind speed. Again questions and answers close that section. Let me unpack the multi-drift method because that’s the heart of the first part. The idea is you take three headings — that’s why it’s sometimes called a three-drift wind. You fly each heading at the same true airspeed, and on each you measure the drift, which is the angle between your heading and your track caused by the wind. So you have three headings and three drift angles. On the wind face, you draw a drift line for each one. The first figure shows you drawing in the first drift line — you rotate the heading to the first reading, keeping 120 knots TAS on the computer, and you draw the drift line. Then you rotate to the second heading, 330°, still at 120 knots, and draw the second drift line. Then the third heading, 090°, again at 120 knots, and you draw the third drift line. Now here’s the key point. If the wind were perfectly consistent and your readings perfect, all three drift lines would intersect at a single point — that point is the wind velocity. But in practice they won’t coincide exactly. Instead, the three lines form a small triangle where they intersect. That triangle is your error — the spread of your readings. The wind velocity is taken as the centre of that triangle. So the multi-drift method gives you both the wind and a visual measure of how reliable your readings are. If the triangle is small, you trust the result; if it’s large, your readings were inconsistent. The two-drift wind is the simpler version — you use only two headings and two drift readings. With two lines you get an intersection point directly, no triangle, but you lose that error check. So two-drift is quicker, three-drift is more robust. Now let me move to wind components, because that’s the second half and it’s a different kind of problem. The requirement for wind component calculations comes from performance and handling. When you’re taking off or landing, what matters isn’t the full wind speed — it’s how much of that wind is blowing along the runway and how much is blowing across it. The component along the runway is the headwind or tailwind; the component across is the crosswind. Aircraft have limits on both, so you need to calculate them. One thing I want to flag clearly: the chapter asks whether wind directions are true or magnetic. In navigation, wind velocity is always given as a direction from which the wind blows, and that direction is referenced to true north unless stated otherwise. But for runway calculations, the runway heading is magnetic, so you must be consistent — you convert the wind direction to the same reference as the runway before you resolve the components. Mixing true and magnetic is a classic error. The calculation itself is done on the wind face of the computer. You set up the wind velocity and the runway direction, and the computer resolves the wind into a headwind or tailwind component and a crosswind component. The crosswind component is the part perpendicular to the runway; the headwind or tailwind is the part parallel. The chapter then gives you the reverse problems: given a maximum crosswind component limit, what’s the maximum wind strength you can accept? That’s "Maximum Wind Strength for a Given Crosswind Component." And given a wind speed, what’s the allowable wind angle — the angle between the wind direction and the runway — before you exceed your crosswind limit? That’s "Allowable Wind Angle for a Given Wind Speed." And there’s a separate treatment of tailwind components, because tailwinds are treated differently from headwinds — many aircraft have a lower tailwind limit than headwind limit, so you calculate the tailwind component separately and check it against its own limit. So the whole chapter is really two skills: reconstructing an unknown wind from drift readings, and then resolving a known wind into the components that matter for take-off and landing performance. Both are done on the wind face of the navigation computer, and both are examinable in the ATPL general navigation paper. Let’s look at the figures I have for this. The first shows you drawing in the first drift line at 120 knots TAS. The second shows the second drift line at heading 330°, still 120 knots. The third shows the final line and the triangle formed by the three intersections — that’s the error triangle I described. So that’s the shape of the chapter. We’ve got multi-drift winds — three headings, three drift lines, error triangle, wind at the centre — and two-drift as the quicker variant. Then wind components: headwind, crosswind, tailwind, resolved on the wind face, with the true-versus-magnetic caution, and the reverse problems for limits. That’s the full scope of what we’re covering.