Let me start with the chapter structure so you know what's ahead — Page 173, Lesson 147

I want to walk you through a new chapter now — Chapter 9 of your General Navigation syllabus. This chapter is called "The Navigation Computer — Multi-drift Winds and Wind Components." It builds on the wind-face work you've already done, but now we're going to use multiple drift readings to find the wind, and then we'll move into calculating wind components for performance and limits. Let me start with the chapter structure so you know what's ahead. The chapter opens with "Further Uses of the Wind Face," then moves into "Multi-drift Winds" starting on page 169. After that comes "Two-drift Winds" on page 172, followed by questions and answers. Then we shift into "Wind Components" on page 175, which covers the "Requirement for Wind Component Calculations" on page 176, "Wind Directions – True or Magnetic?" also on page 176, then "Calculation of Wind Components" on page 177, "Maximum Wind Strength for a Given Crosswind Component" on page 178, "Headwind and Crosswind Limits" on page 180, "Tailwind Components" on page 181, and finally "Allowable Wind Angle for a Given Wind Speed" on page 183. The chapter closes with more questions and answers on pages 185 and 186. Now, let's define the key terms you'll see throughout. "Multi-drift winds" refers to a technique where you take drift readings on more than one heading — typically three or two — and use those measurements to determine the actual wind velocity aloft. A "drift" is the angular difference between the heading you're steering and the track you actually make over the ground, caused by the wind. "Two-drift winds" is a specific case where you only use two drift readings, which can still give you the wind but with less redundancy. "Wind components" are the parts of the total wind that act along and across your runway or flight path. The "headwind component" is the part of the wind blowing directly toward you, opposing your motion. The "crosswind component" is the part blowing perpendicular to your direction, pushing you sideways. The "tailwind component" is the part blowing from behind you, helping you along. "Maximum wind strength for a given crosswind component" means finding the strongest total wind you can accept without exceeding a crosswind limit. "Headwind and crosswind limits" are the maximum values your aircraft or operator allows for takeoff and landing. "Tailwind components" are treated separately because they increase your ground speed and landing distance. "Allowable wind angle for a given wind speed" is the range of wind directions you can accept for a specific wind strength without exceeding your crosswind limit. The chapter also clarifies an important point: "Wind Directions – True or Magnetic?" — you need to know which reference the wind direction is given in, because your heading and runway directions are usually magnetic, but wind forecasts may be given in true. You'll need to apply variation correctly. Now, the first major technique is multi-drift winds. The idea is that you fly three different headings, each for long enough to get a steady drift reading. You plot each drift line on the wind face of your navigation computer. If all three lines intersect at a single point, that point gives you the wind velocity — both direction and speed. If they don't meet at one point, they form a small triangle, and the wind is taken from the centre of that triangle. This gives you a more reliable wind than a single drift reading because it averages out errors. Let me show you the first step with the diagram. Here in Figure 9.1, you see the instruction: "Draw in the 10°P drift line. Now rotate the heading to the second reading of 330°. Ensuring that you still have 120 knots." The "10°P" means 10 degrees of port drift — drift to the left. So you set your first heading on the computer, draw the drift line at 10° port, then rotate to the next heading of 330°, keeping your true airspeed at 120 knots. Then the second diagram. Figure 9.2 says: "Draw in the second drift line. Now rotate the heading to the third reading of 090°. Ensuring that you still have 120 knots TAS." So you draw the drift line for the 330° heading, then rotate to 090° for your third heading, still at 120 knots TAS — true airspeed. Finally, the third diagram. Figure 9.3 says: "Plotting the final line. If the 3 lines do not coincide, a triangle will be formed by the intersection. This triangle is" — and the caption continues that the triangle's centre gives you the wind. So you plot the third drift line, and where all three lines cross, that's your wind. If they form a small triangle, you take the wind from the middle of that triangle. That's the core of multi-drift winds. Next, we'll move into two-drift winds, which is a quicker method using only two headings, and then into wind components for takeoff and landing limits.