The Navigation Computer - Calculation of Heading and Wind Finding — Page 154, Lesson 143

I want to walk you through how we actually find the wind affecting the aircraft in flight using the Navigation Computer. This is a practical skill — the met forecast gives you a general picture over a large area and several hours, but if you want to know exactly what wind is pushing you around right now, at your level, in your local area, the Navigation Computer can give you that answer. Let me set up the situation. You are heading 060° true — that's your heading, the direction the nose is pointing. Your true airspeed, TAS, is 140 knots. You're flying along, trying to follow some planned track on your chart. Once you're airborne, you get a fix at point B, say 20 minutes after passing over a known landmark A. You measure the track angle from A to B on your chart — that's the actual path you've made good over the ground — and you find it is 065° true. You also measure the distance from A to B on your chart and find it is 40 nautical miles. If you've flown 40 nautical miles in 20 minutes, your ground speed must be 120 knots. So now you know four things: heading is 060° true, TAS is 140 knots, track is 065° true, and ground speed is 120 knots. That gives you the triangle of velocities, and Figure 8.13 shows the complete triangle. Now, how do we find the wind velocity from this? On your Navigation Computer, you put the heading — 060° — against the heading index. Then you put the TAS of 140 knots against the blue circle. The blue circle is the TAS reference point on the computer's slide. The track is 065°, which means you have 5° of starboard drift — that's drift to the right, because your track is 5° greater than your heading. On the computer, you make a mark where the 5° starboard drift line crosses the 120 knots ground speed line. That mark is called the wind point. Figure 8.14 shows you plotting that wind point. Now your triangle of velocities is set up on the computer. To measure the wind, you look at the wind face in its present position. You can see roughly that the wind direction is from about 030° and the wind strength is about 20 knots. But we need to measure it more accurately than that. The only reason we rotate the wind vector is to measure it more accurately. So you rotate the wind cross — that's the cross on the wind arm — to the 6 o'clock position. Once it's there, you can read the direction accurately from the 12 o'clock index. You can also read the length of the wind vector accurately. In this example, the wind velocity — which we abbreviate as W/V — is 034° true at 23 knots. That is the wind that has affected you over the last 20 minutes. It is far more recent and far more local than any area met forecast could give you.