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

Right, let's get into wind finding. This is the practical payoff of the triangle of velocities — using the Navigation Computer to discover the actual wind affecting your aircraft right now. The starting point is a problem: the meteorological forecast gives you a wind, but it covers a large area and its period of validity extends over several hours. So it can only be a general guide for your area. If you want to find out exactly what wind is affecting you in your local area at the present time, the Navigation Computer can be used to find the wind. Let me set up the situation. You are heading 060°(T) at a TAS of 140 knots, in order to fly some planned track. Once airborne, you get a fix at B, say 20 minutes after passing over a known landmark A. You measure the track angle from A to B on your chart and find that it is 065°(T). You also measure the distance A to B on your chart and find that it is 40 nautical miles. If you have flown 40 NM in 20 minutes, your ground speed must be 120 knots. So now you know four things: Heading = 060°(T), TAS = 140 knots, Track = 065°(T), Ground speed = 120 knots. That's your triangle of velocities — you have the heading and TAS vector, and the track and ground speed vector, and the wind is the third side that connects them. Now, how do we find the wind on the computer? On your Navigation Computer, put the heading 060° against the heading index and the TAS of 140 knots against the blue circle. The track is 065°, which means you have 5° starboard drift. Make a mark where the 5° S line crosses 120 knots ground speed. That mark is the wind point. Let me be clear about that drift: track 065° is 5° to the right of heading 060°, so that's 5° starboard drift — the wind is pushing you to the right. And the ground speed of 120 knots is less than your TAS of 140 knots, so you have a headwind component. The wind point is where those two lines cross. Now we need to measure the wind. You can see from the wind face in its present position that the wind direction is from about 030° and that the wind strength is about 20 knots. But we need to measure it a bit more accurately than that. This is the only reason we rotate the wind vector — to measure it more accurately. So rotate the wind cross to the 6 o'clock position. You can now read the direction accurately from the 12 o'clock index. You can also read the length of the wind vector accurately. So the W/V is, in this example, 034°/23 knots. That's the wind velocity — 034° true, 23 knots. And that's the wind that has affected you over the last 20 minutes. It will be far more recent and far more local than any area met forecast. Let me just recap the whole process, because it's a sequence you'll use every flight. You fly a heading at a known TAS, you get a fix after a known time, you measure the track and distance on the chart, you compute ground speed from distance over time, then you set heading against the index and TAS against the blue circle, you mark the wind point where the drift line crosses the ground speed, and then you rotate the wind cross to the 6 o'clock position to read the direction from the 12 o'clock index and the speed from the length of the vector. The key insight here is that this wind is local and current — it's what actually pushed you over the last 20 minutes, not what a forecast predicted for a large area hours ago. That's why wind finding is so valuable in flight.