The Navigation Computer - Triangle of Velocities — Page 136, Lesson 125

Let’s pick this up right where the graph paper left off. We just drew the triangle of velocities by hand and found that a heading of 000°(T) at a TAS of 100 knots, in a wind of 240/30, gives you a track of 012°(T) and a ground speed of 118 knots. That works, but it’s long-winded. So now I want to show you the same problem solved on the Navigation Computer, using the Wind Face. Same situation: you’re in a Warrior PA 28, TAS is 100 knots, heading is 000°(T), and the forecast wind velocity, the W/V, is 240/30. We want to find track and ground speed. First step: put the W/V on the computer. Rotate the wind face to bring 240 up to the index — that’s the 12 o’clock position on the disc. So the wind direction is set against the index. Next, put a convenient datum to measure from under the blue circle in the centre of the wind face. I’d suggest 100 knots — that’s a nice round number. Alternatively, you can use the graph-paper square grid at the bottom of the slide as your datum instead. Now plot downwards, in the 6 o’clock direction. Mark off 30 knots on the wind face. That’s the wind speed. And here’s a practical tip: use a soft pencil. You can use a felt-tip pen, provided it’s water-based, non-permanent ink — those have grey barrels. Do NOT use spirit-based, permanent pens — those have black barrels. They’ll ruin the slide. That plotted point is your Wind Vector. Now for the Air Vector. Rotate the wind disc to put the heading, 000°, against the true heading index — again the 12 o’clock position. Then move the slide to put your TAS, 100 knots, under the blue circle in the centre. At this point, the computer should look like Figure 7.9 — it shows the drift and ground speed. And Figure 7.8 shows the W/V vector being plotted. So you’ve now got both vectors on the face: the wind vector you plotted, and the air vector set by the slide and disc. The line from the centre to your plotted wind point gives you the ground vector — and from that you read off your track and ground speed, just like we did on the graph paper, but far quicker.