We’ve been talking about the air mass that your aircraft flies in — Page 130, Lesson 122

Let’s start with the big picture, because this is the heart of all navigation. We’ve been talking about the air mass that your aircraft flies in. I want you to imagine that air mass as a river — a broad river, at least 60 nautical miles by 60 nautical miles in size. That whole body of air moves over the ground, just like a river moves. Your aircraft is supported by this air, exactly like a boat or a twig is supported by the river. So if the air moves, you move with it, just as the twig travels with the river. That movement is the wind, and it’s the reason we can’t just point the nose where we want to go and expect to arrive there. So the problem is: we have to take account of this moving air when we navigate. And the way we solve it is by constructing what we call the Triangle of Velocities. It’s a vector triangle — a drawing of three arrows that represent the three speeds and directions involved. The components are always the same, every single time. Let me name them for you. First, the Air Vector. The Air Vector consists of Heading and True Airspeed, which we abbreviate TAS. Now, TAS is the true speed of the aircraft through the air — not the indicated speed, the true speed. And here’s the key idea: it’s the speed the aircraft would travel over the ground if there were no wind at all. The Air Vector is always drawn with one direction arrow on it. Second, the Wind Vector. The Wind Vector consists of Wind Direction and Wind Speed. And here’s a critical convention you must never forget: wind direction is always given in terms of the direction the wind has come from, not where it is blowing to. So if I give you a wind vector, and it points like the one in Figure 7.3, that indicates a wind direction of 330°(T) — the T meaning true — not 150°(T). The wind is coming from 330°, blowing toward 150°. This actually matches everyday speech. Think about it: a South wind is warm because it has blown up from Morocco. A West wind is warm and wet because it has blown across the Atlantic and over Ireland. So when we say a wind is from the south, we mean it came from the south. The Wind Vector is always drawn with three direction arrows on it. Third, the Ground Vector. The Ground Vector is drawn by joining up the Air Vector and the Wind Vector. The resultant — the result of adding those two together — is your Track and your Ground Speed. The Ground Vector is always drawn with two direction arrows. Figure 7.4 shows the complete Triangle of Velocities. Now, in practice, pilots do not normally draw this out to scale on graph paper. We solve it using an analogue navigation computer. But here’s the thing I want you to understand: the Navigation Computer is merely a device for quickly producing a scale drawing. It is actually drawing the Triangle of Velocities for you, every time you use it. So if you understand this triangle, you understand what the computer is doing. Let me walk you through a practical example so you can see how the pieces fit together. Imagine you are in a Warrior PA 28. Your TAS is 100 knots. Your heading is 000°(T) — that’s due north, true. The forecast wind, the W/V, is 240/30. That means the wind is from 240° true, at a speed of 30 knots. The question is: what will be your track and your ground speed? Let’s draw it out. On a piece of paper — graph paper is ideal — start by drawing the Air Vector. It has a direction of 000°(T), and a vector length equivalent to 100 knots. So if you use a scale of 1 millimetre per knot, you draw it 100 millimetres long, pointing straight up the page, due north. That’s Figure 7.5. Now draw in the Wind Vector. It was 240/30. Remember, it is from 240°. So it will actually point in the direction 060°(T) — because the wind blows from 240° toward 060°, the opposite direction. Whatever units you used for the 100 knots of TAS — say millimetres — use the same units for the wind. So draw the Wind Vector 30 millimetres long, pointing toward 060°(T). That gives you Figure 7.6, with both the Air Vector and the Wind Vector drawn. Now, the final step: join the start of the Air Vector to the end of the Wind Vector. That line you draw is the resultant — the Ground Vector. Its direction is your track, and its length, measured in the same scale, is your ground speed. That’s the answer to the question, and that’s the whole Triangle of Velocities in action.