MAGNETIC — Page 182, Lesson 159

I want to walk you through how we use the Navigation Computer to calculate wind components for takeoff and landing. This is a practical skill you'll use every flight. Let's start with the source of wind information. In aviation, we get wind from different sources, and each one tells us the direction in a different reference system. When you hear wind from ATC or ATIS, it's given in MAGNETIC direction — that's relative to magnetic north. When you get wind from VOLMET, TAF, or METAR, it's given in TRUE direction — relative to true north. You need to know which one you're working with because the runway direction you'll use is always magnetic. Now, let's work through a real example. Suppose we have Runway 27 — that means the runway magnetic direction is 270 degrees. The ATIS gives us the wind as 310/30 — that's wind from 310 degrees magnetic at 30 knots. First, we use the graph paper part of the Navigation Computer — that's the scale at the bottom of the slide. We plot the wind velocity in the normal way. So for a wind of 310 degrees at 30 knots, you'd mark that point on the grid. Next, we rotate the wind disc to put the runway QDM at the 12 o'clock index. QDM is the magnetic bearing to the runway — essentially the runway direction in magnetic terms. So for Runway 27, that's 270 degrees. You rotate the disc until 270 is at the top, at the 12 o'clock position. Now we read the components. Reading vertically down the y-axis — that's the vertical axis — gives you the headwind component. In this example, it's 23 knots headwind. Reading across from the centre line — that's the x-axis, the horizontal axis — gives you the crosswind component. Here it's 19 knots crosswind, and the direction is from right to left. So for this approach, you'd have 23 knots of headwind helping you slow down, and 19 knots of crosswind pushing you from the right. Now, there's an important operational consideration: maximum wind strength for a given crosswind component. Sometimes the wind is blowing from a known direction but it's gusting. A gust is defined as an increase in wind speed lasting less than one minute. Gusts will only be forecast or reported if they are 10 knots or more above the mean wind speed. Here's the scenario: you might want to try to land between gusts. In that case, you continue the approach down to the point where you must commit to a landing or go around. For a visual approach, this committal point will be far lower than an instrument decision height — it may be 50 feet or lower, depending on your aircraft's handling. At that last moment, you request a spot readout of wind from the tower. If the wind strength from the actual direction gives a crosswind component at or below your aircraft's crosswind limit, you can land. Otherwise, you must go around. But here's the key point: the moment of visual committal decision is no time to start calculating crosswind limits. You should have thought about it beforehand. Before you start the approach, you calculate what wind strength from the forecast direction corresponds to your crosswind limit on the runway in use. These calculations are made at the top of descent to allow an instant decision at the visual committal point. Let's take another case. Suppose the TAF gives wind as 180/10G50 — that's wind from 180 degrees true at 10 knots gusting to 50 knots. The variation is 15 degrees West. Variation is the difference between true north and magnetic north. Since it's West variation, you add it to convert from true to magnetic. So the magnetic wind direction would be 180 plus 15, giving 195 degrees magnetic. And the runway in use — well, that would be given in the full scenario, but you'd use the same plotting method to check your crosswind component against your limit before you start down.