The Navigation Computer - Multi-drift Winds and Wind Components — Page 189, Lesson 164

I want to walk you through a technique for handling wind components on the navigation computer that builds on what we've already covered with multi-drift winds. Let's start with the tailwind limit line method. First, you rotate the disc round to the wind direction. Then you read off the value where the intersection occurs. In the example we're looking at, that intersection gives you 16 knots. Now, if you find that whole process too complicated, don't forget you can always treat it as a headwind problem on the reciprocal runway. Try treating it as the same type of problem you saw on pages 176 and 177, with the same wind of 320/10G30 but landing on runway 27. You get the same answer — 16 knots. Now let's move to a different type of problem: allowable wind angle for a given wind speed. This is another problem that arises sometimes and is quickly solved on the computer. It takes the form of determining between what directions a wind of given speed can blow without exceeding a stated crosswind component. Here's an example. You need to find the directions between which a wind of 50 knots may blow without creating a crosswind component of more than 30 knots on a runway of direction 250° magnetic. Step one: with the runway direction set against the true index, mark a cross on the centre vertical line to represent the wind speed of 50 knots. Step two: turn the plastic disc clockwise until the cross indicates a crosswind of 30 knots. Note the direction which now appears against the true index — that's 214° magnetic. Step three: now turn the plastic disc anticlockwise and repeat the procedure, reading off the direction which appears against the true index — that's 286° magnetic. Step four: the same crosswind component will be present if the wind direction is between 034° magnetic and 106° magnetic — those are the reciprocals of the directions you found in steps two and three. But a tailwind component will be present on this runway. So to summarise: you've found that a 50-knot wind blowing from between 214° and 286° magnetic will give you a crosswind component of 30 knots or less on runway 250. And if the wind comes from the opposite directions — 034° to 106° — you'll get the same crosswind component, but you'll also have a tailwind on that runway. Now, the next part of the book presents some practice questions on these concepts. These are the book's practice questions — let's try them one at a time.