The Navigation Computer - Calculation of Heading and Wind Finding — Page 144, Lesson 133

I want to walk you through a key part of using the Navigation Computer — calculating heading and ground speed, and then how we reverse the process to find the heading to fly when we already know the track. Let’s start with a practical scenario. Imagine you have the forecast wind marked on the wind face of your computer before flight — say, 315/15, meaning the wind is coming from 315° true at 15 knots. You set your True Airspeed, or TAS, under the centre circle — let’s say 160 knots. Then you set the heading of 030° and read off the drift and ground speed, exactly as you did in the last lesson. That gives you 5° of starboard drift — drift to the right — which means your track is 030° plus 5°, so 035° true. And your ground speed reads 157 knots. Now, from a position you noted at a specific time — say 1003 — you draw a track line of 035° true on your chart. You’ve been flying for 6 minutes, which is one-tenth of an hour. At a ground speed of 157 knots, in one-tenth of an hour you cover 15.7 nautical miles. That gives you your best estimate of position — what we call a dead reckoning position, or DR position. It’s shown on the chart with a specific symbol: a short line across the track with a dot. From that DR position, you draw a straight line to your destination — in this example, Clacton. That line becomes your new desired track. The point of this example is that sometimes you need to calculate track from a given heading — that’s the less common direction. But in the ATPL examination, you will be tested on all three Navigation Computer operations, and you need to be completely familiar with each one and able to tell them apart. Now, the most common requirement in navigation and planning is actually the reverse: finding the heading to fly. When you plan a trip, you join up your turning points on a map with a ruler and measure the track of the straight line. So you already know your track. What you need is the heading to fly, because the wind will cause drift. That’s the reverse of the problem we just solved. So let’s see how you use the Navigation Computer, given a known track, to find the heading to fly.