The Navigation Computer - Triangle of Velocities — Page 130, Lesson 117

We've finished the slide rule face of the Navigation Computer, so now we're turning to the wind face. But before we touch the computer itself, we have to nail down some definitions and understand how wind affects navigation. That's where the Triangle of Velocities comes in. Let's start with the most fundamental distinction: Heading versus Track. Heading is the direction in which the aircraft is pointing. It's defined as the direction in which the fore and aft axis of the aircraft is pointing, and it may be measured from True, Magnetic, or Compass North. So, think of the nose of the aircraft — that's your heading. Track, on the other hand, is the direction of the aircraft's path over the ground. It may be measured from True or Magnetic North. So, track is where you're actually going over the earth's surface. Now, here's the key point: these are not necessarily the same. If there's any crosswind component, there will be a difference between the direction the aircraft is pointing and the direction it's travelling. The aircraft gets pushed sideways by the wind. That difference is called Drift. Drift is the difference between heading and track. And here's the rule you must remember: you always drift from your heading to your track. Let me give you the examples from the text. If your heading is 090°(T) and your track is 097°(T), you are drifting to the right — from heading to track. The drift is 7° to the right. If your heading is 030°(T) and your track is 026°(T), you are drifting to the left. So the drift is 4° to the left. Now, there's a related term called the Wind Correction Angle, or WCA. The WCA is the same value as drift, but it's applied in the opposite sense. Let me explain. In the first example, heading is 090° and track is 097°. The drift angle is 7° right, or starboard — that is, added to the heading to give track. So heading plus 7° gives you track. The WCA in this example would be -7°. That is, it's applied to the track to give heading. So track minus 7° gives you heading. Same magnitude, opposite sense. Now, let's talk about Port and Starboard. Most countries, and certainly the USA, use the terms 'left' and 'right' in aviation. But the British retain an affection for their maritime tradition, so they use 'Port' and 'Starboard' when referring to the left or right side of the aircraft, looking forwards, or when referring to drift. 'Port' is left — easy to remember because 'Port' has 4 letters and so does 'left'. 'Starboard' is right. Finally, there's the word Course, and this is a word that can lead to enormous confusion. Early British terminology used 'course' to mean heading — the direction in which the aircraft is pointing. That practice ceased around 1950, when 'heading' became the accepted UK term. However, many present American publications use the word 'course' to mean track — the direction of the aircraft's path over the ground. Because of this potential for confusion, we will not use the word 'course' in this book. We'll always use the unambiguous words 'heading' and 'track'. However, you should know that the word 'course' may sometimes be used in formal examinations to mean 'desired track'. So, to summarise: heading is where the nose points, track is where you actually go over the ground, drift is the difference between them, and you always drift from heading to track. The WCA is the same value as drift but applied in the opposite sense. And we avoid the word 'course' because it's ambiguous. Now, let's look at the diagram of the Triangle of Velocities at Figure 7.4 of this chapter — it shows drift to the right. That's the concept we're building toward. That's the foundation. Once you have these definitions clear, we can move on to how the wind face of the Navigation Computer actually solves this triangle.