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

I want to walk you through the next major section of your navigation training: the wind face of the navigation computer and the triangle of velocities. We've finished everything on the slide-rule face, so now we turn to the wind side. But before we can use it, we need to define some essential terms and understand how wind affects navigation. Let's start with heading. Heading is the direction in which the aircraft is pointing. That sounds straightforward, but here's the critical point: the direction the aircraft points is not necessarily the direction it actually moves over the ground. If there's any crosswind component, the aircraft will be pushed sideways, so the nose points one way while the path over the ground goes another. More formally, heading is defined as the direction in which the fore-and-aft axis of the aircraft is pointing. It can be measured from True North, Magnetic North, or Compass North, depending on the reference you're using. Now track. Track is the direction of the aircraft's actual path over the ground. It's the line your flight path traces on the map below. Track can be measured from True North or Magnetic North. The difference between heading and track is called drift. You always drift from your heading to your track. Let me give you two examples. If your heading is 090° True and your track is 097° True, you are drifting to the right — from heading to track. If your heading is 030° True and your track is 026° True, you are drifting to the left. There's a diagram in the book, Figure 7.4, that shows drift to the right, and I'd encourage you to look at it when you have the book open. Next, Wind Correction Angle, often abbreviated WCA. Wind Correction Angle is the same numerical value as drift, but it's applied in the opposite sense. Let's go back to that first example: heading 090°, track 097°. The drift angle is 7° to the right, or starboard — that means you add 7° to the heading to get the track. The Wind Correction Angle in this example would be -7°, meaning you apply it to the track to get the heading. So drift and WCA are mirror images of each other: one tells you how much the wind pushes you off your heading, the other tells you how much to correct your heading to stay on track. Now a quick note on terminology. Most countries, and certainly the USA, use '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 four letters and so does 'left'. 'Starboard' is right. Finally, the word course. This can cause enormous confusion. Early British terminology used 'course' to mean heading — the direction the aircraft points. That practice stopped around 1950, when 'heading' became the accepted UK term. However, many present American publications use '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 training. We will always use the unambiguous words 'heading' and 'track'. That said, you should be aware that in formal examinations, the word 'course' may sometimes appear, and in that context it usually means '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 Wind Correction Angle is the opposite of drift. We'll build on these definitions as we move into the triangle of velocities and the wind face of the computer.