The Navigation Computer: The Wind Face
We have completed all aspects of the slide rule face of the Navigation Computer and we now turn our attention to the wind face. However, before we do so we must define some terms, and then discuss the effect of wind on navigation, and introduce the concept of the Triangle of Velocities.
Heading is the direction in which the aircraft is pointing. This is not necessarily the direction in which it is tracking over the ground. As we shall shortly see, if there is any crosswind component, there will be a difference between the direction in which the aircraft is pointing and the direction in which it is travelling.
Heading is defined as the direction in which the fore and aft axis of the aircraft is pointing; it may be measured from True, Magnetic, or Compass North.
Track is the direction of the aircraft’s path over the ground. It may be measured from True or Magnetic North.
Drift is the difference between heading and track. You always drift from your heading to your track. Thus, if your heading is 090°(T) and your track is 097°(T), you are drifting to the right (from heading to track). If your heading is 030°(T) and your track is 026°(T), you are drifting to the left. Have a look at the diagram of the Triangle of Velocities at Figure 7.4 of this chapter. It shows drift to the right.
Wind Correction Angle (WCA). Wind Correction Angle is the same value as drift but it is applied in the opposite sense. In the paragraph above, the heading is 090° and the track is 097° which means the drift angle is 7° right or starboard i.e. added to the heading to give track. The WCA in this example would be -7° i.e. applied to the track to give heading.
Port and Starboard. Most countries, and certainly the USA, use the terms ‘left’ and ‘right’ in aviation. However, the British retain an affection for their maritime tradition and so use the terms ‘Port’ and ‘Starboard’ when referring either to the left or right side of the aircraft (looking forwards) or when referring to drift. ‘Port’ is left (easy to remember - ‘Port’ has 4 letters and so does ‘left’). ‘Starboard’ is right.
Course 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. This 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’. The unambiguous words ‘heading’ and ‘track’ will always be used. However, the word course may sometimes be used in formal examinations to mean ‘desired track’.
Next: The Effect of Wind