The Earth
The Solar System & Time
Charts
Using Aeronautical Charts
Basics of Navigation
Distance, Speed & Time
Vertical Navigation
Fuel Planning
Practical Navigation Techniques
Radio Navigation
Practice Exam

CRP-1: Calculating Heading & Groundspeed

It is vital to make sure you are using the same north for each component of the calculations. Tracks measured on the chart relative to a line of longitude are in degrees true (°T). Forecast wind velocities are in degrees true (°T). However, some exam questions will require you to convert between degrees true (°T) and degrees magnetic (°M) before using the navigational computer.

Example

The wind is 140/20. Your desired track from A to B is 080°T. Your true airspeed (TAS) is 90 knots. What heading should you fly to arrive overhead B and what will your groundspeed be?

Drawing a diagram can help you to visualise the situation:

1 Put the airspeed dot on the horizontal line at the top of the square grid

2 Put the wind direction (140°T) under the index by rotating the compass rose

3 Move vertically down from the airspeed dot and mark the wind speed (20 knots)

4 Slide the airspeed dot upwards and put it over your true airspeed (90 knots)

5 Rotate the compass rose to set the desired track (080°T) under the index

6 Your mark is now indicating the drift. In this case, it is 12° of drift to port (left)

7 Rotate the compass rose to point the aircraft’s nose further into the wind to compensate for the drift. In this case, we turn the aircraft 12° to the right.

It is easy to accidentally rotate the wrong way! Remember: you will always need to turn the nose into the wind to compensate for drift. In this case, the wind (140°) is from our right, so we must turn the aircraft to the right (i.e. the number under the index must increase)

8 Recheck the drift as it may have changed slightly when you rotated the compass rose. In this case, the drift has changed to 11°. We need to move the compass rose to match this drift value, since this is the amount of drift we will experience on this heading.

This process is often called ‘balancing the drift’ as we need the drift value and the change in heading (from the desired track) to match. You may have to make a few of these small adjustments to get the new heading and drift to balance.

9 Now that we have balanced the drift (11° of drift and heading is 11° right of the desired track), you can read off the heading from under the index:

Required Heading is 091°

and read off the groundspeed from under the cross:

Groundspeed will be 77 knots

Gross Error Checking

Make sure to do a check for any large errors – it is very easy to accidentally rotate the compass rose the wrong way! Make sure that:

The new heading is into the wind. When the wind is from the left the new heading should be to the left of the track, so it will be a smaller number. When the wind is from the right, the new heading will be a larger number.
When there is a tailwind your groundspeed is faster than your airspeed, and when there is a headwind your groundspeed is slower than your airspeed.