Great Circles, Rhumb Lines & Directions on the Earth — Page 28, Lesson 38

I want to walk you through a key idea about how great circle tracks behave, and then we'll shift into the distance units you'll use every day in aviation. Let's start with that diagram reference — Figure 2.7. It gives you a memory aid called DIID, which stands for Decreasing – Increasing – Increasing – Decreasing. Here's how it works. Imagine you're flying a great circle track. The diagram splits the world into hemispheres: North and South. Left and right on the diagram tell you whether your track direction is Easterly (that's to the right) or Westerly (to the left). So DIID spells out the pattern of how the great circle track direction changes as you move along it: it decreases, then increases, then increases again, then decreases. The key rule to remember from all these cases is this: the Great Circle direction always changes towards the Equator. That's a fundamental behaviour — no matter which hemisphere you're in or which direction you're flying, the great circle track will curve so that its direction moves toward the equator. Now, let's move on to Distance on the Earth. In aviation, we use both metric and Imperial measures, so you need to be comfortable with both. I know many of you may not have used the smaller Imperial measures before, so let's go through the conversions you need to remember. Starting with metric: 1 metre (m) equals 100 centimetres (cm), and also equals 1000 millimetres (mm). One centimetre is 10 millimetres. Now the Imperial side: 1 metre equals 3.28 feet (ft). One foot is 12 inches — and you'll see inches written as either 'in' or with a double quote mark ("). One inch equals 2.54 centimetres. And 1 yard (yd) equals 3 feet. For any conversion you need in aviation, the Navigation Computer — your flight computer — is accurate enough. You don't need to memorise every conversion to six decimal places; the computer will handle it. Now let's talk about the Kilometre (km). This is a special unit because it's defined from the Earth itself. One kilometre is 1/10,000th of the average distance on the Earth between the Equator and either Pole. Think about what that means: if you go from the Equator to the North Pole, that distance is 10,000 kilometres. The same from the Equator to the South Pole. That also tells us that the circumference of the Earth is 40,000 kilometres — because you'd go from the Equator to one pole (10,000 km), back to the Equator (another 10,000 km), to the other pole (another 10,000 km), and back to the Equator (another 10,000 km). That's 40,000 km all the way around. Finally, here's one more conversion between kilometres and Imperial units: 1 kilometre (km) equals 3280 feet (ft). And just to reinforce the metre-to-foot relationship we already saw: 1 metre (m) equals 3.28 feet. So to summarise: great circle tracks always change direction toward the Equator, and you can remember the pattern with DIID. For distance, you now have the key conversions between metres, feet, inches, yards, and kilometres — and you know the kilometre is defined from the Earth's own dimensions.