Nautical Air Miles — Page 45, Lesson 57

I want to walk you through the concept of Nautical Air Miles, or NAM. This is a key idea you'll see on many performance graphs for Single-engine Piston (SEP), Multi-engine Piston (MEP), and later the Medium Range Jet Transport (MRJT) aircraft. Let's start with the definition. Nautical Air Miles are a measure of the air distance flown by an aircraft — that is, the distance the aircraft travels through the air itself, measured at its True Airspeed, or TAS. Think of it as the distance the aircraft's wings have actually moved through the air mass. Now, there's a related term: Nautical Ground Miles, or NGM. This is the distance the aircraft travels over the ground, the kind of distance you would measure on a chart between two points on the earth's surface. In perfectly still air — when there is no wind component along the aircraft's heading — the NAM flown equals the NGM flown. The air isn't moving, so the distance through the air and the distance over the ground are the same. But usually, the air is moving. And when an aircraft flies through moving air, it covers a different distance over the ground than it does through the air. Here's the key relationship. If the air is moving in the opposite direction to the aircraft — that's a headwind, or a minus wind component — the aircraft will fly more NAM than NGM. The aircraft is fighting against the wind, so it has to fly further through the air to cover the same ground distance. Take a look at Figure 3.1, which illustrates this: NAM greater than NGM. Conversely, if the wind is blowing in the same direction the aircraft is flying — a tailwind, or a plus wind component — the NAM will be less than the NGM. The wind is helping push the aircraft along, so it covers more ground for each mile it flies through the air. Figure 3.2 shows this: NAM less than NGM. So, the relationship between NAM, NGM, TAS, GS (Ground Speed), and Wind Component (WC) is given by a simple proportion: NAM over NGM equals TAS over GS. In other words: NAM / NGM = TAS / GS. Let's work through the example in the book. An aircraft flies at a TAS of 142 knots for 63 NAM. The wind component is -20 knots — that's a 20-knot headwind. We want to find how many NGM it flies. First, we need the ground speed. GS is TAS plus the wind component. So 142 knots plus -20 knots gives us a ground speed of 122 knots. Now we set up the proportion: 63 over x equals 142 over 122. Solving for x, we get x equals 122 divided by 142, times 63, which equals 54. So the aircraft flies 54 Nautical Ground Miles. You can also do this on your navigation computer, as shown in Figure 3.3. You set the red cursor: 142 on the inner scale against 122 on the outer scale. Then the blue cursor reads 63 on the inner scale against 54 on the outer scale. Now, what if you don't know the TAS or the ground speed? This often happens during a climb or descent. The conversion can still be done using a different approach. Consider an aircraft with a TAS of 100 knots. In one hour, it would fly 100 NAM. But in that hour, the air itself has been moved by the wind component. If the wind component is -25 — that's a 25-knot headwind — then the air has moved 25 nautical miles in the direction the aircraft came from. So the ground distance flown is 100 minus 25, which equals 75 NGM. Similarly, if the wind component is +25 — a 25-knot tailwind — the ground distance is 100 plus 25, which equals 125 NGM. From this, we can see that the difference between air distance and ground distance is the wind component per minutes flown. More precisely, it's plus or minus the wind component divided by 60, multiplied by the minutes flown. This gives us another formula: NGM equals NAM plus or minus the quantity: wind component over 60, times the sector time. So: NGM = NAM +/- (WC / 60 × sector time) Let's apply this to the climb example. An aircraft climbs to cruising level in 11.5 minutes, covering 23.5 NAM. The wind component is -30 knots — a 30-knot headwind. We calculate: NGM = 23.5 minus the quantity: 30 over 60 times 11.5. 30 over 60 is 0.5. 0.5 times 11.5 is 5.75. So NGM = 23.5 minus 5.75, which equals 17.75, or about 18 NGM. That's the core of Nautical Air Miles — understanding the difference between the distance you fly through the air and the distance you cover over the ground, and how to convert between them using either the TAS/GS ratio or the wind component and time method.