Range Profile Figure — Page 58, Lesson 65

I want to walk you through the Range Profile Figure for a single-engine piston aeroplane — what we call an SEP. This is a table or graph that gives you an estimate of the maximum still air range for your aircraft, and it does this for each of four different power settings, at a selected pressure altitude. Let me define those terms clearly. Still air range means the distance the aircraft can fly in zero wind — no headwind, no tailwind. Pressure altitude is the altitude you read on your altimeter when you set the sub-scale to 1013.25 hectopascals (or 29.92 inches of mercury). It's the altitude above the standard datum plane, not necessarily your true altitude above sea level. Now, the range figure doesn't just give you cruise fuel. The calculated range it shows already includes fuel for several phases: climb, cruise, taxi and run-up, and also a reserve fuel for 45 minutes at economy cruise power. So that 45-minute reserve is built into the range number you read off the graph — you don't have to add it separately. Why is this table useful? It enables the pilot — you — to rapidly select a suitable cruise level for a given route distance and your preferred power setting. Alternatively, if you already know the cruise altitude you want to fly and the route distance, you can use the table to pick a power setting that will get you there. Here's how you use it. You enter on the left of the table with your cruising pressure altitude. Then you move horizontally across to the selected power setting curve — one of the four curves. From that point on the curve, you move vertically downwards to read off the range in Nautical Air Miles, or NAM. Now, you'll notice that on each power setting curve, the True Airspeed (TAS) is quoted for three specific pressure altitudes: 4000 feet, 8000 feet, and 12 000 feet. If your actual cruising altitude falls between these values — say you're at FL100, which is 10 000 feet — you will have to interpolate between the quoted TAS values to extract an accurate TAS to use later. Why do you need that TAS? Because range is affected by wind. The range you read from the graph is in Nautical Air Miles — that's distance through the air, not over the ground. To get your actual ground distance, you have to convert NAM into Nautical Ground Miles (NGM) using the formula discussed in Chapter 3 of your book. That conversion uses your TAS and the wind component. Let me now walk through the worked examples you have here. Example 4 asks you to extract range in NAM for three different conditions, all referring to CAP 697 Figure 2.4 — that's the Range Profile Figure for this SEP. First: Full Throttle at 2500 rpm, FL100. The answer given is 839 NAM. Second: Full Throttle at 2100 rpm, FL100. The answer is 904 NAM. Notice that at the same altitude, a lower rpm gives a longer range — that makes sense because you're using less power and therefore less fuel per mile. Third: Full Throttle at 2300 rpm, FL70 — that's Flight Level 70, or 7000 feet pressure altitude. The answer is 832 NAM. Example 5 gives you: Full Throttle at 2500 rpm, FL80, with a 30-knot tailwind component. The question asks: "What is the ground range?" The answer isn't fully written out here, but the principle is: you first read the NAM from the graph for that altitude and power setting, then you convert that NAM into NGM using the wind. With a 30-knot tailwind, your ground range will be greater than the still air range. Let me show you the figure so you can see the curves and the layout. That's the Range Profile Figure. You can see the pressure altitude on the left, the four power setting curves, and the TAS values marked at 4000, 8000, and 12 000 feet on each curve. Practice entering at an altitude, moving across to your power curve, and reading down to the range in NAM. Then remember: that's still air range — you'll need to apply the wind correction to get your actual ground distance.