Single-engine Class B - En Route and Descent — Page 317, Lesson 397

Let’s pick this up right where the range and endurance work begins to get practical. I want to walk you through what these graphs actually do for you as a pilot. The key idea here is that the manufacturer gives you graphs and tables which help you calculate the endurance and range of your aeroplane under various conditions. So endurance is how long you can stay airborne, and range is how far you can go — and both depend on the conditions you’re flying in. Now, let’s look at a typical endurance graph for a light single-engine Class B aeroplane. I’ll give you a concrete example so you can see how to read it. Let’s assume a cruise altitude of 7000 ft, with an outside air temperature of 7°C, at 65% power. Working through the graph, you find the endurance of the aircraft. And here’s the crucial part — the graph gives you two numbers, and the difference between them is the reserve fuel. Allowing 45 minutes of reserve fuel, the endurance is 6.6 hours. Allowing for no reserves, the endurance is 7.4 hours. So the difference between those two — 7.4 hours minus 6.6 hours — is exactly that 45 minutes of reserve fuel you’ve set aside. That’s the whole point of the graph: it lets you read off endurance for a given altitude, temperature, and power setting, and it shows you the penalty of carrying a reserve. Let me make sure the terms are crystal clear. Endurance is the time you can fly. Range is the distance you can cover. The graph in Figure 9.6 is specifically an endurance graph — it’s giving you time, not distance. And the conditions you input are altitude, outside air temperature, and power setting. In our example, that’s 7000 ft, 7°C, and 65% power. One thing I want you to notice: the temperature here is given as outside air temperature — that’s the actual temperature of the air outside the aircraft, not the standard atmosphere value. And the power is given as a percentage of maximum — 65% power. These are the inputs you’d take from your flight plan or from the conditions of the day. So when you’re planning, you find your cruise altitude, you find your outside air temperature at that altitude, you find your power setting, and you read across the graph to get your endurance. Then you decide whether you’re planning with reserves — in which case you use the 6.6-hour figure — or without reserves, in which case you use the 7.4-hour figure. That’s the core of it. The graph is your tool for turning the conditions of your flight into a usable endurance number, and it always reminds you of the cost of that reserve fuel.