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

Let’s pick this up right where the performance charts take over. We’re now looking at the endurance graph for a light single-engine Class B aeroplane, and I want you to see exactly how to read it, because this is the kind of chart you’ll use to plan how long you can stay airborne. First, the term endurance. Endurance is the amount of time your aeroplane can remain in the air on the fuel available. It’s measured in hours, not distance. That’s the key difference from range, which is the distance you can cover. So when we read this graph, we’re answering the question: “How long can I fly?” Now, the graph itself — Figure 9.6 — is a typical endurance graph for a light single-engine Class B aeroplane. It’s a set of curves that relate your fuel flow or endurance to the conditions you’re flying in. To use it, you need to know three things about your flight: your cruise altitude, your outside air temperature, and the power setting you intend to use. Let me walk you through the example given. We assume a cruise altitude of 7000 feet. That’s your pressure altitude at cruise. The outside air temperature is 7°C — that’s the actual temperature of the air outside the aeroplane at that altitude, not the ISA standard, just the real value. And we’re using 65% power, meaning the engine is producing 65% of its maximum rated power. You enter the graph with those three values. You locate the 7000-foot altitude line, you find where the 7°C temperature intersects it, and you read across to the 65% power curve. Working through the graph, you get two endurance figures, and this is the critical part — the graph gives you endurance with and without reserve fuel. With 45 minutes of reserve fuel allowed, the endurance is 6.6 hours. That means you plan to land with 45 minutes of fuel still in the tanks, so the time you can actually spend cruising is 6.6 hours. If you allow no reserves at all — meaning you’re willing to use every last drop of fuel before landing — the endurance rises to 7.4 hours. So the difference between those two numbers, 7.4 minus 6.6, is 0.8 hours, which is exactly that 45-minute reserve you set aside. The graph is telling you: plan for 6.6 hours of cruise if you want to keep a legal and safe reserve, or 7.4 hours if you’re flying right to the edge of empty tanks. That’s the whole point of the endurance graph — it lets you convert your fuel, altitude, temperature, and power into a usable time aloft, with the reserve built into the planning. When you’re actually flying a Class B single, you’ll use this chart to confirm you have enough endurance for the leg you’re about to fly, and you’ll always respect that reserve figure.