General Principles - Cruise — Page 251, Lesson 302

Let’s pick this up right where the drag and power curves left off, because this passage is the payoff of all that. We’ve just seen how gear and flaps wreck your endurance picture, and now I want to walk you through the two big external factors that change the whole game: wind and altitude. First, wind. And here’s the headline: wind has no effect on endurance whatsoever. Let me make sure you really feel why. Endurance is all about minimizing fuel flow — it’s time in the air, not distance covered. Wind doesn’t change how much fuel the engine burns per hour; it just changes how far you travel over the ground for that same hour of fuel. So whether you’ve got a headwind or a tailwind, the aeroplane stays airborne for exactly as long as it has usable fuel in its tanks. That’s the whole story. Endurance is a time game, and wind only plays the distance game. Altitude, though — altitude absolutely does affect endurance, and this is where it gets interesting because the effect is very dependent on engine type. Let’s take them one at a time. For jet aeroplanes, efficiency improves as altitude increases. There are two reasons. First, the ambient temperature decreases with altitude, and that helps the engine. Second, the rpm required to maintain thrust increases. So theoretically, the maximum endurance of a jet is achieved when flying at or above the tropopause — that’s the boundary layer in the atmosphere where the ambient air temperature stops decreasing and reaches its lowest value. So for a jet, you climb high to get the best endurance. Now turbo-propeller aeroplanes. These function in a similar way to a jet because, in essence, they are jet engines with a propeller attached to a geared shaft. The turbo-prop engine itself gains efficiency with altitude, just like a jet. But here’s the catch: the power required increases as altitude rises, because the rising TAS — true airspeed — offsets those efficiency gains. So the net effect is that for the majority of modern turbo-propeller aeroplanes, maximum endurance is achieved at around 10,000 feet or less. That’s a very practical number to remember — it’s not a jet, it’s not sea level, it’s right around 10,000 feet. And finally, piston engine aeroplanes. These are most efficient at sea level, when the manifold pressure is high and the rpm is low — provided, and this is a critical qualifier, that the mixture has been leaned correctly. So for a piston, you want to be down low, at sea level, with high manifold pressure, low rpm, and a properly leaned mixture. So let me tie the whole picture together for you. Wind — ignore it, it does nothing to endurance. Altitude — it matters a lot, but the direction depends on the engine. Jets want to be high, at or above the tropopause. Turbo-props want to be around 10,000 feet or less. Piston engines want sea level with high manifold pressure, low rpm, and a correctly leaned mixture. And one more thing from earlier in this passage that I don’t want you to lose: in the landing configuration, fuel flow can increase by 150% compared to a clean configuration. That’s a huge number. So it’s important not to deploy gear or flaps too early — doing so unnecessarily increases the fuel costs for the flight. That’s the practical operational warning that ties all this theory back to how you actually fly the aeroplane.