We had reached point D, where the tanks are full — Page 256, Lesson 311

We're in the middle of the range-and-payload trade-off graph, so let me pick up right where the marker line left off. We had reached point D, where the tanks are full. From point C to point D, the total mass of the aeroplane stayed constant — we were simply swapping payload for fuel, so the aeroplane got heavier with fuel but lighter with payload, and the net mass didn't change. Now, the only way to increase range beyond point D, even though the tanks are already full, is to remove the rest of the payload. You'll recall the principle: reducing weight increases range. So reducing the payload completely moves the marker line from point D to point E. At point E, the aeroplane has full tanks, maximum range, but no payload at all. That's the extreme end of the trade — you've traded every kilogram of payload for range. In the majority of airlines, this trade-off between range and payload is carried out on initial aeroplane purchase and, thereafter, during in-flight planning. As a pilot, it's unlikely you'll be required to work through these graphs yourself — your job is to check and confirm the data that has been prepared in advance for you. Now let's move to a new factor affecting range: configuration. You'll recall that deploying the flaps and gear increases parasite drag. Parasite drag is the drag that doesn't depend on lift — it's the drag from the aeroplane's shape, the skin friction, the protrusions like gear and flaps. When you deploy them, you increase parasite drag, and that moves the total drag curve and the power required curve up and to the left, as shown in Figure 5.22. Let me explain what "up and left" means on that graph. The drag curve moves up because, at any given speed, you now have more drag — the flaps and gear add resistance. It moves left because the speed at which minimum drag occurs shifts to a lower value. With the gear and flaps deployed, the aeroplane has more drag, and therefore needs more thrust. More thrust requires greater fuel flow. And greater fuel flow decreases the range — you burn more fuel per mile. But here's the interesting part: notice that the speed for maximum range, which is 1.32 times VMD, is now lower. VMD is the speed for minimum drag. For a jet aeroplane, the speed for maximum range is 1.32 times that minimum-drag speed. When you deploy flaps and gear, the whole drag curve shifts, and that 1.32 VMD value drops — the speed at which you get maximum range is now lower than it was in the clean configuration. So the key takeaway: configuration affects range through parasite drag. More drag means more thrust needed, more fuel flow, less range — but the optimum speed for maximum range actually decreases. That's the trade you're managing in cruise.