General Principles - Cruise — Page 256, Lesson 309

We're moving into the cruise phase of the flight now, and I want to walk you through how weight affects fuel flow and range. This is where the performance planning really comes together. Let's start with the propeller aeroplane, because it behaves much the same as the jet we looked at. At higher weights, more power is required. That means the engine has to work harder, so fuel flow increases and range decreases. But here's the key point you need to notice: the speed for best range, which we call VMD, is actually higher at the higher weight. VMD stands for velocity for minimum drag, and that's the speed at which you get the best range. So when the aeroplane is heavier, that optimum speed shifts up. Now, there's another effect at higher weights that's specific to jets. The operating altitudes are reduced. When a jet aeroplane is heavier, it can't climb as high, and at those lower altitudes the jet engine is less efficient. That means the specific fuel consumption increases. Specific fuel consumption is the amount of fuel burned per unit of thrust per hour, and when it goes up, the engine is using more fuel for the same work. Here's a really useful relationship to remember: there is a linear relationship between weight and fuel flow. That's assuming you're comparing identical aeroplanes, at the same altitude, and with the same specific fuel consumption. Linear means if you plot weight against fuel flow, you get a straight line. And because of that, if you know the fuel flow at one weight, you can calculate the fuel flow at any other weight. Let me show you the example. Say an aeroplane weighs 120,000 kilograms and has a fuel flow of 4,400 kilograms per hour. Now take an identical aeroplane, same altitude, same specific fuel consumption, but it weighs 110,000 kilograms. To find its fuel flow, you divide 4,400 by 120,000, which gives you 0.03666. Then you multiply that by 110,000 kilograms, and you get 4,033.33 kilograms per hour. So the percentage change in fuel flow is proportional to the percentage change in aircraft weight. That's the whole principle in one sentence. Now let's move on to something that's central to how an airline chooses its aeroplane: payload versus range. This is one of the most important considerations for operators, and it's best understood by walking through a typical example. I want you to look at the payload range graph for a Boeing 777. On the vertical axis we have payload in thousands of kilograms, and on the horizontal axis we have range in thousands of nautical miles. Let's trace the marker on this graph. As payload is initially added to the aeroplane, the marker moves from point A to point B. Payload will reach its maximum when either there's no more space on the aeroplane, or the aeroplane has reached its zero fuel mass. Zero fuel mass, or ZFM, is the weight of the aeroplane with all payload and crew on board, but before any fuel is added. Notice that at point B, the range is zero, because no fuel has been added yet. Now fuel is added, and the marker moves to the right, showing an increase in range. You can keep adding fuel until you reach the maximum structural take-off mass. That's the heaviest the aeroplane is allowed to be at take-off, and it's shown by point C on the graph. Here's the interesting part: even though you've reached maximum mass, it's unlikely the tanks are full at this stage. So to increase range further, you need more fuel. But you can't add mass beyond the maximum. The only way to add more fuel is to exchange some payload for fuel. So the marker starts to move down and to the right. Range is increasing, but payload is decreasing. This swapping of payload for fuel can continue only until the tanks are full, and that's point D. From point C to point D, the total mass of the aeroplane has remained constant. You're just trading payload weight for fuel weight, keeping the total the same. So you can see the whole story of the graph: from A to B you're adding payload, from B to C you're adding fuel up to the structural limit, and from C to D you're trading payload for fuel until the tanks are full. That's the complete payload range picture.