General Principles - Cruise — Page 256, Lesson 309

Let's pick up with the cruise phase and look at how weight affects a propeller aeroplane's performance. We just saw the jet, and now for the propeller aeroplane, it's much the same story. At higher weights, more power is required. That means the fuel flow increases, and consequently, the range decreases. But here's the key difference I want you to notice: the speed for best range, which we call VMD, is higher at the higher weight. So for a propeller aeroplane, when you're heavy, you need to fly faster to get your best range. Now, let's also remember something important about operating altitudes. At higher weights, the operating altitudes are reduced. For a jet aeroplane, this is a double-edged sword because at lower altitudes, the jet engine is less efficient. That means the specific fuel consumption increases. So not only are you heavier and burning more fuel, but the engine is also working less efficiently at the lower altitude you're forced to fly at. There's a very useful linear relationship between weight and fuel flow. This assumes we're talking about identical aeroplanes, at the same altitude, and with the same specific fuel consumption. If we know the fuel flow for an aeroplane at one weight, we can calculate the fuel flow at an alternate weight using simple proportion. Let me walk you through the example. An aeroplane has a weight of 120,000 kilograms with a fuel flow of 4,400 kilograms per hour. Now, an identical aeroplane at the same altitude and specific fuel consumption, but weighing 110,000 kilograms, would have a fuel flow of... we take 4,400 divided by 120,000, which gives us 0.03666. Then we multiply that by 110,000 kilograms, and we get 4,033.33 kilograms per hour. In other words, the percentage change in fuel flow is proportional to the percentage change in aircraft weight. Now, let's move on to one of the most important issues an airline operator faces: the choice of aeroplane. This choice is mainly based on the required payload and range. The best way to understand this is through a typical example. Shown in Figure 5.21 is the payload range graph for a Boeing 777. On the vertical axis, we have the payload in thousands of kilograms. On the horizontal axis, we have the aeroplane range in thousands of nautical miles. Let's trace through this graph. As payload is initially added to the aeroplane, the marker moves from point A to point B. However, payload will reach a maximum when either there is no more space on the aeroplane, or the aeroplane has reached its zero fuel mass, which we call ZFM. Notice that the range at point B is zero because no fuel has been added yet. Now, fuel is added to the aeroplane, and the marker line moves to the right, showing an increase in range. Adding fuel can continue until the maximum structural take-off mass is achieved. This is shown by point C on the graph. Here's the crucial point: although maximum mass has been reached, it's unlikely that the tanks are full at this stage. To increase range further, more fuel must be added. But since the maximum mass has already been achieved, the only way to add more fuel is to exchange some of the payload for fuel. Now the marker starts to move down and to the right. This shows that range is increasing, but the payload is decreasing. Swapping payload for fuel in this way can continue only until the tanks are full, as shown by point D. From point C to point D, the total mass of the aeroplane has remained constant. So you see, the payload-range graph tells the whole story of an aeroplane's capability. It shows you the trade-off between carrying things and going far. The maximum range is only achievable by sacrificing payload, and the maximum payload is only achievable by sacrificing range.