Class A - En Route — Page 452, Lesson 554

Let’s pick this up right where the cost index idea starts to do its real work. I want to walk you through what ECON speed actually is, because that’s the heart of this page. Look at Figure 17.4 with me. It’s a graph plotting the operating costs of the aeroplane against the aeroplane speed. There are several cost curves on it, but the one that matters most is the total cost curve. From that total cost curve you can see that there is one particular speed which gives the minimum total operating cost. That speed is the most economical speed to fly, and it has a proper name: ECON. In other words, ECON is the minimum cost speed. Now, here’s the key point — the aeroplane doesn’t just pick ECON by magic. The value of the ECON speed is worked out by the flight management system, and it’s based entirely on the value of the cost index. So the cost index is the input, and ECON speed is the output. Let me give you the exact formula for the cost index, because it’s a ratio. The cost index is the ratio of the cost of time, which we write as CT, to the cost of fuel, which we write as CF. So cost index equals CT divided by CF. Think of it as the airline telling the flight management system: how much is my time worth compared to how much is my fuel worth? Now let’s see what happens when you change that ratio, because that’s where the graphs in Figures 17.5 and 17.6 come in. First scenario: fuel costs are high and time costs are very low. In that case the cost index would be almost zero. On the graph, the blue total cost line is moved to the left. The intersection point of the other cost lines will lie very close to the maximum range cruise speed, giving a cost index of zero. So the ECON speed — which you find at the bottom of the blue line — would now be at the maximum range speed. That makes sense: if fuel is expensive and time is cheap, you want to fly slowly and efficiently to save fuel. Second scenario, the opposite: time costs are high and fuel costs are low. Then the cost index would be very high, and the blue total cost line moves to the far right of the graph. The ECON speed, found at the bottom of the blue curve, would now be very close to the maximum operating speed. Again, logical: if time is expensive and fuel is cheap, you want to get there fast. So let me summarize that relationship for you, because it’s the single most important takeaway. Increasing the cost index from zero to maximum will increase the ECON speed from the maximum range speed up to the maximum operating speed. So the cost index is a dial: turn it up, and the aeroplane flies faster; turn it down, and it flies slower and more fuel-efficiently. One last practical detail. For most aeroplanes, the cost index doesn’t run on an infinite scale. It varies from zero to 99, or from zero to 999, depending on the aeroplane type. So when you’re setting up the flight management system, you’ll be entering a number in one of those ranges, and that number directly sets your ECON speed. That’s the whole mechanism: cost index in, ECON speed out, and the two extremes are maximum range speed at a cost index of zero, and maximum operating speed at the maximum cost index.