Class A - En Route — Page 452, Lesson 554

Let’s pick this up right where the cost index idea starts to do real work. I want to walk you through what ECON speed actually is, because it’s the heart of Class A en-route economy flying. Look at Figure 17.4 with me. It plots the operating costs of the aeroplane against the aeroplane’s speed. There’s a total cost curve on that graph, and the key idea is this: the speed that gives the minimum total operating cost is the most economical speed to fly. That speed is called ECON — in other words, the minimum cost speed. Now, ECON isn’t just picked out of the air. 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 let’s define that properly. As a formula, the cost index is a ratio of the cost of time, abbreviated CT, to the cost of fuel, abbreviated CF. So cost index = CT divided by CF. That ratio is what the flight management system uses to compute your ECON speed. Let me make that concrete with two extreme cases, because they show you exactly how the system behaves. First, suppose fuel costs are high and time costs are very low. Then the cost index would be almost zero. What happens on the graph? The blue total cost line moves 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 — found at the bottom of the blue line — would now be at the maximum range speed. That’s Figure 17.5. Now flip it. Suppose 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. That’s Figure 17.6. So here’s the summary rule, and I want you to hold onto this: increasing the cost index from zero to maximum will increase the ECON speed from the maximum range speed up to the maximum operating speed. In other words, the cost index is a dial — turn it up, and you fly faster; turn it down, and you fly slower and more fuel-efficiently. One more practical detail. For most aeroplanes, the cost index varies from zero to 99, or from zero to 999, depending on the aircraft type. So when you’re setting up the flight management system, you’re entering a number in that range, and that number directly determines your ECON speed. So the chain is: cost index (CT/CF) → flight management system → ECON speed → your most economical cruising speed for that flight. That’s the whole logic in one line.