Class A - En Route — Page 443, Lesson 552

Let's start with the big picture. When you're flying en route, you have a choice of speeds, and that choice is a direct trade-off between fuel and time. I want to walk you through that trade-off, because it's the heart of the "Class A – En Route" performance philosophy. Look at Figure 17.3, which plots cost against speed. The graph shows two key speeds. First, the maximum range cruise speed, which we abbreviate as MRC. The advantage of flying at MRC is simple: the aeroplane uses the least amount of fuel, and therefore has the least fuel cost for a given distance. That's the pure fuel-efficiency point. But operationally, we don't actually fly at MRC. We fly at a faster speed called the long range cruise, or LRC. Why? Because by getting to the destination more quickly, more revenue-earning flights can be carried out in any given period. Here's the striking figure: over a given time period, you can carry out 4% more flights with only a 1% increase in fuel consumption. That's the operational argument for LRC. However, the long range cruise speed has limitations. It does not take into account the variable cost of fuel from day to day or month to month, and neither does it account for operational costs. Think about that: if fuel prices are high, that extra fuel consumption can dramatically increase the overall cost of the flight. In that situation, a more operationally economical speed may need to be flown. So the relationship of these costs is explained by the use of a cost index, and the speed flown based upon the cost index is called "ECON". We'll get to that in a moment. Now let's dig into the cost index concept itself. The fundamental rationale is to achieve minimum operation trip cost by means of a trade-off between time-related costs and fuel-related costs. So the cost index is used to take into account the relationship between those two categories. Let's look at time-related costs first. The faster the aircraft is flown, the more money is saved in time costs. Why? Because the faster you fly, the more miles can be flown for time-related components. It also means more miles can be flown between inspections when considering maintenance costs. These time-related costs are minimum at the maximum operating speed, which we write as VMO/MMO — that's the maximum operating indicated airspeed and the maximum operating Mach number. But here's the catch. If the aircraft is flown at such a high speed, the fuel burn increases, and the total fuel cost for the trip increases. So fuel costs are minimum at the maximum range cruise speed, MRC, and maximum at the maximum operating speed. You see the inverse relationship: time costs want you fast, fuel costs want you slow. Adding the time-related costs and fuel-related costs together produces a direct operating cost, or more simplistically, a total operating cost. And this is where the flight management system comes in. The FMS uses the time and fuel-related costs to help select the best speed to fly. That's the essence of the cost index — it's the mechanism that balances these two opposing cost curves to find the speed that gives you the minimum total operating cost for the trip. So to tie it together: MRC gives you minimum fuel, LRC gives you the operational compromise of 4% more flights for 1% more fuel, and the cost index refines that choice by weighing today's actual fuel price and time costs, producing the ECON speed. That's the en-route speed selection logic you'll be flying with.