Class A - En Route — Page 443, Lesson 552

Let’s start with the big picture. When you’re en route, you have two competing speeds to think about: the maximum range speed and the long range cruise speed. The maximum range speed, or MRC, is the speed that gives you the least fuel burn for a given distance. That sounds perfect, but operationally we don’t actually fly at MRC. We fly at the long range cruise speed, or LRC, which is a bit faster. Why? Because getting to the destination sooner means the aeroplane can do more revenue-earning flights in the same period. The trade-off is small: over a given time period, you can carry out about 4% more flights with only a 1% increase in fuel consumption. That’s the whole reason LRC is the operational choice. But LRC has limitations. It doesn’t account for the variable cost of fuel from day to day or month to month, and it doesn’t account for operational costs either. If fuel prices are high, that extra fuel burn can dramatically increase the overall cost of the flight. In that case, you might need to fly a more operationally economical speed instead. This is where the cost index comes in. The speed flown based on the cost index is called “ECON.” Let me explain the cost index concept now. The fundamental rationale of the cost index is to achieve minimum operational trip cost by trading off time-related costs against fuel-related costs. Think of it as a balance. With time-related costs, the faster you fly, the more money you save in time costs. That’s because the faster the aircraft flies, the more miles you cover for the same time-related components. It also means more miles can be flown between inspections when you consider maintenance costs. These time-related costs are at their minimum at the maximum operating speed, which we call VMO/MMO. VMO is the maximum operating speed in knots indicated airspeed, and MMO is the maximum operating speed in Mach number. At that high speed, time costs are lowest. However, there’s a catch. If you fly at VMO/MMO, the fuel burn increases, and the total fuel cost for the trip goes up. Fuel costs, on the other hand, are at their minimum at the maximum range cruise speed, MRC, and at their maximum at the maximum operating speed. So you have two opposing forces: time costs want you fast, fuel costs want you slow. When you add the time-related costs and the fuel-related costs together, you get a direct operating cost, or more simply, a total operating cost. The flight management system uses these time and fuel-related costs to help select the best speed to fly. That selected speed, based on the cost index, is the ECON speed. Let me show you the graph that ties this together. This figure plots cost against speed, and you can see the relationship between the long range cruise speed, LRC, and the maximum range cruise speed, MRC. The key takeaway is that the cost index is the tool that balances these two cost components to find the most economical speed for the trip.