What is Cost Index?

Cost Index
You’ve probably heard the term being thrown around the forum or when reading or watching aviation related articles or videos. But what is cost index, and how is it used in real world aviation. In a definition, cost index is a number used in the Flight Management System (FMS) to optimize an aircrafts speed. It gives the ratio between the unit cost of time and the unit cost of fuel.

With this number along with knowledge of the aircrafts performance, it is possible to find out the optimal speed of the aircraft which will result in the lowest cost of that particular flight. Speeds slower than that optimal speed result in lower fuel burn but more flying time. This extra flying time outweighs fuel savings at speeds below the optimal speed. Speeds faster than the optimal speed will result in less flying time but more fuel burn. Again, the savings of less flying time do not outweigh the fuel burn at speeds above the optimal.

So what do different cost indexes tell us?
A low cost indicates the cost of time is low, or that fuel is expensive. It will result in a low speed. A high cost index means cost of time is high (e.g. passengers about to miss their connection flight) or low fuel price which is rare nowadays.

Airlines typically have a standard cost index they use for planning and can adjust them for different flights.

To find out more about cost index, read the Airbus article here. If you have any other interests in cost index we can discuss it down below. Hope you enjoyed reading!


Thank you for the succinct explanation!


Here’s another explanation for it, hopefully can add more clarity


Pilots also use cost index accordingly with their airline. For example, Jetstar uses a low cost index as it is a LCC. Qantas uses a higher cost index as they are a premium airline. It doesn’t severely affect flight time, but all in all can ultimately define major differences in airlines.

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