Definition
A numerical value entered into a flight management system that expresses the ratio of time-related operating costs to fuel costs for a given flight. The cost index is used by the FMS to calculate the most economical climb, cruise, and descent speeds for the airplane on that flight.
Plain English
A number the crew loads into the flight computer that tells it how much the airline values saving time versus saving fuel. The computer then picks the speeds that match that balance.
Context Anchor
Seen in airline and turbine aircraft operations when setting up the flight management computer before or during a flight.
Derivation
From 'cost' (what is spent) and 'index' (a number used to represent a ratio or relationship). The term reflects its function: a single number that indexes the relationship between two costs — time and fuel.
Why Pilots Care
It directly controls fuel burn versus block time and therefore total trip cost for commercial operations.
Analogy
It is like choosing between driving slower to save gas or driving faster to arrive sooner. The Cost Index tells the aircraft computer which choice matters more for that flight.
Intuition Check
Cost Index is not the total cost of the flight. It is a setting that compares time cost with fuel cost so the aircraft can choose economical speeds.
Example Sentence 1
Before departure, the captain entered a cost index of 30 into the FMS, which set the cruise speed for an economical fuel burn.
Example Sentence 2
Raising the cost index to 100 caused the FMC to command a higher Mach number and shorter flight time.