Cost Index

Cost Index (CI) is a numerical value entered into the Flight Management System (FMS) that represents the ratio of time-related costs to fuel costs. It is used to compute the most economical climb, cruise, and descent speeds.

• CI = 0: Maximum Range speed — minimizes fuel burn (flies slower)
• CI = max (e.g., 999): Maximum Speed — minimizes flight time regardless of fuel
• Optimal CI: Balances crew costs, maintenance costs, passenger time value, and fuel price

Airlines adjust Cost Index based on fuel prices, schedule constraints, and delay costs. A delayed flight may fly with a higher CI to recover schedule, while a fuel-expensive route may use a lower CI.