Cost Index and Economic Cruise

Understanding Cost Index in Flight Planning

The cost index (CI) is a numerical value used in airline flight planning and Flight Management Systems (FMS) to weigh the cost of fuel against the cost of time-dependent factors like crew wages, maintenance, and aircraft utilization. A low cost index (e.g., 0) prioritizes fuel savings, resulting in slower, more fuel-efficient cruise speeds. A high cost index prioritizes time savings, commanding the FMS to fly faster at the expense of higher fuel consumption.

Economic Cruise Explained

Economic cruise is the practice of selecting a cruise speed that minimizes the total operating cost for a flight. This speed is not always the same as the speed for minimum fuel burn (long range cruise), as it also considers time-related costs. The FMS can automatically calculate and fly the economic speed based on the entered cost index, adjusting for factors such as wind, temperature, and aircraft weight.

Cost Index Calculation and Effects

The cost index is calculated as the ratio of time cost per hour to fuel cost per hour. Airlines set this value based on current fuel prices and operational needs. For example:

• CI = 0: Maximum range, minimum fuel use
• CI = Maximum: Minimum time, higher fuel use

By changing the cost index, pilots can adapt to operational changes, such as delays, fuel price fluctuations, or the need to meet a specific arrival time.

Practical Application in Economic Speed Planning

During flight, the FMS uses the cost index to select the optimal cruise speed. Pilots can modify the CI in response to changing conditions, and the FMS will recalculate the economic speed instantly. This flexibility supports real-time optimization of fuel consumption and arrival times, helping airlines control costs and maintain schedules.