What does Cost Index do in an FMC?

Cost Index tells an aircraft’s FMC/FMS how to balance fuel burn against flight time. A low value favours fuel-saving ECON speeds; a high value favours faster climb, cruise and descent, at greater fuel cost. In Aviation & Real-World Flying and flight simulators, it is an optimisation input—not a throttle or direct speed command.

What does Cost Index actually control?

Cost Index primarily determines the aircraft’s ECON speed schedule. In airline operations, it represents the relationship between time-related operating costs and fuel cost. Time-related costs may include crew, maintenance, aircraft utilisation and disruption from delays.

The FMC combines the entered Cost Index with aircraft weight, altitude, temperature, forecast winds, performance data and speed restrictions. It then calculates economical targets for climb, cruise and descent. Changing the value may also update fuel predictions, estimated arrival time and the calculated descent profile.

A Cost Index of zero does not command idle thrust or the slowest speed at which the aircraft can fly. It tells the optimisation model to place no cost on time. Likewise, the highest accepted value does not override VMO, MMO, flap limits, route constraints or other aircraft protections.

Cost Index also does not select a take-off derate or create extra engine thrust. For a temporary ATC speed instruction, use selected speed or an appropriate speed constraint rather than changing Cost Index.

Does a higher Cost Index always make the aircraft faster?

A higher Cost Index usually produces higher managed or VNAV target speeds, but the difference may be small or temporarily hidden by another limit. In cruise, it commonly raises ECON Mach; during climb and descent, it can produce faster speed schedules where the flight profile permits them.

The time saving is often less dramatic than simmers expect. On a short sector, a large increase may save very little time while adding a noticeable fuel penalty. On a long flight the speed difference has longer to accumulate, but the benefit still diminishes as the aircraft approaches its normal operating limits.

Why is the aircraft not speeding up when I change Cost Index?

If changing Cost Index produces no visible response, either the new schedule has not been applied or another system is controlling the speed.

• Selected speed is active. The FMC can calculate a new ECON target without commanding it. Check the flight-mode annunciator rather than assuming the aircraft is in managed speed or VNAV.
• The entry was not activated. Some FMCs require the modified performance page to be confirmed or executed before recalculating the profile.
• A speed restriction is active. SID, STAR, waypoint and terminal-area restrictions take priority over the ECON schedule.
• The aircraft is already at a limit. VMO, MMO, turbulence limits or an aircraft-specific performance limit can prevent a higher target.
• The performance setup is incomplete. Missing gross weight, cruise altitude, winds or other required entries can leave the FMC using defaults or producing unreliable predictions.
• The value is outside the permitted range. Depending on the unit, it may be rejected, capped or silently replaced with a valid value.
• The simulated FMC has limited modelling. Simpler implementations may use Cost Index only for cruise, approximate its effect or accept the entry without changing the flight profile.

For Airbus operation, our A320 MCDU setup walkthrough for Microsoft Flight Simulator shows where Cost Index fits into INIT and performance preparation. Simmers learning a more traditional FMC layout can also use our screenshot-led FMC programming tutorial for FS2004 to see the surrounding flight-planning entries.

Why does the same Cost Index differ between aircraft?

The same numerical Cost Index can produce different speeds because its valid range, scaling and performance calculations are not universal. A value copied from one Airbus, Boeing or add-on should not be assumed to mean the same thing in another.

Aircraft aerodynamics, engine performance, FMC software and operator-configurable limits all affect the resulting ECON schedule. Some simplified systems treat Cost Index as a general slow-to-fast control, while fuller simulations calculate a specific economy speed; our explanation of Cost Index and ECON cruise in Universal FMC illustrates that relationship.

What Cost Index should I use in a flight simulator?

Use the planned Cost Index for the aircraft and flight being simulated whenever one is provided.

1. Start with the dispatch value. If the operational flight plan or scenario includes a Cost Index, enter that figure.
2. Follow aircraft-specific guidance. Use the add-on’s documented default or operator profile when no dispatch figure is available.
3. Choose according to the objective. Use a relatively low value when fuel or landing reserve is the priority; use a higher value when modelling schedule recovery or a time-sensitive operation.
4. Check the result. Review ECON climb speed, cruise Mach, descent speed, arrival time and predicted landing fuel before accepting the profile.

Do not assume that 50 is a universal middle setting, or that 100 is appropriate simply because it sounds like a percentage. Cost Index is not a percentage, and each FMC has its own accepted range.

Can Cost Index be changed during flight?

Most capable FMC/FMS implementations allow Cost Index to be changed in flight. Once the entry is accepted, the system recalculates ECON speeds and may revise ETA, fuel-at-destination and top-of-descent predictions.

The aircraft will not necessarily accelerate immediately: selected speed, constraints and operating limits still take precedence. In real operations, crews follow company and dispatch procedures before changing it; in a simulator, recheck predicted landing fuel and reserve before using a higher value to recover time.