What does Cost Index do in an FMC?
In aviation and flight simulators, Cost Index tells the FMC how to trade fuel against time. A higher Cost Index usually commands faster ECON climb, cruise and descent speeds, but only within the aircraft's normal limits. It does not create extra thrust, ignore ATC restrictions or let the jet exceed maximum operating speed or Mach.
What does Cost Index actually control?
Cost Index is the FMC's way of balancing fuel cost against time cost. In airline terms, it is the ratio of time-related cost to fuel cost. You do not need the exact maths to use it in a sim; what matters is the direction of the trade-off.
With a low Cost Index, the FMC favours fuel efficiency and tends to choose a slower ECON profile. With a high Cost Index, it favours saving time and tends to choose faster speeds, accepting more fuel burn for a smaller reduction in trip time.
| Cost Index | FMC tendency | Usual trade-off |
|---|---|---|
| Low | Slower ECON climb, cruise and descent speeds | Less fuel burn, longer flight time |
| Medium | Balanced ECON profile | Middle ground on fuel and time |
| High | Faster ECON climb, cruise and descent speeds | More fuel burn, shorter flight time |
A mistake we see often is treating Cost Index like a direct speed command. It is not. The FMC still has to respect aircraft weight, altitude, temperature, winds, route constraints and certified speed limits.
Does a higher Cost Index make the flight faster?
Usually, yes. If the FMS is properly managing the flight, a higher Cost Index normally gives higher target speeds in climb, cruise and descent. In cruise that often means a higher ECON Mach or indicated airspeed, and in descent it can produce a less fuel-saving schedule.
The catch is that the time saving is often smaller than people expect. On short sectors, a large Cost Index increase may shave off very little time while burning noticeably more fuel. On longer sectors, the extra speed can help more, but once the aircraft is already near its efficient cruise Mach, pushing the number higher often gives diminishing returns.
Why isn't the aircraft speeding up when I raise Cost Index?
If you increase Cost Index and little changes, the FMC is usually being limited by something else.
- Managed or VNAV mode is not active. If you are flying selected speed or manual pitch/thrust, the FMS cannot apply its ECON schedule.
- Speed restrictions are active. SID, STAR and approach constraints can hold the aircraft below the speed the FMC would otherwise choose.
- You are already at a limit. The aircraft may already be close to maximum climb speed, maximum cruise Mach or another phase-of-flight limit.
- Performance data are incomplete. Missing weights, cruise level or other PERF initialisation entries can lead to odd predictions or fixed speeds.
- The add-on models Cost Index only partly. Some simpler FMCs barely use it, or only use it in cruise.
If you are flying an Airbus, our A320 MCDU/FMS setup guide for Microsoft Flight Simulator shows where Cost Index is entered and how it affects managed speeds. For a classic Boeing-style workflow, our beginner FMC programming walkthrough for FS2004 is a useful reference for the performance pages that need to be filled in before ECON speeds make sense.
Why does the same Cost Index behave differently in different aircraft?
Because the number itself is not universal. A Cost Index of 30, 50 or 100 does not map to the same speeds in every Boeing, Airbus or third-party add-on. The aircraft type, performance database and operating assumptions inside that FMC all affect the result.
That is why copying a Cost Index from one aircraft to another can produce strange behaviour. In some simplified FMCs, the value acts as a rough slow-to-fast bias. In fuller simulations, it feeds specific ECON calculations; our note on how the Universal FMC uses Cost Index in ECON cruise speed calculations is a good example of that logic.
What Cost Index should I use in a flight simulator?
The best Cost Index is the one that matches the flight you are trying to reproduce.
- Use the planned value if you have one. Airline dispatch paperwork or a prebuilt route may already include a Cost Index.
- Choose a low value when fuel efficiency matters. This usually suits longer flights and gives more conservative ECON speeds.
- Choose a middle value for a balanced profile. If you do not have a dispatch figure, this is often the safest starting point.
- Use a high value only for a reason. Trying to recover a delay or simulate a time-sensitive operation makes sense; setting an extreme number just to make the aircraft feel quicker usually does not.
If the aircraft still feels too slow or too fast, check the basics before blaming Cost Index. Gross weight, cruise altitude, winds, anti-ice use and route constraints often change the result more than the Cost Index entry itself.