Cost Index — What Does It Do?

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📍 Route: LIRF → LEMD⏱ ~3 minutes📊 5 steps🎓 Performance & Fuel

Most pilots type the cost index off the OFP and never think about it again. Try three values and watch what the number actually controls.

What this scenario covers

Cost index is the ratio the FMGS uses to trade time against fuel. A cost index of zero tells the box that time is free and fuel is everything, so it flies the maximum-range speed. A high cost index says the opposite — hurry, fuel is cheap relative to the cost of the delay — and the computed speeds rise accordingly.

Entering 0, then 30, then 80 on INIT makes the trade visible: the same aircraft on the same route flies different speeds and predicts different arrival times and fuel figures, purely because of one number.

How it applies in flight ops

Airlines compute a cost index from the actual cost of time — crew, maintenance, lease, the cost of a missed connection — against the price of fuel that week. It is an economic setting, not a performance one, and it is why the same operator will fly the same route at different speeds in different seasons.

Crews are occasionally asked to fly a specific cost index by operations to recover a delay, and it is one of the few levers available in the cruise. Knowing what it does — and that it changes the descent profile as well as the cruise speed — stops it being a magic number.

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Step-by-step: Cost Index — What Does It Do?

  1. Set up the route. Type LIRF/LEMD (Rome Fiumicino to Madrid) and press 1R.

    Cost Index (CI) is a single number that tells the FMS how to balance two costs: the cost of fuel, and the cost of time (crew, maintenance, schedule). The FMS uses CI to compute one optimum cruise speed (ECON SPD) and one optimum descent profile.

  2. Try CI = 0 first. Type 0 and press 5L.

    CI 0 means "fuel is everything, time is free." The FMS picks the slowest cruise speed that still gets the aircraft there — Maximum Range Cruise. You burn the least fuel possible but the flight takes longer. Almost no airline uses CI 0 in real operations because crew/maintenance costs make time too valuable.

  3. Now try CI = 30 — a typical airline value. Type 30 and press 5L.

    CI 30 is roughly where most short/medium-haul airlines operate. The FMS picks an ECON speed about Mach 0.78 in cruise — fast enough to keep the schedule, slow enough to save real fuel. The exact number per airline depends on their fuel-vs-crew cost ratio, fleet age, and route mix.

  4. Now try CI = 80 — pushing the speed up. Type 80 and press 5L.

    CI 80 means "we are late, get us there." ECON cruise climbs to roughly Mach 0.80 — pushing toward the A320's MMO of Mach 0.82. Fuel burn rises sharply because drag scales with speed squared. Used to recover from delays, ATC speed restrictions, or airline-mandated catch-up. The maximum CI an Airbus FMS accepts is 999, but values above 100 are rare in practice.

  5. Set CI back to 30 — the value you would actually fly. Type 30 and press 5L.

    The OFP your dispatcher prints will have a CI on it (e.g. CI 32, CI 28, CI 45). You enter exactly what they wrote — they have already balanced the day's fuel price and any schedule pressure. Your job is to type it accurately, not to guess a "good" value.

What to take away

  • Cost index trades fuel against time; 0 is maximum range, high values are speed.
  • It is an economic input set by the airline, not a performance limit.
  • Changing it changes the whole speed schedule, including descent.
  • Recovering a delay by raising CI has a fuel price — that is the entire point of the number.

The rest of Performance & Fuel

Background reading

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