Cost Index — What Does It Do?

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

Three cost index values on PERF INIT — 0, 30 and 80 — and the speed schedule changes under you.

What this scenario covers

Cost index expresses how much time is worth relative to fuel. Zero means fly for maximum range; higher values tell the FMC to spend fuel to save minutes, and the computed climb, cruise and descent speeds rise accordingly.

Cycling three values shows that it is not a cruise-speed knob — the entire speed schedule moves, and with it the predicted arrival time and fuel.

How it applies in flight ops

Airlines derive the figure from the real cost of time — crew, maintenance, aircraft utilisation, missed connections — divided by the cost of fuel. It changes with fuel prices and with the season, and different operators fly the same route at noticeably different speeds because of it.

Operations sometimes asks for a specific cost index to recover a delay. It works, and it costs fuel; the number simply makes that trade explicit rather than leaving it to the crew's judgement.

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

  1. Open the PERF INIT page so you can edit COST INDEX.

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

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

    CI 0 means "fuel is everything, time is free." The FMC picks the slowest cruise speed that still makes the route — Long Range Cruise. You burn the least fuel possible but the flight takes longer. Almost no airline operates CI 0 in real flying because crew and 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 and medium-haul airlines operate the 737. The FMC picks an ECON speed around Mach 0.78 in cruise — fast enough to hold the schedule, slow enough to save real fuel. Exact numbers vary per airline based on fuel-vs-crew cost ratios and fleet age.

  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 737NG'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 schedule-catch-up. Boeing's FMC accepts CI 0–999; values above 100 are rare in operations.

  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

  • CI is a time-versus-fuel ratio, not a speed setting.
  • It moves climb, cruise and descent speeds together.
  • Airlines set it economically and revise it as fuel prices move.
  • Recovering time by raising CI has a fuel price. That is the point of the number.

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