05/30/2026
Why Two Planes Fly the Same Route at Different Speeds
Commercial flight operations require a continuous balance between two primary cost drivers: fuel expenditure and time-related operating costs. The mechanism airlines use to quantify and manage that balance is called the cost index.
The cost index is calculated by dividing time-related costs by fuel cost per unit of fuel. Time-related costs encompass crew compensation, airframe maintenance, airport and handling fees, and the downstream costs of schedule disruption — including missed connections and repositioning. The resulting figure is entered into the Flight Management System prior to departure, where it drives the FMS's ECON speed calculations for all phases of flight: climb, cruise, and descent.
A cost index of zero instructs the FMS to minimize fuel burn entirely, producing maximum range cruise speeds and profiles optimized purely for fuel efficiency. In practice, most airlines operate somewhere between these extremes, with the specific value reflecting current fuel pricing, schedule pressure, and operational priorities.
It's worth noting that on short-haul sectors, a high cost index often yields minimal time savings while producing a disproportionate increase in fuel burn — making the tradeoff less favorable than it might appear on longer routes.
The cost index is not a static value. Airlines adjust it between flights and sometimes mid-route based on fuel price fluctuations, weather deviations, ATC-driven delays, and load factors. The FMS recalculates ECON speeds accordingly.
The practical result is that two aircraft operating the same city pair on the same day may cruise at meaningfully different speeds — not due to aircraft performance differences, but because the economic inputs driving their FMS calculations are different.
Bonus Question: What's the highest cost index you can enter into a 737's FMC?
