Cost index (CI) standardization at major U.S. carriers represents one of the more consequential—and least publicly discussed—operational decisions airline flight operations departments make, and Southwest Airlines' reported use of a CI near 20 on the Boeing 737-800 has drawn recurring attention among pilots and aviation enthusiasts. Cost index is a dimensionless number input into the Flight Management System (FMS) that expresses the ratio of time-related operating costs to fuel costs, expressed in units of (100 lbs fuel/hour) or (kg fuel/minute) depending on the FMS manufacturer. On Boeing aircraft equipped with the Smiths or Honeywell FMS, a CI of 0 commands maximum range cruise (minimum fuel burn), while a CI of 999 commands maximum speed cruise (minimum time, maximum fuel burn). A CI of 20 on the 737-800 would produce cruise speeds modestly above long-range cruise (LRC) but well below high-speed cruise—broadly consistent with a carrier that historically prioritized fuel efficiency and low unit costs as structural competitive advantages.
Southwest has never officially published its cost index policy, and airline-specific FMS procedures fall under proprietary operational documentation not subject to public disclosure. However, claims of a CI around 20 are broadly plausible given Southwest's well-documented emphasis on fuel hedging, fuel efficiency, and low CASM (cost per available seat mile). Southwest's fleet homogeneity—operating exclusively 737 variants—also makes a standardized, fleet-wide CI policy administratively straightforward to implement and enforce. Carriers operating diverse fleets must manage CI differently across aircraft types because the same numerical CI produces different speed schedules depending on aircraft weight, altitude, and the specific FMS installed. On a single-type operator like Southwest, a standard CI value can be embedded in SOPs and route-specific departure documents with minimal ambiguity.
From a practical standpoint for working airline pilots, cost index directly shapes the ECON cruise speed commanded by the FMS, affecting block time, fuel burn, and schedule recovery capability. A CI of 20 on the 737-800 typically yields cruise Mach numbers in the range of roughly M.78 to M.79 at typical cruise altitudes and weights—slightly below the common industry-standard LRC of approximately M.785 but within a narrow band that varies dynamically as the FMS recalculates optimal speed based on winds, weight, and altitude. Pilots operating under low CI regimes must be mindful that schedule recovery through speed increase comes at a steeply nonlinear fuel cost, since drag rises with the square of velocity. Dispatchers and flight operations controllers at carriers running standardized CI must therefore be more proactive with gate departure planning rather than relying on crews to absorb delays airborne.
The question of cost index transparency also reflects a broader industry dynamic in which airline crews may know their company's target CI from informal channels—logbooks, line indoctrination, union materials, or oral tradition—rather than from formal, version-controlled SOPs. This creates risk in training and standardization environments, particularly when new hires or check airmen from legacy carriers with different CI cultures enter the operation. The FAA does not mandate CI disclosure or standardization in operator manuals, leaving it to carriers to integrate cost index guidance into Flight Operations Manuals (FOMs), Route Books, or Dispatch Release documentation as they see fit. For business aviation operators and Part 135 certificate holders, cost index concepts are increasingly relevant as modern FMS installations on business jets (including the 737 BBJ variants, Gulfstreams, and Bombardier Global series) now incorporate ECON cruise modes that function on the same principles as their airline counterparts, making cost index literacy an increasingly expected competency across all segments of professional aviation.