From the Honeywell Boeing 777 Flight Management System - Pilot’s Guide:
COST INDEX
The FMS normally flies the aircraft in the economy (ECON) mode. The
computed ECON speed results in minimum cost per mile flown or
maximum distance per pound of fuel. ECON Mach is calculated in the
performance database and is a function of gross weight, selected
altitude, temperature, and cost index. Cost index is a number that
governs the speed the aircraft flies. The higher the number the faster
the speed, which saves time. The lower the number the slower the
speed, which saves fuel. Valid entries are 0 to 9999.
Cost index is the ratio of the flying time to the cost of fuel. It is
determined by dividing the dollar cost per hour to operate the aircraft,
excluding fuel, by the cost of fuel in cents per pound.
For example, if it costs $1200 per hour for flying time and 10 cents per
pound for fuel, the cost index is 120.
If the cost of fuel increases to 20 cents per pound the cost index is 60.
The aircraft would fly slower to save fuel.
If in the example, flying time per hour increased to $1500 per hour, the
cost index would then be 150. The faster speed would save time.
Determining an airlines’ cost of flying time per hour depends on the
airline’s economic situation and how they figure operating expenses. It
can include insurance, crew costs, maintenance, passenger handling,
etc. Each airlinemust decide its particular priorities and use a cost index
that achieves the desired results. Segment costs can vary with the
direction of flight over a specific route, and whether the flight is domestic
or international.
If an airline is not certain what cost index to use over a new route
segment, a good starting point is to select a cost index that produces
a cruise Mach number close to long range cruise (LRC). This can be
determined prior to departure by first entering a cost index on thePERF
INIT page and then checking the ACT CRZ page for the resultingMach
number. After flying the route several times, the cost index can then be
adjusted as needed to better fit the route segment.
Cost index is only associated with ECON speed mode. When flying
ECON speed, cost index may vary the speed slightly due to changing
wind conditions. This is a normal function of cost index since its major
purpose is to constantly optimize economy of flight.