Thats actually an interesting point rivetgun,
johns7022 cost index is a number entered into the FMS at the flight planning stage to determine the efficiency or speed of the flight. Best endurance and LRC will always occur at the same angle of attack but at different speeds for different weights.
Typically if 0 is entered as the cost index number the FMS will calculate speeds etc to fly for minimum fuel burn (and usually speed) and generally this will be as slow as possible. If you entered 999 (the cost index usually goes from 0-999) the FMS will calculate the flight to be flown at max speed and not worry at all about fuel flow.
Can anyone with answer rivetguns Q?? would be interested to know also.
rocket