a navigational aid that is overlaid on the attitude indicator that shows the pilot of an aircraft the attitude required to follow a certain trajectory.
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The flight director computes and displays the proper pitch and bank angles required in order for the aircraft to follow a selected path.
The FD is generally used in direct connection with the Autopilot, where the FD commands the AP to put the aircraft in the attitude necessary to follow a trajectory. The FD/AP combination is typically used in autopilot coupled instrument approaches such as CAT II and CAT III ILS instrument approaches. (same source)