Flight director (aeronautics)
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The flight director computes and displays the proper pitch and bank angles required for the aircraft to follow a selected path.
A simple example: The aircraft is in level flight on a heading of 045 degrees and at an altitude of 15,000 feet maintaining a speed of 260 kts, the FD bars are thus centered. Then the flight director is set to a new heading of 090 degrees and a new altitude of 20,000 feet. The aircraft must thus turn to the right and climb. This is done by rolling to the right and pulling up. The roll bar will deflect to the right and the pitch bar will deflect upwards. The pilot will then pull back on the control column while rolling the aircraft to the right. Once he reaches the proper pitch and bank angle the FD bars will again center and remain centered until it is time to roll back to wings level (when the heading starts to approach 090). When the aircraft approaches 20,000 feet the pitch bar will deflect downwards thus commanding the pilot to reduce pitch in order to level off at the new altitude.
The FD is generally used in direct connection with the Autopilot (AP), 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 low instrument approaches (below 200 feet AGL), or CAT II and CAT III ILS instrument approaches.
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