What is a Flight Director, and how does it differ from Autopilot?
The flight director is related to the autopilot system. It displays a guide on the artificial horizon, which shows the attitude of the airplane, but does nothing to control the plane. The guide represents a reference of an airplane attitude that will follow the parameters set for the autopilot. The pilot can manually fly the plane directly where the flight director indicates, and by doing so the plane will follow the parameters set for the autopilot.
If the autopilot is engaged, autopilot flies the plane to follow the flight director. The flight director serves as a visual indication of where the autopilot wants the plane to go. Although a flight director typically accompanies an autopilot system, some aircraft have a flight director without an autopilot.
The procedure to engage them is to first turn on the flight director, which will show where the autopilot wants the plane to be, and then to engage the autopilot, which will then automatically fly the plane.
This is what the autopilot controls can look like.
Here is the Boeing flight director, visible as the crossed magenta lines in the center of the screen.
It lets the pilot know what the autopilot would do if it were flying instead, by displaying an indicator, usually a miniature pink plane or line, on the artificial horizon.
The difference between the autopilot and flight director is that the autopilot flies the plane, the flight director gives the pilot an idea of what the autopilot would like to do if it was in charge.
It's the traditional flight instruments, integrated to work in a complementary way. For example the steering bars moving across the face of the compass so as to give the pilot visual ques for turning onto a desired heading and/or altitude.
The flight director gives a visual display of the overall aircraft attitude in space. The flight director display reacts to inputs coming from, for example, heading dials, electronic flight plans, and actual aircraft movement. The flight director does not physically control the aircraft.
The autopilot physically controls the aircraft in response to essentially the same inputs. The flight director essentially does not care if a human or the autopilot is actually in control. The autopilot adds intelligence which allows for various levels of pilot non-intervention up to the point of virtually hands-off control from takeoff to landing.
Together, the flight director and autopilot permit the pilot to let his skills atrophy while inducing a sense of inferiority on the part of the pilot in command; thus permitting well controlled crashes.
Simply: Flight Director is a system that computes desired pitch and roll from parameters like heading, altitude, vertical speed. Then an autopilot computes control surfaces deflection from given parameters: pitch and roll.
So: Pilot sets altitude, heading for Flight Director. Flight director then computes pitch and roll for Autopilot (and human pilot, too). If Autopilot is engaged, then it computes control surfaces deflections and moves the surfaces.
Flight Director is also displayed on PFD in some way, so if Autopilot is disengaged, the human pilot can still fly to follow FD's directions.