Auto-Pilot vs Flight Director
Auto-Pilot in its most basic version is a Flight Director plus servos.
In either system, the computers have many different inputs available, some of which are indicated.
The gyro inputs are always in use, since they are basic to the operation.
The inputs in use by an autopilot or flight director depend upon its mode of operation.
Modes of operation in the roll channel could be following
a selected heading
a selected radio path
merely maintaining wings level
A pitch channel mode might maintain a certain
certain vertical speed
follow a glide path
Whatever the mode of autopilot operation, there is a set of servos of some kind operating the control surfaces as required to achieve the needed aircraft attitude. The computers compute the necessary servo commands which operate the control surfaces.
Airplane attitude changes are reflected in the gyro output signals. This is called aerodynamic response. Gyro signals to the computers are follow-ups to commands. Control surface position information is also follow-up (feedback).
The same set of computers could be used as a flight director.
The difference is that the flight director does not have a set of servos to operate the aircraft control surfaces.
All the flight director can do is command the pilot to operate the control surfaces.
If the flight director wants the elevators raised, it moves the horizontal command bar, called pitch command bar, up. If it wants the elevators lowered, it moves the pitch command bar down. The pilot is then supposed to operate the control column accordingly.
If the flight director wants the right wing moved down, it moves the vertical command bar, called roll command bar, to the right. If it wants the left wing down, it moves the command bar to the left.
The pilot then operates his control wheel in accordance with that command.
The job of the pilot is to keep the two command bars centered. Whenever one moves away
from the center position, he operates the control column or the control wheel as directed by the command bars.
In earlier airplanes, the autopilot system and the flight director system have separate
computers and the flight director computers are generally less sophisticated than the autopilot computers.
In airplanes built today, the autopilot computers are also used as flight director computers.
The pilot can, at his discretion, allow the autopilot to operate the controls, or he can follow the commands of the flight director.
Typically, he uses the autopilot to control the airplane, and the flight director becomes a monitor of autopilot operation.