so, the system would continuously overcorrect. Surface  deflection would occur until the desired attitude is attained.  But movement would still occur as the surface(s) returned  to pre-error position. The attitude sensor would once again  detect an error and begin the correction process all over again.  Various electric feedback, or follow-up signals, are generated  to progressively reduce the error message in the autopilot so  that continuous over correction does not take place. This is  typically done with transducers on the surface actuators or  in the autopilot servo units. Feedback completes a loop as  illustrated in  Figure 10-111 . commands. The rudder channel receives two signals that  determine when and how much the rudder moves. The first  signal is a course signal derived from a compass system. As  long as the aircraft remains on the magnetic heading it was  on when the autopilot was engaged, no signal develops. But,  any deviation causes the compass system to send a signal  to the rudder channel that is proportional to the angular  displacement of the aircraft from the preset heading. A rate system receives error signals from a rate gyro that are  of a certain polarity and magnitude that cause the control  surfaces to be moved. As the control surfaces counteract the  error and move to correct it, follow-up signals of opposite  polarity and increasing magnitude counter the error signal  until the aircraft’s correct attitude is restored. A displacement  follow-up system uses control surface pickups to cancel  the error message when the surface has been moved to the  correct position. The second signal received by the rudder channel is the  rate signal that provides information anytime the aircraft is  turning about the vertical axis. This information is provided  by the turn-and-bank indicator gyro. When the aircraft  attempts to turn off course, the rate gyro develops a signal  proportional to the rate of turn, and the course gyro develops  a signal proportional to the amount of displacement. The  two signals are sent to the rudder channel of the amplifier,  where they are combined, and their strength is increased. The  amplified signal is then sent to the rudder servo. The servo  turns the rudder in the proper direction to return the aircraft  to the selected magnetic heading. The following autopilot system description is presented  to show the function of a simple analog autopilot. Most  autopilots are far more sophisticated; however, many of the  operating fundamentals are similar.  As the rudder surface moves, a follow-up signal is developed  that opposes the input signal. When the two signals are equal  in magnitude, the servo stops moving. As the aircraft arrives on  course, the course signal reaches a zero value, and the rudder  is returned to the streamline position by the follow-up signal. The automatic pilot system flies the aircraft by using  electrical signals developed in gyro-sensing units. These units  are connected to flight instruments that indicate direction,  rate of turn, bank, or pitch. If the flight attitude or magnetic  heading is changed, electrical signals are developed in the  gyros. These signals are sent to the autopilot computer/ amplifier and are used to control the operation of servo units. The aileron channel receives its input signal from a transmitter  located in the gyro horizon indicator. Any movement of the  aircraft about its longitudinal axis causes the gyro-sensing  unit to develop a signal to correct for the movement. This  signal is amplified, phase detected, and sent to the aileron  servo, which moves the aileron control surfaces to correct for  the error. As the aileron surfaces move, a follow-up signal  builds up in opposition to the input signal. When the two  signals are equal in magnitude, the servo stops moving. Since  the ailerons are displaced from the streamline, the aircraft  now starts moving back toward level flight with the input  signal becoming smaller and the follow-up signal driving the  control surfaces back toward the streamline position. When  the aircraft has returned to level flight roll attitude, the input  signal is again zero. At the same time, the control surfaces  are streamlined, and the follow-up signal is zero. A servo for each of the three control channels converts  electrical signals into mechanical force, which moves the  control surface in response to corrective signals or pilot  The elevator channel circuits are similar to those of the  aileron channel, with the exception that the elevator channel  detects and corrects changes in pitch attitude of the aircraft.  For altitude control, a remotely mounted unit containing an  altitude pressure diaphragm is used. Similar to the attitude  and directional gyros, the altitude unit generates error signals  when the aircraft has moved from a preselected altitude. This  10-65 Figure 10-111.  Basic function of an analog autopilot system  including follow-up or feedback signal.  Autopilot Functions Gyro input signal Control surface feedback signal Feedback circuit Amplifier A B Servo