Aircraft with cable actuated control surfaces use two basic  types of electric motor-operated servos. In one, a motor is  connected to the servo output shaft through reduction gears.  The motor starts, stops, and reverses direction in response  to the commands of autopilot computer. The other type of  electric servo uses a constantly running motor geared to the  output shaft through two magnetic clutches. The clutches  are arranged so that energizing one clutch transmits motor  torque to turn the output shaft in one direction; energizing  the other clutch turns the shaft in the opposite direction.  [Figure 10-109]  Electropneumatic servos can also be used  to drive cable flight controls in some autopilot systems.  They are controlled by electrical signals from the autopilot  amplifier and actuated by an appropriate air pressure source.  The source may be a vacuum system pump or turbine engine  bleed air. Each servo consists of an electromagnetic valve  assembly and an output linkage assembly. Aircraft with hydraulically actuated flight control systems have  autopilot servos that are electro-hydraulic. They are control  valves that direct fluid pressure as needed to move the control  surfaces via the control surface actuators. They are powered by  signals from the autopilot computer. When the autopilot is not  engaged, the servos allow hydraulic fluid to flow unrestricted  in the flight control system for normal operation. The servo  valves can incorporate feedback transducers to update the  autopilot of progress during error correction.  The command unit, called a flight controller, is the human  interface of the autopilot. It allows the pilot to tell the autopilot  what to do. Flight controllers vary with the complexity of the  autopilot system. By pressing the desired function buttons,  the pilot causes the controller to send instruction signals to the  autopilot computer, enabling it to activate the proper servos  to carry out the command(s). Level flight, climbs, descents,  turning to a heading, or flying a desired heading are some  of the choices available on most autopilots. Many aircraft  make use of a multitude of radio navigational aids. These  can be selected to issue commands directly to the autopilot  computer.  [Figure 10-110] In addition to an on/off switch on the autopilot controller,  most autopilots have a disconnect switch located on the  control wheel(s). This switch, operated by thumb pressure,  can be used to disengage the autopilot system should a  malfunction occur in the system or any time the pilot wishes  to take manual control of the aircraft. As an autopilot maneuvers the flight controls to attain  a desired flight attitude, it must reduce control surface  correction as the desired attitude is nearly attained so the  controls and aircraft come to rest on course. Without doing  Figure 10-109.  reversible motor with capstan and bridle cable (left), and a single-direction constant motor with clutches that drive the  output shafts and control cable in opposite directions (right). Figure 10-110.  An autopilot controller of a simple autopilot system. Command Elements Feedback or Follow-up Element  10-64 PITCH UP DOWN TURN R L ON ON OFF NAV ALT MASTER DC motor Clutches Capstan Capstan Reversible motor Bridle cable Control cable Clutch activation signal from amplifier