Engine-driven vacuum pump Suction from controller Diaphragm Cable clamp Wing leveler vacuum lines Turn coordinator Figure 10-107.  The wing leveler system on a small aircraft is a vacuum-operated single-axis autopilot. Only the ailerons are controlled. The aircraft’s turn coordinator is the sensing element. Vacuum from the instrument vacuum system is metered to the diaphragm cable  actuators to move the ailerons when the turn coordinator senses roll.  the aircraft’s controls so that any deviation from the desired  attitude of the aircraft is corrected. This is done by memorizing  the desired aircraft attitude and moving the control surfaces so  that the aircraft returns to that attitude. Rate based autopilots  use information about the rate of movement of the aircraft  and move control surfaces to counter the rate of change that  causes the error. Most large aircraft use rate-based autopilot  systems. Small aircraft may use either. mounted. Remote gyro sensors drive the servo displays in  the cockpit panel, as well as provide the input signals to the  autopilot computer. Modern digital autopilots may use a variety of different  sensors. MEMS gyros may be used or accompanied by the  use solid state accelerometers and magnetometers. Rate  based systems may not use gyros at all. Various input sensors  may be located within the same unit or in separate units  that transfer information via digital data bus. Navigation  information is also integrated via digital data bus connection  to avionics computers.  Most autopilot systems consist of four basic components,  plus various switches and auxiliary units. The four basic  components are: sensing elements, computing element, output  elements, and command elements. Many advanced autopilot  systems contain a fifth element: feedback or follow-up. This  refers to signals sent as corrections are being made by the  output elements to advise the autopilot of the progress being  made.  [Figure 10-108] The computing element of an autopilot may be analog or  digital. Its function is to interpret the sensing element data,  integrate commands and navigational input, and send signals  to the output elements to move the flight controls as required  to control the aircraft. An amplifier is used to strengthen the  signal for processing, if needed, and for use by the output  devices, such as servo motors. The amplifier and associated  circuitry is the computer of an analog autopilot system.  Information is handled in channels corresponding to the  axis of control for which the signals are intended (i.e., pitch  channel, roll channel, or yaw channel). Digital systems use  solid state microprocessor computer technology and typically  only amplify signals sent to the output elements.  The attitude and directional gyros, the turn coordinator, and  an altitude control are the autopilot sensing elements. These  units sense the movements of the aircraft. They generate  electric signals that are used by the autopilot to automatically  take the required corrective action needed to keep the aircraft  flying as intended. The sensing gyros can be located in the  cockpit mounted instruments. They can also be remotely  10-62 Computer and Amplifier Sensing Elements Autopilot Components