Here's how an aircraft's autopilot system works ¹:

*Sensing 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 and 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 or remotely mounted

*Computer and Amplifier:*
- 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

*Output Elements:*
- The output elements of an autopilot system are the servos that cause actuation of the flight control surfaces
- They are independent devices for each of the control channels that integrate into the regular flight control system
- Autopilot servo designs vary widely depending on the method of actuation of the flight controls

*Command Elements:*
- 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)

*Feedback or Follow-up Element:*
- 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 4