Refer to figure.
EASA learning objective 022.06.01.01.04 states: List the following elements of a closed-loop control system and explain their basic function: input signal; error detector; signal processor providing a measured output signal according to set criteria or laws; control element such as an actuator; feedback signal to error detector for comparison with input signal

All aircraft automatic control systems employ the principle where feedback from an action or state is compared to the desired action or state, this is known as a 'closed loop'.

Modern autopilot systems use two loops, the closed or inner control loop senses displacements from a set parameter and corrects it. The open or outer control loop takes inputs from the mode control panel (MCP) or flight management computer (FMC) and adjusts the inner loop to acheive the new parameter. The following example illustrates how a closed-loop system uses feedback loops to correct errors in pitch.

• A rate gyro (or an inertial reference unit) senses a change in pitch attitude
• Error signal is sent to the autopilot computer (signal processor) which compares this input with the reference attitude and computes an appropriate elevator command
• Correction input passed to elevator actuator
• Elevator actuator moves elevator and a feedback loop compares the actual and commanded movement of the elevator.
• Autopilot computer continuously compares rate gyro signal with required attitude
• Correction signals reduce as difference between the current and required pitch attitude reduces.
• When the correction signal is zero the elevator will have returned to its steady state.

Autopilot control laws built into the signal processor take into account aircraft speed and configuration to prevent flight control deflections exceeding structural limits, this is controlled by a rate feedback loop NOT the control surface position feedback loop.