Aircraft Control Systems: Autopilot and Feedback Mechanisms

Autopilot System

• Control surface movement causes a change in aircraft attitude, which is then corrected by the sensor output, returning the control surface to its neutral position.

Autopilot Elements

• A basic autopilot consists of sensing, command, and output elements.
• The sensing element detects uncommanded movement of the aircraft about its three flight axes and the rate of that movement.
• The command element includes the autopilot mode select panel, flight controller, control wheel steering, and navigation systems.
• The output element consists of units that move the control surfaces of the aircraft in response to computer demands.

Sensing Elements

• A minimum of pitch and roll attitude, heading information, and airspeed and altitude data are required for autopilot function.
• Additional sensors can improve autopilot function, including pitch, roll, and yaw rate sensing, vertical acceleration, and air data information.
• A fully operational AFCS (Automatic Flight Control System) uses data from inertial navigation systems, radio receivers, global positioning systems, flight management systems, and radar altimeter information.

Command Elements

• The autopilot mode select panel is the primary interface between the flight crew and the automatic flight control systems.
• The panel contains switches and logic circuits for on/off control and mode selection for autopilot/flight director channels and the Autothrottle system.
• The mode-select panel autopilot engage switches have CWS (Control Wheel Steering) in place of MAN (Manual).

Control Wheel Steering

• The pilot can set the autopilot to control wheel steering mode, where the pilot's input is measured and used as an input signal to the flight control computer.
• The autopilot stays engaged and helps to move the control surfaces.
• The transducer on the control wheel converts the input force into an electrical signal.

Autopilot Override

• The pilot can overpower the autopilot at any time by applying a reasonable amount of pressure on the respective flight control.
• A typical overpowering force would be 25 to 35 pounds of turning force on the control wheel or 40 to 50 pounds of force on the control column.

Trim Indicator

• The trim indicator provides a visual display of the magnitude and polarity of the signal being applied to the flight control actuator or servomotor.
• Each pointer represents a primary flight control, and a pointer is deflected each time a servo amplifier command signal is applied to the actuator.
• The pilot can manually trim the aircraft until the pointer returns to the zero position.

Understand how autopilot systems work in aircraft, including the role of control surfaces, sensors, and feedback mechanisms in maintaining stable flight. Learn about the process of correcting aircraft drift and returning to a selected attitude.