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A320 Flight Control System Overview

Oct 2, 2025

Overview

This lecture explains the Airbus A320 flight control system, focusing on the fly-by-wire concept, computer-based control, redundancy, actuation of control surfaces, system displays, and high-lift devices.

Fly-By-Wire System Overview

  • The A320 replaces traditional cables and pulleys with electrical wires, reducing aircraft weight and allowing for advanced system integration.
  • Pilot inputs via the sidestick generate electrical signals, which are sent to flight control computers and then to hydraulic actuators (servo controls) that move the control surfaces.
  • Computers interpret pilot commands, ensuring the aircraft remains within safe flight limits and preventing excessive maneuvers.
  • There is no direct mechanical link between the sidestick and the control surfaces; instead, the system uses feedback loops (servo loops) for precise control.
  • This design increases aircraft stability, enhances safety, and reduces pilot workload by automating many control functions.

Flight Control Computers & Redundancy

  • The system is managed by seven computers:
    • 2 Elevator and Aileron Computers (ELAC)
    • 3 Spoiler and Elevator Computers (SEC)
    • 2 Flight Augmentation Computers (FAC), which manage rudder movements
  • Each computer is split into two physical units, programmed in different software languages, with separate power supplies and signaling paths for redundancy.
  • The system is designed so that no single failure—whether in a computer, electrical system, hydraulic system, or sensor—will degrade normal flight control functions.
  • Flight Control Data Concentrators (FCDC) collect data from ELAC and SEC computers for maintenance, recording, and failure indication. FCDCs interface with the Centralized Fault Display Interface Unit (CFDiu) and the ECAM system, storing maintenance data, failure history, and troubleshooting information.

Flight Control Surfaces & Actuation

  • All flight control surfaces are powered by three independent hydraulic systems: green, blue, and yellow.

  • Ailerons: Each aileron is powered by two servo controls from separate hydraulic systems. Normally, one servo is active (via ELAC), and the other is damped. Surface position is shown on the ECAM display.

  • Elevators: Each elevator is actuated by two independent hydraulic servo controls. In normal operation, one is active (via ELAC), and the other is backup. If electrical control is lost, the servos center the surface automatically. Elevator position is displayed on ECAM.

  • Trimmable Horizontal Stabilizer (THS): Used for pitch trim, actuated by a failsafe ball screw jack with two independent hydraulic motors. Manual control is via hand wheels on the center pedestal. On the ground, trim is set manually; in flight, 3 electrical motors

    control takes over, but manual override is possible. THS position is shown on ECAM and on a scale near each trim wheel.

  • Rudder: The single-piece rudder is actuated by three independent hydraulic servo controls, signaled mechanically from the pedals and yaw damper actuators. Rudder position is displayed on ECAM. A rudder travel limiter restricts movement at high speeds, controlled by the FACs.

  • Ground Spoilers and Speed Brakes: Each spoiler is powered by a single servo control, with hydraulic supply distributed for redundancy. If a failure is detected, the affected spoiler retracts and stays retracted. Spoiler position is shown on ECAM.

ECAM Display & Control Inputs

  • The ECAM (Electronic Centralized Aircraft Monitor) displays all flight control surface positions and system statuses, including hydraulic supply and actuator availability.
  • Ailerons and Elevators: Movements are shown by green indices on white scales. Actuator status appears beside the scale and turns amber if unavailable.
  • Rudder: Movements are shown by a green symbol on a white scale, with trim indicated by a blue line. Rudder travel is limited at high speeds, with limits shown by white ticks.
  • THS: Pitch trim position is displayed in degrees up or down, with hydraulic motor status shown above the indication.
  • Spoilers: Extended positions are indicated by arrows. Speed brakes use the three central spoilers, and their deployment is shown on ECAM. Computers automatically retract speed brakes in certain conditions (e.g., high angle of attack, flaps full, landing, or computer failure).
  • Sidesticks: Each pilot has a sidestick that sends independent electrical signals to the computers. If both are used, inputs are algebraically added. When autopilot is engaged, sidesticks and rudder pedals are locked in neutral by a solenoid.
  • Manual Controls: Manual pitch trim is via hand wheels; rudder trim is via a switch on the center pedestal, with automatic reset available. Speed brake and ground spoiler controls are on the center pedestal. The speed brake lever must be pushed down to select speed brakes and pulled up to arm ground spoilers.

High-Lift & Lift Augmentation Devices

  • Each wing has five leading-edge slats and two trailing-edge flaps, plus an aileron with a droop function. The A321 has double-slotted flaps.
  • Slats and flaps are powered by hydraulic power control units and electrically controlled by two Slat/Flap Control Computers (SFCC), each with separate channels for slats and flaps.
  • The system includes protections against asymmetry, runaway, overspeed, and uncommanded movement. Position feedback is provided by pickoff units.
  • Slat and flap positions are displayed on the engine warning display and ECAM, with white dots indicating extension. The flap lever on the right pedestal has positions: zero, one, two, three, and full, with detents and stops to prevent excessive movement.
  • The SFCCs and other flight control computers are located in specific avionics racks, and their built-in test memories can be accessed for maintenance.

Key Terms & Definitions

  • Fly-by-Wire: Aircraft control system using electrical signals and computers instead of mechanical linkages.
  • Sidestick: Joystick-style pilot controller replacing the traditional yoke.
  • ECAM: Electronic system displaying aircraft systems status, warnings, and control surface positions.
  • ELAC/SEC/FAC: Dedicated computers managing different flight control surfaces and functions.
  • THS: Trimmable Horizontal Stabilizer, the main device for pitch trim.
  • SFCC: Slat/Flap Control Computer, managing high-lift device operations.
  • FCDC: Flight Control Data Concentrator, collecting and relaying system data for maintenance and display.

Action Items / Next Steps

  • Review ECAM indications for all flight control and high-lift devices, including actuator and hydraulic status.
  • Practice identifying and operating sidestick, trim wheels, rudder trim, and speed brake controls in cockpit mockups or simulators.
  • Study the procedures for manual override and system redundancy in case of failures.
  • Complete assigned reading on A320 fly-by-wire, flight control computers, and high-lift systems for deeper understanding.

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