Airbus A320 Primary Flight Controls
In contrast with the flight control system installed on conventional aircraft, the Airbus A320 is fitted with a fly-by-wire flight control system. This means that the mechanical linkage between control column and control surface has been replaced by electrical wires. Just like the Boeing 737, the Airbus A320 flight controls are divided into primary and secondary flight controls. Both the primary and secondary flight controls are controlled by a total of 7 computers. The primary flight controls fitted on the aircraft are controlled by sidestick inputs and digital processing by the Elevator Aileron Computer (ELAC), the Spoiler Elevator Computer (SEC) and Flight Augmentation Computer (FAC). When the primary flight controls on the Airbus A320 are being operated, electrical signals from the sidestick or Flight Management and Guidance System (FMGS) are send to the flight control computers before being passed to the flight control hydraulic actuator.

Roll Control
Roll control on Airbus A320 is accomplished by use of sidestick movements or autopilot com-mands. When the ailerons are moved by operating the sidestick, electrical signals are being sent to the active ELAC computer. The Airbus A320 has two operational ELAC computers available, one operating in active mode while the other operates in damping mode and serves as back-up in case of failure. The ELAC then sends a signal to both the SEC computer, which controls the flight spoilers, and the FAC computer which sends turn coordination orders for the rudder. Unlike the SEC, which consists of three independent computers, the FAC has two work-ing computers in the same order as both ELAC’s. The computers then process these signals into an output which activates the hydraulic system actuators connected to the control surfaces. The control surfaces will then deflect according to sidestick input. For safety matters, the proc-essing of signals by the flight control computers use pre-set limitations and instructions called laws. This means that pre-scribed limitations can not be exceeded. When the aircraft is controlled by the autopilot, the ELAC’s and FAC’s receive electric signals generated by the FMGS. Normally ELAC one (green) is in control. In case ELAC one fails, ELAC two automatically takes over control. Similarly the FAC and SEC computers are being backed-up.

Figure 1.1 Flight Control Computer Overview

  1. Aileron hydraulic actuator
  2. Rudder hydraulic actuator
  3. Spoiler 2 hydraulic actuator
  4. Spoiler 5 hydraulic actuator
  5. Spoiler 3-4 hydraulic actuator

Normally ELAC one (green) is in control. In case ELAC one fails, ELAC two automatically takes over control. Similarly the FAC and SEC computers are being backed-up.

The ailerons have two electrically controlled hydraulic actuators connected to each aileron. One of these actuators is in control while the other actuator is in damping mode. The actuators are connected to the Green and Blue hydraulic systems.

Pitch Control
On the Airbus A320, pitch control is maintained in two ways. Primarily the elevators are used to pitch the aircraft. Besides the elevators there is the Trimmable Horizontal Stabilizer (THS), where the elevators are attached to, which is used to trim the aircraft.

The elevators are operated by either sidestick movement or, in case of autopilot engagement, by the FMGS. In manual control when the sidestick is used to operate the elevators, electrical signals are generated by the sidestick and send to both ELAC computers. The signals received by the ELAC computers are then converted into outputs which drive the hydraulic system actuators connected to the elevator. Normally ELAC one is in control with ELAC two serving as back-up. In case of dual ELAC failure, SEC one or two automatically takes over control.

Figure 1.2 Elevator Control

  1. Elevator Hydraulic Actuators
  2. Trimmable Horizontal Stabilizer

When the autopilot is in command, this means the FMGS is maintaining pitch control. It sends electrical signals to both ELAC’s which produce an output activating the elevator actuators. Each elevator is powered by two hydraulic actuators, one in active mode while the other serves as back-up. Both actuators become active in case of large pitch demands.

Trimmable Horizontal Stabilizer - THS
The THS is positioned by a screw actuator and driven by two hydraulic motors. In turn the hydraulic motors are driven by one or three electric motors. Only one electric motor can be operative at a time while the other two are in a standby role. The electric motors are being controlled by either ELAC or SEC computers.

Operating the THS by using the mechanical trim wheel has priority over the electrical trim (figure 1.3).

Figure 1.3 THS Trim Wheel

Directional Control
The rudder is being controlled in three different ways, namely; manually by rudder pedal movement, rudder trim or, in case of autopilot engagement, by autopilot commands.

Rudder Pedal Control
There are two pairs of rudder pedals installed and are connected mechanically to each other. They are linked to the artificial feel unit by a cable loop, which in turn is connected to the hydraulic rudder actuators via a differential unit. Movement of the pedals generates a signal which is send to the active ELAC. The ELAC then calculates yaw damping turn coordination and rudder trim orders and sends these to the FAC computers. Both FAC computers control the yaw damper servos, rudder trim and rudder limit motors.

Rudder Trim
Rudder trim is achieved by two electric motors, each controlled by its associated FAC. Rudder trim is done manually by operation of the rudder trim control (figure 1.4) located on the over-head panel.

Figure 1.4 Rudder Trim Control

Autopilot Rudder Movement
When the autopilot is engaged, both FAC computers receive commands from the FMGS for rudder trimming and yaw control (figure 1.5).

  1. Variable Stop Unit
  2. Yaw Damper Servos
  3. Rudder Trim
Figure 1.5 Autopilot Rudder Movement

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