Airbus A320 Flight Control Systems
The Airbus A320 flight control surfaces are all electrically controlled. When the electric system approves the action, the flight controls are hydraulically activated. Thereby, the THS and the rudder can also be mechanically controlled as a back-up system.

Electronic Systems

The A320 is an example of a civil electrical flight controlled aircraft. The distinctive feature of this aircraft is that all control surfaces are controlled electrically by high-level control laws in normal operation and that the system is designed to be available under all circumstances. This system was built to very stringent dependability requirements both in terms of safety (the system must not output erroneous signals) and availability. The basic building blocks are the fail-safe control and monitoring computers. These computers have stringent safety requirements and are functionally composed of a control channel and a monitoring channel. The control channel ensures the function allocated to the computer (for example, control of a control surface). The monitoring channel ensures that the control channel operates correctly.

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

With this respect, the A320 flight controls are composed of seven computers, and the auto-pilot of two. The flight control computers are of a control and monitoring type. The aircraft’s re-sponse to surfaces movement is feedback to both auto-pilot and flight controls computers. Through specific sensors (ADIRU, accelerometers) and dedicated screens it will be displayed to the crew.

Figure 1.2 Electronic Flight Controls

 
Hydraulic Systems

The A320 has three independent hydraulic systems, the green, blue and yellow system (figure 1.3). The green and yellow systems are each pressurized by an engine driven pump (1). The yellow hydraulic system can also be pressurized by an electrical pump (2). A PTU (3) enables the green system to be pressurized by the yellow system and vice versa. Fire shut off valves (4) are located between the reservoirs and the engine driven pumps. The blue hydraulic system is pressurized by an electric pump. In an emergency the blue system can be pressurized by the RAT (5). Each hydraulic system delivers a constant pressure to the users.

  1. Engine driven pump
  2. Electrical pump
  3. Power Transfer Unit
  4. Fire shut off valves
  5. Ram Air Turbine
Figure 1.3 A320 Hydraulic Systems
 

The hydraulic system is fully automatic, however each pump and the PTU has an associated pushbutton switch for abnormal operation. These buttons are located on the overhead hydraulic control panel (figure 1.4).

Figure 1.4 Hydraulic System Control

 
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