Airbus A320 Back-up Systems
An A320 uses digital computers for controlling its flight controls. Therefore a system was introduced where different flight control laws are responsible for a safe flight envelope. One of the contributions of the electrical flight controls to the safety of the aircraft is the protections which are an integral part of the flight control laws. Through these laws the way some systems monitor and control the flight controls become different during specific flight situations and the backup systems are activated.
Flight laws and protections
During flight, a sidestick or an autopilot sends a message to the flight control computers demanding an aircraft manoeuvre. The flight control computer processes the demand and sends it to the control surfaces. The processing uses preset limitations and instructions called laws. The most comment law is normal law. It is modified depending on the phase of flight. It operates in three modes (figure 1.1). Together with these laws there are also all sorts of protections. Such as g-load factor, speed and high angle-of-attack that prevents the aircraft from stalling. These protections lighten the pilot’s workload, in particular, during avoidance maneuvers whether for an obstacle or windshear. These protections enhance safety.

Figure 1.1 - Airbus flight modes

Backup laws come in to play when problems and failures occur during one of the three modes. A single failure can not result in the loss of normal law. However, multiple failures of flight control, hydraulic or electrical systems may result in a degradation of the flight control normal law. The level of degradation will depend on the severity of the failures. Degradation of the flight control laws is sometimes referred to as reconfiguration. Control law reconfigurations are divided into the following two families. The alternate law and the direct law. If multiple failures of redundant systems occur, the flight controls revert to alternate law. After the alternate law comes the direct law and it is the lowest level of computer flight control and occurs with certain multiple failures.
Back-up flight controls
The first type of failure to be taken into account is the hardware failure of the system’s equipment. As the computers are controlling and monitoring computers, due this it makes control surface runaway by a computer extremely improbable. The failure of a computer will therefore lead to it being shut down. The actuators are monitored by the computers both by the monitoring channels and the control channels (figure 1.2). Each actuator (1) is controlled by a differ-ent computer. One actuator for each surface is always is active mode (2). The other stays in damping mode (3) and is monitored by its associated computer.

Figure 1.2 - Elevator ailerion control

The various sensors in the system comprise another runaway source. Each sensor is at least duplicated so that all information used is consolidated by comparison between at least two different sources of information. The electrical power is normally supplied by two generators each driven by a different engine. Also, an auxiliary generator, batteries and a RAT are available. In case a multiple engine failure occurs, the RAT is automatically extended. It then pressurizes the yellow hydraulic system which drives a third electrical generator. The computers are connected to at least two power sources. The aircraft has three hydraulic systems. One of which is sufficient to control the aircraft. Two systems pressurized by each engine, the third one being pressurized either by an electric pump or by the RAT.

For every flight control there is a different solution in the way of backing it up. For example if on the Engine/Warning Display E/WD the message occurs that one of the ELACs is not working properly, the other ELAC will attempt to take over his function. If that fails, it means that the ailerons are not active anymore. They are automatically switched in damping mode. The roll function is still available via the spoilers. SEC2 computer will now take over the control over the elevator and the THS. If SEC2 fails it will be taken over by SEC1. In case of total loss of normal control or hydraulic supply, the elevators are automatically set to and maintained in the neutral position. Mechanical control of the THS is always available via the pitch trim wheels. And for the rudder, mechanical control via the rudder is always available.

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