Airbus A320 Secodary Flight Controls
The secondary flight controls, used during flight phases which require an increase in lift or drag, are commanded by either SEC or by the Command Sensor Unit (CSU). Just like the primary flight controls the secondary flight controls are controlled by FCC’s and powered by independent hydraulic systems, which in case of a failure, function as backup. The LE and TE devices of an A320 are operated by different servojacks and are also hydraulically operated.

Leading and Trailing Edge devices
High lift augmentation is achieved on each wing (figure 1.1) by the five LE slats (1), in conjunction with the two single element Fowler flaps on the TE (2) of the wing. Airbus delivers the flaps single slotted, but most airliners choose to modify them into multiple slotted flaps because of the higher take-off performance. The slats and flaps are hydraulically actuated like all the other moveable surfaces on the wings.

The positioning of the slats and flaps can be accomplished using the flap lever located in the cockpit on the centre pedestal (figure 1.2). The lever selects simultaneous operation of the slats and flaps and the collar must be lifted before selection of any position. The signal from the flap control lever is signalled via a CSU to two identical computers, the Slat Flap Control Computers (SFCC). Each SFFC has two channels, one for the slats and one for the flaps. Each of these channels can drive its associated surfaces. The slat and flap drive systems are controlled and monitored by the SFCC, each of them controls one slat and one flap hydraulic motor. In the SFCC is a valve block. Each valve block consists of three solenoid valves. Two of the valves, the directional valves operate the Control Valve Spool (CVS) for the retraction and the extension. The third, the Enable Solenoid Valve (ESV), oper-ates the Pressure Off Brake (POB).

Position feedback, instrumentation and recording is provided by Position Pick-off Unit (PPU) located in PCU of each drive system. PPU’s are also used for both detection of asymmetric operation and for system monitoring. The desired position and the actual position of the slats and flaps given by one of the PPU’s, the Feedback Position Pick-off Unit (FPPU), is being compared in the SFFC. Subsequently after the comparison and the matching of data, a signal is transmitted to one of the solenoid valves. This makes sure that the flaps and slats are put in the right positions. Finally if they are in the right position, the POB takes care that the slats and flaps remain in the same position.

The spoilers used on the A320 are divided into two main activities. For roll control; the roll spoilers are the most significant surface controls used. The spoilers are also used for decelerating the aircraft during flight and during landing using the speed brakes.

Roll Spoilers
Roll control is provided on each wing by one aileron, augmented by four roll spoilers mounted on the upper wing surfaces. Surface controls are actuated by independently supplied, electrically signalled, hydraulic servojacks. The position of the roll spoilers is based on signals from the sidestick controllers processed by the electrical FCC or by the automatic pilot. If the sidestick is moved it generates a signal towards the SEC, and to one of the ELACs. Then the SEC passes the signal through to one of the servojacks which makes sure the spoilers are in the right position.
During the flight the two outer spoilers can be used in combination with the aileron to compensate vertical acceleration due to gusts. This Load Alleviation Function (LAF) is accomplished by the electrical flight control system. The LAF is implemented in the ELAC and SEC. There are four specific accelerometers that are installed to provide the flight computers with vertical acceleration values. These sense the up gust and deploy the spoilers to smooth out the normal result of an up gust of wind.

Ground Spoilers and Speed Brakes
There are three speed brakes located on the upper surface of each wing. Each speed brake group is operated by one servojack controlled by the corresponding electrical flight computer, which is also used for the roll spoilers. All of the five spoilers mounted on each wing of the A320 are used as ground spoilers during landing. They are automatically extended after touchdown when specific ground conditions are fulfilled.
Conditions such as the selection of one engine thrust reverser or the speed brake lever being armed to the deploy position (figure 1.3).

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.

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