High Lift Devices On Aircraft
HIGH LIFT
Slats and flaps achieve the high lift function. There are two flaps, inboard and outboard, and five slats on each wing, numbered from wing root to wing tip. The A321 has double slotted flaps. The slats and flaps are electrically controlled and hydraulically operated. Two Slat Flap Control Computers (SFCCs) do the control and monitoring. Each computer has one slat and one flap channel.
PCU
The slat and flap systems are similar. A Power Control Unit (PCU) drives each system with two hydraulic motors coupled to a differential gearbox. Torque shafts and gearboxes transmit the mechanical power to the actuators, which drive the surfaces.
MOTOR
Each motor is powered by a different hydraulic system and has its own valve block and Pressure Off Brake (POB). Valve blocks control the direction of rotation and the speed of their related PCU output shaft. The POB locks the transmission when the slat and flap surfaces have reached the selected position or if hydraulic power fails.
Wing Tip Brake
Wing Tip Brakes (WTBs) are given in order to stop and lock the system when major failures are detected. They are hydraulically activated and can only be reset on ground. Position Pick-Off Units (PPUs) send slat and flap position feedback to the SFCCs and ECAM. Flap sensors installed between inboard and outboard flaps inhibit further flap operation when a flap attachment failure is detected.
The signal is sent to the SFCCs via the Landing Gear Control and Interface Units (LGCIU). To prevent an aircraft stall, slats cannot be fully retracted at high angles of attack or low speeds (Alpha/speed lock function). The FLAPS lever has five positions: 0, 1, 2, 3 and FULL.
Two configurations correspond to position 1: Configuration 1 and Configuration 1+F. These are selected as shown on the graphic. The flaps lever selects simultaneous operation of the slats and flaps. The five lever positions correspond to the surface positions as shown on the graphic.
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