An elastically expandable hollow displacement element is secured to the rear surface of an aircraft wing slat. When the displacement element is contracted, the slat may be retracted onto the wing's leading edge. When the slat is extended, the displacement element is expanded to protrude convexly from the slat, thereby preventing vortex formation in the slat air gap and reducing aero-acoustic noise. A pressure control system for inflating and deflating the displacement element includes a bleed air line connected from the aircraft engine bleed air system to the displacement element, a shut-off valve and a pressure control valve interposed in series in the bleed air line, and a slat contour controller connected by respective signal lines to the valves, which control the quantity and the pressure of the bleed air supplied into the displacement element, for properly inflating the same.