Fluid from the selector valve enters port “A” and leaves through port “B” to the down side of the actuating cylinder. When the flaps move downward into the slip stream of the aircraft, the aerodynamic force against the downward movement of the flaps increases, and this causes the pressure in the down line and the valve to increase. This increased pressure is transmitted to the bottom face of the piston through drilled passages in the lower end of the piston. If flaps are lowered at a higher speed, the pressure transmitted to the bottom face of the piston becomes great enough to overcome the force of the spring before the flaps are all the way down. The piston then moves up, and the port “A” is closed and the fluid can no longer flow to or from the actuating cylinder; hence the flaps remain in place unless the speed of aircraft increases or decreases.
A valve in the flap system that prevents the flaps from being lowered at speeds that may cause structural damage. If the pilot tries to lower the flaps at higher than permissible speeds, causing excessive air loads on the flap, the valve acts to prevent the flaps from being lowered. Also known as a blow-back valve