blow-back valve

blow-back valve

The valve in the flap system that prevents the flaps from being lowered at too high an air speed. If the flaps are down and the air speed increases to too high a value, the flaps will retract automatically. Also called a flap-overload valve.
Blow back valve or flap overload valve: Fluid from the selector valve enters port A, flows around cutaway section of the piston, and leaves through port B to the down side of the actuating cylinder. The flaps move downward into the slip stream of the aircraft, but the force that resists the downward movement of the flaps increases, and this causes the pressure in the down line and the valve to increase also. The 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 speed that is too great for this operation to be accomplished safely, 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 by a shoulder on the bottom end of the piston. 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.