Moving it to center position holds the brakes at any desired angle. Moving it aft opens the brakes. The switch is spring-loaded to neutral from the aft position only. Selecting the open position energizes the selector valve, porting hydraulic pressure from the combined hydraulic system to the extended side of the actuating cylinder. When the switch is positioned to closed, the opposite solenoid energizes.
Pressure is ported to the retract side of the actuating cylinders.
With the switch in neutral, hydraulic fluid is blocked from both the extend and retract sides of the speed brake cylinders. This action hydraulically locks the speed brakes. The wingtip speed brake control system normally depends upon the hydraulic flow regulators to maintain symmetrical extension of the left and right brakes. If a malfunction causes asymmetry of extension, an electrical disparity signal is sensed by the speed brake null detector. When the disparity between the extension of the left and right brake reaches 8 degrees, the null detector de-energizes the selector valves and causes the speed brakes to close.