Eight cyclic valves are used in the deice system. Two cyclic valves are located in each outboard wing, one in each inboard wing, and one in each horizontal stabilizer. The cyclic valve is normally spring-loaded closed, electrically controlled, and pneumatically actuated. The cyclic valve controls the cyclic distribution of bleed air to the appropriate segment of the deice system piccolo tubes. The cyclic valve is pneumatically controlled by a bleed-air solenoid valve (fig. 1-13). When the solenoid is not energized, the spring keeps the cyclic valve closed, and inlet pressure is vented to ambient and to the closing side of the valve (chamber A). When the solenoid is energized, the inlet pressure is blocked, and when it builds to

Figure 1-13.- Bleed-air solenoid valve.

16 psi, it overcomes the spring and opens the cyclic valve.

Wing and Empennage Deice Timing Controller

The timing controller is located in the left load center. This device collects logic information necessary to provide control signals to the cyclic valves through solid-state sequence switching. The controller provides 30-second sequential control signals to each cyclic valve. It also receives the 60 temperature sensor indication and interrupts the valve-open cycle when aircraft skin and structural temperatures exceed 60 3 F (15 1.6 C). The timing controller collects logic signals to control deice operations in various failure modes.