The pressure-operated sequence valve, also called a priority valve, looks like a check valve externally. Like a check valve, the installation position is indicated by an arrow. Figure 8-14 shows this valve installed in a wing fold system.

During the wing folding cycle, pressure-operated (priority) valves sequence the movement of the lockpins and fold actuators. These valves ensure lockpin actuation before fold actuator operation. This completely automatic valve consists of a body containing a spool, seat, poppet, related springs, seals, and an end cap.

When the wing fold selector valve is in the fold position, it directs fluid both to the wing lockpin and to the pressure-operated sequence (priority) valve. System pressure drops in the wing fold system because of the amount of pressurized fluid needed to actuate the lockpins. This lowers pressure below that needed to open the pressure-operated (priority) valve. View A of figure 8-14 shows insufficient pressure to unseat the spool. When lockpins have completed their travel, system pressure builds until it overcomes spring tension and causes the poppet to unseat the spool (view B of fig. 8-14). Fluid then flows freely through the valve to the wing fold actuators. View C of figure 8-14 shows the free-flow position of the valve. When spreading the wings, return fluid moves the seat from the spool compressing the poppet spring, which causes the poppet to bottom and allows free flow of fluid through the valve.

All aircraft incorporate emergency systems that provide alternate methods of operating essential systems required to land the aircraft safely. These emergency systems usually provide pneumatic or hydraulic operation of the essential systems; however, in some cases due to the design, they maybe operated satisfactorily through mechanical linkage. When using the pneumatic or hydraulic emergency system, that pressure must be directed to the unit concerned; emergency pressure must not enter the normal system, especially if the pneumatic type system is used. To allow operating pressure to reach the actuating unit and still not enter the other system, a shuttle valve is installed in the working line to the actuating unit. The main purpose of the shuttle valve

Figure 8-14.View of priority valve.

is to isolate the normal system from the emergency system.

Shuttle valves are located close to the actuating unit concerned. This location reduces to a minimum the units to be bled and isolates as much of the normal system from the emergency system as possible. In some installations, the shuttle valve is an integral part of the actuating unit.

A typical shuttle valve is shown in figure 8-15. The body contains three ports-the normal system inlet port, the emergency system inlet port, and the unit outlet port. A shuttle valve used to operate more than one actuating cylinder may contain additional unit outlet ports.

Enclosed in the body is a sliding part called the shuttle. It is used to seal one of the two inlet ports. A shuttle seat is installed at each inlet port. During operation, the shuttle is held against one of these seats, sealing off that port. These parts are held in the body by end caps. External leakage is prevented by an O-ring gasket at each end cap.