Main Types of Valves Within the Aircraft Hydraulic System

The aircraft hydraulic system is a critical element to a number of planes, allowing for pilots to operate landing gear, brakes, flaps, and other control surfaces with ease. As the system requires pressurized liquids that can transmit power through compression for functionality, a number of valves are needed in order to control speed, direction, and flow of fluids. Depending on the operation, a number of valve types may be utilized within a hydraulic system, and common valves include selector valves, sequence valves, check valves, shuttle valves, priority valves, quick disconnect valves, and hydraulic fuses.

The selector valve is a hydraulic component that allows for the governing of actuating cylinder movement, allowing for liquids to pass in and out of the unit. Selector valves may be either open-center or closed-center, each changing the flow of liquids based on a state of actuation. With an open-center selector valve, hydraulic fluids can pass through the valve while the selector is not in actuation position. Meanwhile, closed-center selector valves impede the flow of liquids while the valve is in either a neutral or off position. Depending on the need, selector valves come in the form of poppet-type, plug-type, rotary-type, or spool-types valves.  

With a sequence valve, branches of circuits are governed in order to achieve a sequence of operations, such as having landing gear doors open for the subsequent deployment of the landing gear with one control. To achieve such results, a sequence valve is installed in each of the actuating lines of paired equipment and systems. Similar to a relief valve, sequence valves  are capable of diverting fluids to additional actuators or motors when a set pressure is reached, allowing for operations to be continued in other parts of the hydraulic system. For sequence valves to properly function, they may be operated by pressure, mechanical means, or electric switches.

The check valve is a common hydraulic valve type, and they are implemented in order to provide unimpeded flow for fluids in a single direction, ensuring that fluids are unable to move in the opposite direction. Check valves may operate independently within the aircraft hydraulic system, or they may be a part of a greater component as an integral check valve. For the construction of a check valve, a spring loaded ball and seat are placed within a housing, and the spring compresses in order to allow for the flow of fluids in a specified direction. If fluids cease to pass through the valve, the spring forces the ball against the seat, closing off the orifice to impede the backwards flow of fluids.

Shuttle valves are most commonly used in systems that require a supply of fluids from multiple sources. Within such systems, a shuttle valve can be used to separate the primary source from the emergency or alternate system, allowing for control during failures. While shuttle valves are simplistic in their design, they serve as a critical asset for the safety of aircraft hydraulic systems. Within a typical shuttle valve, a normal system inlet, alternate/emergency system inlet, and outlet are present in order to provide valve functions.

When a critical hydraulic subsystem needs to have dedicated operation during times of low pressure, priority valves are utilized in order for critical systems to have predominance over noncritical systems. During standard operations, if pressure drops below a specified psi, the priority valve will close, impeding the flow of fluid pressure to noncritical systems so that critical hydraulic subsystems can continue to operate efficiently. For redundancy, some aircraft hydraulic systems utilize electrical shutoff valves and pressure switches so that critical systems will have priority.

During times in which units are removed, it is important that fluids are not lost as to ensure the maintained functionality of the system. To prevent fluid loss, quick disconnect valves are implemented on both the upstream and downstream of a power pump, allowing for its removal or disconnection for maintenance, testing, and more. To provide their functionality, the quick disconnect valve features a piston and poppet assembly that is spring loaded, resulting in a closed position when units are unattached.

Hydraulic fuses are another form of safety device, and they are employed in various locations throughout the hydraulic system. If there is a sudden burst in the stream, or a general high increase of fluid flow, fuses shut off flow in order to maintain an optimal amount of fluids for the entire system. Hydraulic fuses are typically placed in various flight surfaces and systems, including the landing gear, flaps, and brakes. The automatic resetting type of fuse is a special hydraulic fuse, and they permit specific amounts of fluid to pass through them on a timely basis.

The aircraft hydraulic system is a complex and advanced part of aircraft operation, allowing for the control of many flight critical systems. If you are in need of hydraulic valves and components that you can steadily rely on, NSN Supplies is your sourcing solution. NSN Supplies is a trusted distributor of aircraft valves and other aerospace components, providing customers access to over 2 billion items listed across our robust catalogues. Get started on the purchasing process today and see how we can help you fulfill all of your operational needs!


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