Definition
A hydraulic system in which fluid is under pressure only when an actuator is being operated. When no hydraulic unit is in use, the pump output flows through the selector valves and back to the reservoir at low pressure. The selector valves are arranged in series with each other so that fluid passes through one valve to reach the next.
Plain English
A hydraulic setup where the fluid is only pressurised when something is actually being moved. The rest of the time the pump just circulates the fluid back to the tank at low pressure.
Context Anchor
Seen in aircraft maintenance descriptions of hydraulic power systems, especially when comparing open systems with systems that hold pressure all the time.
Derivation
Called 'open' because the path back to the reservoir is left open whenever no actuator is being commanded, so fluid flows freely rather than being held under pressure. Contrast with a 'closed-centre' system, where the fluid path is closed off and pressure is maintained at all times.
Why Pilots Care
Knowing whether a system is open or closed helps a technician or pilot understand why pressure rises only during operation, and why only one hydraulic unit at a time can typically be operated in this type of system.
Grounding Statement
In an open-hydraulic system, fluid is always being circulated, but it is not doing major work until the flow is directed to move a part.
Intuition Check
Do not read open as leaking, uncovered, or unsafe. Here, open means the fluid has a normal path to flow back to the reservoir when the system is not moving anything.
Example Sentence 1
In an open-hydraulic system, the pump unloads back to the reservoir whenever the landing gear and flap selectors are in neutral.
Example Sentence 2
During the annual inspection the technician confirmed that the open-hydraulic system returned fluid to the reservoir through the neutral position of each selector valve.