Definition
A closed loop of components — typically a reservoir, pump, lines, valves, and one or more actuators — through which pressurized fluid flows to transmit force and operate an aircraft system such as landing gear, flaps, or brakes.
Plain English
The complete path that hydraulic fluid travels around to do a job in the aircraft, from the storage tank, through the pump and lines, out to the part it moves, and back again.
Context Anchor
Seen in aircraft systems discussions, especially when learning how brakes, landing gear, and some flight controls are powered.
Derivation
‘Hydraulic’ comes from the Greek hydraulikos, meaning ‘worked by water,’ from hydor (water). ‘Circuit’ comes from the Latin circuitus, meaning ‘a going around.’ Together: a path the fluid travels around to do work — even though in aircraft the fluid is oil, not water.
Why Pilots Care
A properly functioning hydraulic circuit supplies the force needed for reliable operation of critical systems; loss of pressure can disable landing gear, flaps, or flight controls.
Analogy
Think of it like a loop of garden hose connected to a pump and a tool. The pump pushes fluid through the hose, the fluid makes the tool move, and the path gives the fluid somewhere to go.
Intuition Check
Do not think of circuit as only an electrical path. In a hydraulic circuit, the path is for fluid under pressure, not electricity.
Example Sentence 1
When the pilot lowered the landing gear handle, fluid flowed through the hydraulic circuit and the gear extended.
Example Sentence 2
A ruptured line in the hydraulic circuit caused the landing gear to fail to retract after takeoff.