Definition
A landing gear retraction system that uses pressurized fluid, driven by an engine-driven or electrically-driven hydraulic pump, to extend and retract the landing gear. The pump pressurizes hydraulic fluid in a reservoir; selector valves direct that pressurized fluid to actuators (cylinders) at each gear leg, which mechanically push the gear up into its wells or down into the locked-down position. Check valves, sequencing valves, and gear-down locks ensure the gear travels in the correct order and stays locked when fully extended.
Plain English
A system that raises and lowers the landing gear by using a pump to push fluid through small cylinders, which then move the gear up or down on command from the cockpit.
Context Anchor
Encountered when studying retractable landing gear operation, gear-up/gear-down checks, and abnormal landing gear procedures.
Derivation
Hydraulic comes from the Greek hydor, meaning water. The word originally described systems that moved water through pipes, but in aviation it refers to any system that uses pressurized fluid to do mechanical work. Knowing that origin helps: a hydraulic system is essentially a fluid-powered system, even though the fluid today is oil rather than water.
Why Pilots Care
Understanding the system helps pilots recognize normal operation, identify hydraulic failures, and correctly use emergency extension procedures.
Analogy
It is similar to using a small hand pump to move something heavy with fluid pressure. The pilot moves a control, and the pressurized fluid does the heavy work of moving the gear.
Intuition Check
Do not think of “gear” here as engine gears or mechanical cogs. In this context, landing gear means the wheels, struts, and related parts that support the airplane on the ground.
Example Sentence 1
After takeoff and a positive rate of climb, the pilot selected gear up, and the hydraulic landing gear retraction system pulled all three legs into their wells within a few seconds.
Example Sentence 2
During the preflight inspection the student checked the hydraulic fluid reservoir that supplies the landing gear retraction system.