Definition
The branch of engineering and the system technology that uses pressurized fluid (typically a special oil) confined within sealed lines and components to transmit force and move mechanical parts. In aircraft, hydraulic systems power devices that require large forces, such as landing gear extension and retraction, wheel brakes, flaps, and flight control boost.
Plain English
Using oil under pressure to push things and move them. Because liquid does not squash, pressing on it at one end moves a piston at the other end with a lot of force.
Context Anchor
Seen in aircraft systems discussions, preflight inspections, brake operation, landing gear operation, and abnormal or emergency procedures involving hydraulic pressure or fluid loss.
Derivation
From the Greek 'hydor' meaning water and 'aulos' meaning pipe. Originally referred to water moving through pipes; now applies to any pressurized fluid used to transmit force through lines and pistons.
Why Pilots Care
Hydraulic systems supply the force required to move large surfaces and heavy components that pilots could not operate manually at speed.
Analogy
Think of squeezing a tube of toothpaste. A small push at one end produces a strong, controlled push at the other. Hydraulic systems do the same thing with oil in metal lines instead of toothpaste in a tube.
Grounding Statement
When you press a brake pedal in many airplanes, hydraulic fluid carries that push to the brake at the wheel.
Intuition Check
Hydraulics does not mean the fluid itself only. It means the whole idea or system of using liquid under pressure to transmit force.
Example Sentence 1
The landing gear retracted normally once hydraulic pressure built up after takeoff.
Example Sentence 2
A hydraulic leak forced the crew to land with manual gear extension.