Definition
Aircraft systems that use pressurized fluid, contained in sealed lines and components, to transmit force and motion in order to operate flight controls, landing gear, brakes, flaps, and other moving parts.
Plain English
Systems that move parts of the aircraft by pushing oil-like fluid through pipes under pressure. When the pilot moves a lever or switch, the fluid pushes a piston, which moves the part being controlled.
Context Anchor
Seen in aircraft maintenance, preflight discussions, brake checks, landing gear operation, and system failure procedures.
Derivation
From the Greek 'hydor' meaning water, and 'aulos' meaning pipe. Originally referred to anything operated by water flowing through pipes. In aviation, the principle is the same — force transmitted through a confined fluid — but the fluid is a specialized oil rather than water.
Why Pilots Care
Failure can remove control of flight surfaces, prevent gear extension, or disable brakes, directly affecting safety and aircraft handling.
Analogy
Like the brakes in a car. Pressing the pedal pushes fluid through a line, and the fluid pushes the brake pads onto the wheel. The pedal does not touch the wheel — the fluid does the work.
Intuition Check
Do not assume hydraulic means water-powered. In aircraft, it means force is carried by liquid under pressure, usually special hydraulic fluid.
Example Sentence 1
Before the first flight of the day, the technician checked the hydraulic systems for proper fluid level and any sign of leaks.
Example Sentence 2
During preflight the mechanic verified pressure in the hydraulic systems that operate the brakes.