1.8 - Hydraulics

Nearly all airplanes are equipped with a hydraulic system.
In smaller general aviation airplanes, hydraulic systems tend to be small and simple, primarily powering brakes, extending/retracting landing gear, and changing blade angle on some speed propellers.
In larger airplanes (airlines), the hydraulic system powers a majority of the airplane, including flight controls and flaps.
A typical hydraulic system consists of:
- a reservoir where the hydraulic fluid is stored,
- a pump that moves the fluid,
- a filter to keep contaminants out of the system,
- a relief valve in case of a hydraulic malfunction,
- and actuators which the hydraulic system operates.

Reservoir: stores hydraulic fluid.
Pump: moves the fluid through the system.
Filter: removes contaminants from the hydraulic fluid to keep the system clean.
Relief valve: protects the system by releasing excess pressure during a malfunction or overload.
Actuators: hydraulic actuators convert fluid pressure into mechanical motion (extend/contract pistons).

The system pumps incompressible fluid through hydraulic lines from one actuator into another, causing actuator pistons to extend or contract.
The hydraulic pressure exerted throughout the actuators is significant, making hydraulic systems very powerful.
Core principle: pressure applied to a confined fluid is transmitted undiminished throughout the fluid (Pascal's principle).

Small/general aviation airplanes: hydraulic systems used for brakes, landing gear extension/retraction, and propeller blade angle adjustment (where applicable).
Large airliners: hydraulics power flight controls (e.g., ailerons, elevators, rudder), flaps, landing gear, brakes, and other systems.
Overall, larger aircraft rely on hydraulics for a broad set of critical control and operating surfaces, contributing to system redundancy and reliability.

When the pilot presses the brakes:
- A piston drives hydraulic fluid from the brake actuator on the pedal through hydraulic lines.
- Fluid travels to the actuator near the wheels.
- The fluid pressure pushes the wheel-side actuator piston.
- The piston mechanically squeezes the brake pads against the brake disc, generating friction that slows the aircraft.
This example illustrates how hydraulic pressure is converted into braking force via actuators and friction surfaces.

Filtration: A filter keeps contaminants out of the hydraulic fluid to prevent damage and maintain smooth operation of actuators.
Relief valve: Prevents dangerous pressure buildup, protecting components and maintaining safe operation during malfunctions.
Incompressible fluid behavior: Ensures that pressure applied at one point is transmitted to all points in the fluid, enabling coordinated actuation of multiple components.

Hydraulic systems enable powerful and compact actuation, allowing small input forces to control large mechanical loads (e.g., braking force, control surface movement).
The reliability of hydraulic systems hinges on clean fluid, proper filtration, and functioning relief valves.
Understanding the basic principles helps in diagnosing malfunctions, planning maintenance, and appreciating how flight controls and braking are managed in various aircraft sizes.

Hydraulic systems exemplify Pascal’s principle: pressure applied to a confined fluid is transmitted to every part of the fluid, allowing small control inputs to effect large outputs.
The choice of components (reservoir, pump, filter, relief valve, actuators) reflects a design emphasis on reliability, contamination control, and safety in aviation contexts.
Real-world relevance includes the critical role of hydraulics in braking, landing gear operation, and flight control surfaces, especially in larger, commercial aircraft where redundancy and precision are essential.