Hydraulic System Components and Operation

Working Principle of Aircraft Hydraulic Systems

Hydraulic systems operate by transmitting force through an incompressible fluid, typically a specialised hydraulic oil. When a pilot moves a control, a selector valve directs pressurised fluid from the pump to an actuator, converting fluid pressure into mechanical movement. This allows relatively small pilot inputs to control large forces needed for flight controls, landing gear, and brakes.

Key Hydraulic System Components

• Reservoir: Stores hydraulic fluid. Can be pressurised (to prevent cavitation at altitude) or unpressurised.
• Pumps: Provide system pressure. Types include engine-driven (constant pressure or gear-type), electrically driven, manual hand pumps, air-driven (ram air turbine), and power transfer units.
• Accumulators: Store pressurised fluid, dampen pressure fluctuations, and provide limited emergency power.
• Valves: Control direction, flow, and pressure. Includes selector (electro, rotary, linear), check (non-return), relief, restrictor, bypass, shuttle, priority, fuse, and fire shut-off valves.
• Actuators: Convert fluid pressure to mechanical motion. Single-acting actuators move in one direction; double-acting can move in both.
• Hydraulic Motors: Convert fluid power into rotary motion for specific applications.
• Filters: Remove contaminants, protecting sensitive components.
• Case Drain and Cooler Lines: Return excess or heated fluid to the reservoir, maintaining temperature and system health.

System Types and Operation

• Active Systems: Use pumps to provide continuous pressure. Most modern aircraft use multiple active systems for redundancy.
• Passive Systems: Rely on manual or gravity-fed pressure, suitable for non-critical or backup functions.

Hydraulic Failure Symptoms

Common signs include unusual noises, slow or unresponsive actuators, fluid leaks, and warning indications. Accumulators can temporarily mask pump failures, but only for limited operations.

Hydraulic System Diagrams

Understanding a typical hydraulic system diagram helps you trace fluid flow, identify component locations, and diagnose faults efficiently.