Summary of Aircraft Hydraulic and Pneumatic Systems

## Introduction Hydraulic systems power many aircraft control surfaces and landing gear by using fluid under pressure to transmit force. This study material explains key components, fluid types, valves, pumps, reservoirs, and maintenance practices you need to understand for safe operation and servicing. > **Definition:** A hydraulic system uses an incompressible fluid to transmit power from a source to actuators that perform work. ## Hydraulic Fluids Hydraulic fluid choice affects compatibility, fire resistance, and maintenance procedures. ### Main fluid types - **Mineral-base fluids** — common, used in many older and some modern systems. Compatible with many elastomers used in those systems. - **Phosphate-ester base fluids (e.g., Skydrol)** — fire-resistant fluids used in many transport-category aircraft; require compatible materials and careful handling. > **Definition:** Phosphate-ester fluids are synthetic, fire-resistant hydraulic fluids that require special cleaning and materials compatibility. Practical note: Always use the fluid specified on the aircraft maintenance manual or the identification plate on the reservoir or unit. ## Reservoirs and Pressurization Reservoirs store fluid and help ensure continuous pump supply. - Reservoirs may be pressurized to ensure pump inlet supply at high altitudes where atmospheric pressure is low. - Two common ways to pressurize reservoirs: 1. An aspirator (venturi) in the fluid return line 2. Bleed air from an engine compressor (note: this involves pneumatic/bleed air sources but only as a pressurization method; do not study pneumatic system details here) > **Definition:** A reservoir aspirator uses the return-flow velocity to create low pressure and assist reservoir pressurization. ## Pumps - **Main (engine-driven) pumps** normally draw fluid from a standpipe in the reservoir. The standpipe helps maintain a constant fluid supply to the pump under varying flight attitudes. - **Emergency pumps** often take fluid from the bottom of the reservoir. - **Double-action pump:** delivers fluid during both directions of handle movement (useful in hand-pump emergency systems). Why shear sections? Most engine-driven pumps include a shear section in the drive coupling so the pump can disconnect from the engine if the pump seizes, preventing engine damage. ## Pressure Control and Protection - **Unloading valve (pressure regulator):** shifts pump outlet fluid back to the reservoir when system pressure is at or above a set limit. This reduces pump load when actuators are not being used. - **Snubbers on pressure gauges:** installed between the gauge and the pump to prevent rapid fluctuation of the gauge pointer and give a steady reading. > **Definition:** An unloading valve is a pressure-regulating device that returns pump flow to the reservoir when the system reaches regulator set pressure. ## Accumulators - Purpose: store hydraulic energy as pressurized fluid. A compressed gas (usually nitrogen) acts on the fluid via a bladder, diaphragm, or piston. - Uses: - Damp pressure spikes - Provide emergency actuator movement when the pump is offline - Maintain system pressure between pump cycles ## Valves: Selector, Check, and Orifice Check ### Selector valves - **Open-center selector valve:** installed in series; fluid flows through the center when no component is actuated. Acts as part of a pump unloading scheme. - **Closed-center selector valve:** installed in parallel; when off, they trap fluid in the actuator lines and direct pressurized fluid to one side of the actuator and return from the other side to the return manifold. Table: Open-center vs Closed-center Selector Valves | Feature | Open-center | Closed-center | |---|---:|---:| | Installation | Series | Parallel | | Fluid flow when idle | Flows through center (pump unloading) | Traps fluid between valve and actuator | | Typical use | Systems that unload pump when idle | Systems that hold actuator position when idle | > *