Hydraulic & Pneumatic Systems
Hydraulic & Pneumatic Systems is a core knowledge area on the AMT — Airframe FAA written exam. This hub collects our 16 in-depth, ACS-aligned hydraulic & pneumatic systems articles — each written in plain English and grounded in the official FAA handbooks. Work through them below, then drill the topic with practice questions.
Hydraulic System Pressure Generation and Pump Types
Aircraft hydraulic systems rely on engine-driven, electric, and hand pumps to generate pressure for flight controls, landing gear, and brakes — each pump type has distinct operating principles, uses, and failure modes that every AMT must know.
Hydraulic Fluid Types and Compatibility (MIL-PRF-5606, 5780, and Skydrol)
Aircraft hydraulic fluids differ critically in chemistry, fire resistance, and compatibility — mixing types can destroy seals, corrode components, and create serious safety hazards that every AMT must know.
Hydraulic Actuators: Linear and Rotary Operation
Hydraulic actuators convert fluid pressure into mechanical force, with linear actuators producing straight-line movement and rotary actuators producing controlled rotation — both essential for aircraft control surfaces, landing gear, and brakes.
Hydraulic Flow Control Valves: Selector, Check, and Priority Valves
Hydraulic flow control valves—selectors, check valves, and priority valves—direct, restrict, and protect hydraulic fluid to ensure aircraft systems operate safely and in the right sequence.
Hydraulic Pressure Relief and Thermal Relief Valves
Pressure relief valves protect hydraulic systems from dangerous overpressure by limiting maximum system pressure, while thermal relief valves guard against pressure spikes caused by thermal expansion of trapped fluid.
Hydraulic System Filters and Contamination Control
Hydraulic system filters remove solid particles and contaminants that cause component wear and failure; understanding filter types, micron ratings, and contamination control is essential for every airframe technician.
Hydraulic System Reservoirs: Pressurized vs. Unpressurized
Aircraft hydraulic reservoirs store and supply fluid to the system; pressurized reservoirs maintain positive head pressure to prevent cavitation, while unpressurized reservoirs rely on other means—knowing the difference is critical for AMT certification.
Hydraulic Accumulators: Function, Precharge, and Servicing
Hydraulic accumulators store pressurized fluid energy, dampen pressure surges, and provide emergency backup power; correct nitrogen precharge and servicing procedures are critical to safe airframe maintenance.
Power Transfer Units and Hydraulic Motor-Pump Combinations
Power transfer units (PTUs) and hydraulic motor-pump combinations allow aircraft hydraulic systems to share energy across independent circuits without transferring fluid, maintaining redundancy and pressure when a primary source fails.
Open-Center vs. Closed-Center Hydraulic Systems
Open-center and closed-center hydraulic systems differ fundamentally in how fluid flows when actuators are idle — a distinction that drives pump design, pressure management, and system efficiency across aircraft.
Aircraft Pneumatic System Components and Ground Air Sources
Aircraft pneumatic systems use pressurized air to power brakes, deicing, pressurization, and more. This article covers every major component and explains how ground air sources integrate with onboard systems.
Pneumatic System Desiccants, Filters, and Moisture Separators
Aircraft pneumatic systems rely on desiccants, filters, and moisture separators to remove water vapor and contaminants before compressed air reaches sensitive components, preventing corrosion, freezing, and valve failure.
Emergency and Backup Hydraulic Power Systems (RAT, Accumulators, Hand Pumps)
Emergency and backup hydraulic power systems—including ram air turbines, accumulators, and hand pumps—keep critical flight controls and brakes functional when primary hydraulic power is lost.
Hydraulic System Seals and O-Ring Material Compatibility
Hydraulic system reliability depends on choosing seals and O-rings made from materials chemically compatible with the fluid they contact — a mismatch causes leaks, system failure, and potential loss of flight-critical controls.
Hydraulic System Troubleshooting: External Leaks, Spongy Controls, and Pressure Loss
External leaks, spongy controls, and pressure loss are the three most common hydraulic faults on aircraft; understanding their root causes and diagnostic steps is essential for safe airframe maintenance.
Bleeding and Purging Procedures for Aircraft Hydraulic Systems
Bleeding and purging hydraulic systems removes trapped air and contamination that can cause spongy controls, actuator drift, or complete system failure — a critical AMT skill grounded in FAA maintenance standards.
More AMT — Airframe subjects
Articles are original summaries grounded in the public-domain FAA handbooks and cite their source. ACS-aligned study aids — not a substitute for the official handbooks or regulations.