Aircraft Hydraulic System (ATA 29)

Vocabulary flashcards covering key terms, components, principles, and fluids related to ATA 29 Aircraft Hydraulic Systems.

Hydraulics

The science of liquids under pressure in motion used to transmit power.

Pneudraulics

The combined use of liquids and gases within the same system or container.

Pressure (PSI)

Force applied per unit area, measured in pounds per square inch in hydraulic systems.

Viscosity

A fluid’s internal resistance to flow; low viscosity flows easily, high viscosity flows slowly.

Fire Point

Temperature at which a substance’s vapors ignite and continue burning when exposed to flame.

Flash Point

Temperature at which a liquid gives off vapor sufficient to ignite momentarily when a flame is applied.

Pascal’s Law

Principle stating that pressure applied to a confined fluid is transmitted undiminished in all directions.

Hydraulic System Purpose

To develop, limit, and direct pressure to actuate aircraft mechanisms such as landing gear, brakes, and flight controls.

Basic Hydraulic System Components

Includes pump, reservoir, directional valve, check valve, pressure-relief valve, selector valve, actuator, and filter.

Constant-Displacement Pump

A pump that delivers a fixed volume of fluid per revolution regardless of system pressure demands.

Vane-Type Power Pump

A constant-displacement pump using vanes in a rotor to move hydraulic fluid.

Variable-Displacement Pump

Pump whose fluid output automatically varies via an internal compensator to meet system pressure demands.

Reservoir

Tank that stores hydraulic fluid, compensates for leakage, and allows for thermal expansion overflow.

Selector Valve

Valve that directs hydraulic fluid to control the movement direction of an actuator.

Check Valve

Allows free fluid flow in one direction while preventing reverse flow.

Sequence Valve

Controls the order of operation between two circuit branches, diverting fluid after a set pressure is reached.

Priority Valve

Ensures critical hydraulic subsystems receive pressure before non-critical ones during low-pressure conditions.

Shuttle Valve

Isolates the normal hydraulic system from an alternate or emergency hydraulic source.

Relief Valve

Safety valve that limits maximum system pressure to prevent component failure or line rupture.

Actuator (Hydraulic Cylinder)

Device that converts hydraulic pressure into mechanical linear motion to perform work.

Filter

Screening device that removes contaminants from hydraulic fluid to protect system components.

Accumulator

Sphere divided by a diaphragm; stores pressurized fluid in one chamber and gas in the other for surge damping and emergency power.

Master Caution Light

Cockpit indication that illuminates when hydraulic overheat or low pressure is detected.

Hydraulic Fluid Characteristics

Must resist compression, flow at extreme temperatures, and be non-corrosive.

Mineral Oil-Based Fluid (MIL-H-5606)

Red hydraulic fluid used since the 1940s where fire hazard is low; MIL-H-6083 is a rust-inhibited version.

Synthetic Oil-Based Fluid (MIL-H-83282)

Fire-resistant hydrogenated polyalphaolefin hydraulic fluid; more flame-resistant than MIL-H-5606 but viscous at low temperatures.

Ester Fluid (Skydrol)

Purple, highly fire-resistant hydraulic fluid widely used in commercial transport aircraft but toxic and not entirely fireproof.

Operating Pressure Range

Aircraft hydraulic systems may operate from a few hundred PSI to over 5,000 PSI in large jets.