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Lesson 11: Aircraft Hydraulic Systems

Aircraft Hydraulic Systems

• The Average modern aircraft utilizes hydraulic systems to operate several systems.

• Landing gear• Wing flaps• Speed and wheel brakes• Flight controls


• Pascal’s Law• Pressure exerted on a fluid in an enclosed

container is transmitted equally and undiminished to all parts of the container and acts as right angles to the enclosing walls.


• Hydrostatic Paradox• The pressure exerted by a column of liquid is

determined by the height of the column and is independent of its volume.


• Neither the shape nor the volume of a container affects the pressure. Only the height of the column does this.

Changes In Velocity and Pressure (Bernoulli)

• When a fluid or gas is supplied at a constant flow rate through a duct, the sum of the pressure energy and velocity energy is constant.

• If pressure increases, velocity decreases proportionally or, if pressure decreases, velocity increases proportionally.

• Kinetic Energy = Velocity (Ram Pressure)• Potential Energy = Pressure (Static Pressure)

Changes In Velocity and Pressure (Bernoulli)


• Relationship Between Pressure, Force, And Area

• Pressure is a measure of the amount of force that acts on a unit of area.

• Pressure is measured in pounds per square inch (psi).

• Force = Pressure x Area


• Relationship Between Pressure, Force, And Area


• Relationship Between Area, Distance, And Volume

• The area of the piston, the distance it moves, and the volume of the fluid displaced.

• Volume = Area x Distance


• Relationship Between Area, Distance, And Volume


• Mechanical Advantage In A Hydraulic System

• Two major advantages

–Ease with which force can be transmitted over large distances and into sealed compartments.

– Large gain in mechanical advantage made possible by varying the size of pistons.





• If a large amount of movement is need but only a small amount of force we can use a large piston to drive a smaller one.

• The fluid moved by the large piston will enter the cylinder with the small piston and move it a distance equal to the volume of fluid divided by the area of the small piston.


• All hydraulic systems are essentially the same, whatever their function.

• Regardless of application, each hydraulic system has a minimum number of components, and some type of hydraulic fluid.

Hydraulic Fluid

• The fluid used in aircraft hydraulic systems is one of the system’s most important parts.

• The fluid must flow with a minimum of opposition.

• Must be incompressible• Good lubricating properties• Inhibit corrosion and not attack seals• Must not foam in operation

Hydraulic Fluid

• Some characteristics that must be considered.

• Viscosity• Chemical Stability• Flash Point• Fire Point

Hydraulic Fluid

• Viscosity is the internal resistance to flow.

• Gasoline flows easily (has a low viscosity)• Tar flows slowly (has a high viscosity)

• A satisfactory liquid for a hydraulic system must have enough body t give a good seal at pumps, valves and pistons; but it must not be so thick that it offers excessive resistance to flow.

• The average hydraulic liquid has a low viscosity.

Hydraulic Fluid

• Chemical Stability is the ability of the liquid to resist oxidation and deterioration for long periods.

• Excessive temperatures have a great effect on the life of a liquid.

• Liquids may break down if exposed to air, water, salt, or other impurities.

Hydraulic Fluid

• Flash Point is the temperature at which a liquid gives off vapor in sufficient quantity to ignite momentarily when a flame is applied.

• High flash point is desirable for hydraulic fluids.

Hydraulic Fluid

• Fire Point is the temperature at which a substance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame.

• High fire point is required of desirable hydraulic fluids.

Types Of Hydraulic Fluid

• Vegetable-base

• Mineral-base

• Synthetic Fluid


• Mineral-base Fluid• MIL-H-5606 is the most widely used hydraulic

fluid in general aviation aircraft.

• Kerosene-type petroleum product.

• Dyed red for identification


• Synthetic Fluid

• Non-petroleum base hydraulic fluid for use in high performance piston engine and turbine powered aircraft.

• Most commonly used fluid of this type is Skydrol

• Colored light purple.

Basic Hydraulic Systems

• Open Hydraulic System


• Closed Hydraulic Systems


• More modern brake system


Power-pack Hydraulic System


• System Components

• Reservoirs• Pumps• Selector Valves• Check Valves• Hydraulic Fuses• Accumulators• Actuators

System Components

• Reservoirs

• In an in-line reservoir, space is provided in the reservoir for fluid expansion and the escape of entrapped air.

• Jet aircraft that operate at altitudes where there is not enough air pressure to assure a positive feed of fluid to the pump have hydraulic reservoirs pressurized.

System Components

• Hydraulic reservoir pressurized by hydraulic system pressure.

System Components

• Pumps

• Powered Pumps

–Constant displacement moves a specific volume of fluid each time its shaft turns.

–Variable displacement does not move a constant amount of fluid each revolution, but only the amount the system will accept.

System Components

• Constant Displacement Pumps

• Gear Pump

• Gerotor Pump

• Vane Pump

System Components

• Gear Pump

• Medium volume of fluid

under pressure.

System Components

• Gerotor Pump

System Components

• Vane Pump

• Moves a large

volume of fluid

under low pressure

System Components

• Valves

• Selector valve• Sequence valve• Priority valve• Pressure Control valves

Plug-Type Selector Valve

• Low pressure

systems

Popper-Type Selector

Sequence Valves

Priority Valves

Hydraulic Fuses

System Components

• Accumulators

• All accumulators consist of a high strength container divided by some form of movable partition into toe sections, or compartments.

• One compartments connected to the hydraulic pressure manifold, and the other is filled with compressed air or with nitrogen.

System Components

• Accumulators

System Components

• Actuators

• Single-Acting Linear

• Double-Acting Unbalanced Linear

• Double-Acting Balanced Linear

System Components

• Single-Acting Linear (Brakes)

System Components

• Double-Acting Unbalanced Linear (Landing Gear)

System Components

• Double-Acting Balanced Linear (Flight Controls)