Hydraulic system

INTRODUCTION

Aircraft hydraulic systems found on most aircraft perform many functions. Some of the

systems operated by hydraulics are flight controls, landing gear, speed brakes, auxiliary

systems, and brakes. Hydraulic systems have many advantages as power sources for operating

various aircraft units; they combine the advantages of light weight, ease of installation,

simplification of inspection, and minimum maintenance requirements. Hydraulic operations

are also almost 100 percent efficient, with only negligible loss due to fluid friction. There are

disadvantages as well, such as the possibility of leakage, both internal and external that can

lead to complete system failure. One of the major reasons for failure of the hydraulic system

is contamination of fluid due to foreign material.

INFORMATION: Hydraulic System

Hydraulic System

The hydraulic system develops, controls, and directs the fluid under pressure to move various

mechanisms on the aircraft. Mechanisms such as flight controls, landing gear, and nose wheel

steering are all controlled with hydraulic pressure. The terms below (Table 3-3) will help to

understand the hydraulic system.

Table 3-3, Hydraulic Terms

Term Description

Hydraulics The science of liquids in motion

Pneumatics The science of gases in motion

Pneudraulics The use of liquids and gases within the same system

Pressure Force applied to a unit of area and measured in pounds per square inch (PSI)

Hydraulic fluid types (fig 3-67)

• Petroleum

• Synthetic

• May be colored red or purple for

identification

Used to create a complete and closed path

throughout the aircraft. Lines are identified

with labels that are colored blue, yellow,

and white (white bands contain black

circles), (fig. 3-68).

Table 3-4, Hydraulic Lines (fig.3-69)

Line Description

Supply Line Supplies fluid from the reservoir to the pump

Pressure Lines Delivers fluid from pumps to control valves

Alternating Lines Lines that carry fluid flow to/from control valves and actuating units

Return Lines Returns fluid back to reservoir from control valve

Hoses (fig.3-70)

Used where flexibility is required and can be reinforced with braided metal wire cover when

needed, there is also an indicator stripe to show if hose is

twisted.

Tubing (fig. 3-71)

Aluminum alloy used for low pressure. Titanium or

stainless steel are used for high pressure or exposed areas.

Reservoir (fig. 3-72)

Used to store hydraulic fluid for system use and may be pressurized or non-pressurized:

• Pressurized

o Pressurized by engine driven pump or engine bleed air for flight operation

o Hydraulic test stand for ground maintenance

o Ensures a constant flow of fluid to the pumps

• Non-pressurized

o Relies on gravity to feed pumps

o Open air vent located at top

o Air filter in vent line prevents fluid contamination

• Quantity Indicating System

o Indicates direct amount of fluid in the reservoir

The basic function of a pump is to pressurize and supply fluid to the system. May be operated

by the following:

• Rotation of main engine

• Electrically using motors

• Manually by hand

With the pumps being operated by the methods listed above they can also be broken down

into different types which are:

• Constant Volume (fig. 3-73)—Maintains a constant flow of fluid at any given pump

speed regardless of system demands

Variable Volume (fig. 3-74)—Output volume varies with system demand but the

pressure remains steady

• Hand Pump (fig. 3-75)—Used for ground operations

Actuating units

Used to convert hydraulic pressure into mechanical movement and consist of:

• Linear actuating (fig. 3-76) cylinder converts pressure to linear movement

• Hydraulic motor (fig. 3-77) converts pressure to rotary movement

Accumulator (fig. 3-78)

Have either a spherical or cylindrical design with two chambers. Filled with either hydraulic

fluid or compressed nitrogen and are separated by a movable piston, diaphragm, or bladder:

• Stores hydraulic fluid under pressure

• Aids the pump at peak operating loads

• Dampens pressure surges

• Emergency source of pressure if a hydraulic system fails

Valves (Table 3-5)

Movable devices used to start, stop, or regulate fluid flow through the system. They can be

controlled:

• Manually

• Electrically

• Pneumatically

• Mechanically

• Hydraulically

• Combinations of two or more methods

Table 3-5, Hydraulic Valves

Valve Description

Check Valve

(fig. 3-79)

Designed to only allow fluid flow in one direction

Pressure relief Valve

(fig. 3-80)

Prevents pressure build up within system caused system

malfunctions

Relieves excess system pressure by returning fluid to the

reservoir

Dump Valve

Dumps system pressure for maintenance (operation is like

pressure relief valve, it is just manually operated instead of

relying on an over pressure scenario)

Control/Selector/Servo

Valve (fig. 3-81)

Directs flow of fluid to and from an actuating unit and

controlled differently depending on the application, those

methods include the following:

• Mechanically (control valve)

• Manually (selector valve)

• Electrically (servo valve)

Shut-off Valve

(fig. 3-82)

Stops fluid flow

Restrictor Valve

(fig. 3-83a, b, c)

Slows the movement of fluid in one or both directions

Shuttle Valve

(fig. 3-84)

Used to separate normal from emergency systems

Allows system with the highest pressure to operate the

actuating unit

Internal mechanism blocks pressure from weaker system

Charging Valve

(fig. 3-85)

Provides connection for servicing hydraulic and pneumatic

components

Hydraulic Fuse (fig. 3-86)

• Automatically shuts off fluid flow if a line should break

Hydraulic Fluid Coolers (fig. 3-87)

• Cools hydraulic fluid by ram air or aircraft fuel

Filters

• Removes foreign particles from hydraulic fluid

• Filter By-Pass Valve—Allows fluid to by-pass the filter element if it becomes clogged

• Differential Pressure Indicator (fig. 3-88a, b) (DPI/Delta-P)—A red indicator pin

extends to indicate a clogged filter

Pressure Indicating System

• Direct Reading Pressure Gauge (fig. 3-89)

o Attached directly to system components to indicate system pressure

• Indirect Reading Pressure Gauge (fig. 3-90)

o Located in the cockpit, receives an electric signal to indicate system pressure

Hydraulic Schematic/Diagram

Uses graphics/symbols to represent system components and are used to trace the flow of fluid

which helps with troubleshooting and repair. They can also be used to learn overall system

operation, troubleshooting and repair.

• Schematic—A simple way to show all the components and connections in a hydraulic

system (fig 3-91)

• Diagrams—These aids in hydraulic component recognition (location) and

identification (fig 3-92)

Servicing Safety

Hydraulic fluid servicing

• Hydraulic fluid is toxic which poses a fetal hazard

• Glove and goggles are required as PPE

Pneumatic Servicing

• Always use proper eye protection

• Inspect servicing equipment and charging valve for serviceability

• Purge air servicing hose before use

• Never service low pressure systems with high pressure air