© October 2019| IJIRT | Volume 6 Issue 5 | ISSN: 2349-6002
IJIRT 157626 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 6
An overview on aircraft hydraulic system
D.B. Jani1, Shah Ashish2, Singh Aditya3, Singh Yash4, Singh Bishambhar5,
Singh Nikhil6, Singh Manmohan7
1,2,3,4,5,6,7GEC, Dahod-389151, Gujarat Technological University, GTU, Gujarat, India
Abstract—Hydraulic systems in aircraft provide a means
for the operation of aircraft components. The operation
of landing gear, flaps, flight control surfaces, and brakes
is largely accomplished with hydraulic power systems.
Hydraulic system complexity varies from small aircraft
that require fluid only for manual operation of the wheel
brakes to large transport aircraft where the systems are
large and complex. To achieve the necessary redundancy
and reliability, the system may consist of several
subsystems. Each subsystem has a power generating
device (pump) reservoir, accumulator, heat exchanger,
filtering system, etc. System operating pressure may vary
from a couple hundred pounds per square inch (psi) in
small aircraft and rotorcraft to 5,000 psi in large
transports.
Key words—Hydraulic fluid, Hydraulic system,
Hydraulic reservoirs, Hydraulic pump, Hydraulic
accumulators, Hydraulic actuators.
I. INTRODUCTION
The state-of-the-art in aircraft systems architectures
consists of complex integration of various
technologies which make up the equipment used to
power and fly an aircraft in the open sky. An
Equipment System fulfils a major functional aspect of
an aircraft and an architecture is defined as the overall
way in which Systems are assembled within the
Aircraft. In a conventional architecture (a basic
schematic layout is shown in Fig. 1), the fuel is
converted into power by the engines. Most of this
power is expended as propulsive power (thrust) to
propel the aircraft. The remainder is transmitted via,
and converted into, four main forms of non-propulsive
power.
Fig. 1. Power distribution in an aircraft.
Air is bled from the engine high-pressure
compressor(s). This pneumatic power is
conventionally used to power the Environmental
Control System (ECS) and supply hot air for Wing Ice
Protection System (WIPS).
A mechanical accessories gearbox transfers
mechanical power from the engines to central
hydraulic pumps, to local pumps for engine equipment
and other mechanically driven subsystems, and to the
main electrical generator.
The central hydraulic pump transfers hydraulic power
to the actuation systems for primary and secondary
flight control, to landing gear for deployment,
retraction and braking, to engine actuation, to thrust
reversal systems and to numerous ancillary systems.
The main generator provides electrical power to the
avionics, to cabin and aircraft lighting, to the galleys,
and to other commercial loads (entertainment systems,
for example). This conventional distribution of energy
is fully reflected in the way aircraft systems are
classified and procured today.
II. HYDRAULIC FLUIDS
Hydraulic system liquids are used primarily to
transmit and distribute forces to various units to be
actuated. Liquids are able to do this because they are
almost incompressible. Pascal’s Law states that
pressure applied to any part of a confined liquid is
transmitted with undiminished intensity to every other
part. Thus, if a number of passages exist in a system,
pressure can be distributed through all of them by
means of the liquid. Manufacturers of hydraulic
devices usually specify the type of liquid best suited
for use with their equipment in view of the working
conditions, the service required, temperatures
expected inside and outside the systems, pressures the
liquid must withstand, the possibilities of corrosion,
and other conditions that must be considered. If
incompressibility and fluidity were the only qualities
required, any liquid that is not too thick could be used
in a hydraulic system. But a satisfactory liquid for a
© October 2019| IJIRT | Volume 6 Issue 5 | ISSN: 2349-6002
IJIRT 157626 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 7
particular installation must possess a number of other
properties. Some of the properties and characteristics
that must be considered when selecting a satisfactory
liquid for a particular system. To assure proper system
operation and to avoid damage to nonmetallic
components of the hydraulic system, the correct fluid
must be used. When adding fluid to a system, use the
type specified in the aircraft manufacturer’s
maintenance manual or on the instruction plate affixed
to the reservoir or unit being serviced. The three
principal categories of hydraulic fluids are:
1. Minerals
2. Polyalphaolefins
3. Phosphate esters
When servicing a hydraulic system, the technician
must be certain to use the correct category of
replacement fluid. Hydraulic fluids are not necessarily
compatible.
Hydraulic systems require the use of special
accessories that are compatible with the hydraulic
fluid. Appropriate seals, gaskets, and hoses must be
specifically designated for the type of fluid in use.
Care must be taken to ensure that the components
installed in the system are compatible with the fluid.
When gaskets, seals, and hoses are replaced, positive
identification should be made to ensure that they are
made of the appropriate material.
Experience has shown that trouble in a hydraulic
system is inevitable whenever the liquid is allowed to
become contaminated. The nature of the trouble,
whether a simple malfunction or the complete
destruction of a component, depends to some extent on
the type of contaminant. Two general contaminants
are:
• Abrasives, including such particles as core sand,
weld spatter, machining chips, and rust.
• Non-abrasives, including those resulting from oil
oxidation and soft particles worn or shredded from
seals and other organic components.
To control the particulate contamination in the system,
filters are installed in the pressure line, in the return
line, and in the pump case drain line of each system.
The filter rating is given in microns as an indication of
the smallest particle size that is filtered out. The
replacement interval of these filters is established by
the manufacturer and is included in the maintenance
manual. In the absence of specific replacement
instructions, a recommended service life of the filter
elements is:
• Pressure filters—3,000 hours
• Return Filters—1,500 hours
• Case drain filters—600 hours
II. HYDRAULIC SYSTEM COMPONENTS
Hydraulic Reservoirs
The hydraulic reservoirs are pressurized by bleed air
through a pressurization module. The standby
reservoir is connected to the system B reservoir for
pressurization and servicing. The positive pressure in
the reservoir ensures a positive flow of fluid to the
pumps. The reservoirs have a standpipe that prevents
the loss of all hydraulic fluid if a leak develops in the
engine-driven pump or its related lines. The engine-
driven pump draws fluid through a standpipe in the
reservoir and the AC motor pump draws fluid from the
bottom of the reservoir. The system A, B, and standby
reservoirs are located in the wheel well area as shown
in Fig. 2.
Fig. 2. Hydraulic reservoirs.
Hydraulic pumps
All aircraft hydraulic systems have one or more
power-driven pumps and may have a hand pump as an
additional unit (Fig. 3) when the engine-driven pump
is inoperative. Power-driven pumps are the primary
source of energy and may be either engine driven,
electric motor driven, or air driven. As a general rule,
electrical motor pumps are installed for use in
emergencies or during ground operations. Some
aircraft can deploy a ram air turbine (RAT) to generate
hydraulic power.
© October 2019| IJIRT | Volume 6 Issue 5 | ISSN: 2349-6002
IJIRT 157626 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 8
Fig. 3. Hydraulic pump.
Flow control valves
Flow control valves control the speed and/or direction
of fluid flow in the hydraulic system. They provide for
the operation of various components when desired and
the speed at which the component operates. Examples
of flow control valves include: selector valves, check
valves, sequence valves (Fig. 4), priority valves,
shuttle valves, quick disconnect valves, and hydraulic
fuses.
Fig. 4. Mechanically operated sequence valves.
Pressure control valves
The safe and efficient operation of fluid power
systems, system components, and related equipment
requires a means of controlling pressure. There are
many types of automatic pressure control valves (Fig.
5). Some of them are an escape for pressure that
exceeds a set pressure; some only reduce the pressure
to a lower pressure system or subsystem; and some
keep the pressure in a system within a required range.
Fig. 5. Pressure relief valve.
Hydraulic pressure must be regulated in order to use it
to perform the desired tasks. A pressure relief valve is
used to limit the amount of pressure being exerted on
a confined liquid. This is necessary to prevent failure
of components or rupture of hydraulic lines under
excessive pressures. The pressure relief valve is, in
effect, a system safety valve.
Actuators
An actuating cylinder transforms energy in the form of
fluid pressure into mechanical force, or action, to
perform work. It is used to impart powered linear
motion to some movable object or mechanism. A
typical actuating cylinder consists of a cylinder
housing, one or more pistons and piston rods, and
some seals. The cylinder housing contains a polished
bore in which the piston operates, and one or more
ports through which fluid enters and leaves the bore.
The piston and rod form an assembly. The piston
moves forward and backward within the cylinder bore,
and an attached piston rod moves into and out of the
cylinder housing through an opening in one end of the
cylinder housing. Seals are used to prevent leakage
between the piston and the cylinder bore and between
the piston rod and the end of the cylinder. Both the
cylinder housing and the piston rod have provisions
for mounting and for attachment to an object or
mechanism that is to be moved by the actuating
cylinder as shown in Fig. 6.
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IJIRT 157626 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 9
Fig. 6. Linear actuator.
Hydraulic motor
Piston-type motors are the most commonly used in
hydraulic systems. They are basically the same as
hydraulic pumps except they are used to convert
hydraulic energy into mechanical (rotary) energy.
Hydraulic motors are either of the axial inline or bent-
axis type. The most commonly used hydraulic motor
is the fixed-displacement bent-axis type (Fig. 7).
These types of motors are used for the activation of
trailing edge flaps, leading edge slats, and stabilizer
trim. Some equipment uses a variable-displacement
piston motor where very wide speed ranges are
desired. Although some piston-type motors are
controlled by directional control valves, they are often
used in combination with variable-displacement
pumps. This pump-motor combination is used to
provide a transfer of power between a driving element
and a driven element. Some applications for which
hydraulic transmissions may be used are speed
reducers, variable speed drives, constant speed or
constant torque drives, and torque converters.
Fig. 7. Bent axis piston motor.
Some advantages of hydraulic transmission of power
over mechanical transmission of power are as follows:
• Quick, easy speed adjustment over a wide range
while the power source is operating at a constant
(most efficient) speed
• Rapid, smooth acceleration or deceleration
• Control over maximum torque and power
• Cushioning effect to reduce shock loads
• Smoother reversal of motion
Ram Air Turbine (RAT)
The RAT is installed in the aircraft to provide
electrical and hydraulic power if the primary sources
of aircraft power are lost. Ram air is used to turn the
blades of a turbine that, in turn, operates a hydraulic
pump and generator. The turbine and pump assembly
is generally installed on the inner surface of a door
installed in the fuselage. The door is hinged, allowing
the assembly to be extended into the slipstream by
pulling a manual release in the flight deck. In some
aircraft, the RAT automatically deploys when the
main hydraulic pressure system fails and/or electrical
system malfunction occurs (Fig. 8).
Fig. 8. Ram air turbine.
Power Transfer Unit (PTU)
The PTU is able to transfer power but not fluid. It
transfers power between two hydraulic systems.
Different types of PTUs are in use; some can only
transfer power in one direction while others can
transfer power both ways. Some PTUs have a fixed
displacement, while others use a variable displacement
hydraulic pump. The two units, hydraulic pump and
hydraulic motor, are connected via a single drive shaft
so that power can be transferred between the two
systems. Depending on the direction of power transfer,
each unit in turn works either as a motor or a pump.
III. CONCLUSIONS
Hydraulic systems have many advantages as power
sources for operating various aircraft units; they
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IJIRT 157626 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 10
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. Furthermore,
an aircraft hydraulic system is a very high
performance system with a high risk both in human
life and financial cost when failures occur while in
flight. Therefore, efficient control must be a major
concern to the aircraft designers and maintenance
personnel associated with the hydraulic systems on
aircraft.
R
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