Hydraulic and Pneumatic
Aircraft Hydraulic Systems
The word “hydraulics” is based on the Greek word for water
and originally meant the study of the physical behavior of
water at rest and in motion. Today, the meaning has been
expanded to include the physical behavior of all liquids,
including hydraulic fluid. Hydraulic systems are not new
to aviation. Early aircraft had hydraulic brake systems. As
aircraft became more sophisticated, newer systems with
hydraulic power were developed.
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.
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.
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 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 are discussed in
the following paragraphs.
One of the most important properties of any hydraulic fluid is
its viscosity. Viscosity is internal resistance to flow. A liquid
such as gasoline that has a low viscosity flows easily, while
a liquid such as tar that has a high viscosity flows slowly.
Viscosity increases as temperature decreases. A satisfactory
liquid for a given hydraulic system must have enough body
to give a good seal at pumps, valves, and pistons, but it must
not be so thick that it offers resistance to flow, leading to
power loss and higher operating temperatures. These factors
add to the load and to excessive wear of parts. A fluid that is
too thin also leads to rapid wear of moving parts or of parts
that have heavy loads. The instruments used to measure
the viscosity of a liquid are known as viscometers or
viscosimeters. Several types of viscosimeters are in use today.
The Saybolt viscometer measures the time required, in
seconds, for 60 milliliters of the tested fluid at 100 °F to
pass through a standard orifice. The time measured is used to
express the fluid’s viscosity, in Saybolt universal seconds
or Saybolt furol seconds. [Figure 12-1]
Figure 12-1. Saybolt viscosimeter.
Chemical stability is another property that is exceedingly
important in selecting a hydraulic liquid. It is the liquid’s
ability to resist oxidation and deterioration for long periods.
All liquids tend to undergo unfavorable chemical changes
under severe operating conditions. This is the case, for
example, when a system operates for a considerable period
of time at high temperatures. Excessive temperatures have
a great effect on the life of a liquid. It should be noted that
the temperature of the liquid in the reservoir of an operating
hydraulic system does not always represent a true state of
operating conditions. Localized hot spots occur on bearings,
gear teeth, or at the point where liquid under pressure is
forced through a small orifice. Continuous passage of a liquid
through these points may produce local temperatures high
enough to carbonize or sludge the liquid, yet the liquid in the
reservoir may not indicate an excessively high temperature.
Liquids with a high viscosity have a greater resistance
to heat than light or low-viscosity liquids that have been
derived from the same source. The average hydraulic liquid
has a low viscosity. Fortunately, there is a wide choice of
liquids available for use within the viscosity range required
of hydraulic liquids.
Liquids may break down if exposed to air, water, salt, or
other impurities, especially if they are in constant motion or
subject to heat. Some metals, such as zinc, lead, brass, and
copper, have an undesirable chemical reaction on certain
liquids. These chemical processes result in the formation
of sludge, gums, and carbon or other deposits that clog
openings, cause valves and pistons to stick or leak, and give
poor lubrication to moving parts. As soon as small amounts
of sludge or other deposits are formed, the rate of formation
generally increases more rapidly. As they are formed, certain
changes in the physical and chemical properties of the liquid
take place. The liquid usually becomes darker in color, higher
in viscosity, and acids are formed.
Flash point is the temperature at which a liquid gives off vapor
in sufficient quantity to ignite momentarily or flash when a
flame is applied. A high flash point is desirable for hydraulic
liquids because it indicates good resistance to combustion and
a low degree of evaporation at normal temperatures.
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. Like flash point, a high
fire point is required of desirable hydraulic liquids.
Types of Hydraulic Fluids
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:
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. For example,
contamination of the fire-resistant fluid MIL-H-83282 with
MIL-H-5606 may render the MIL-H-83282 non fire-resistant.
Mineral oil-based hydraulic fluid (MIL-H-5606) is the oldest,
dating back to the 1940s. It is used in many systems, especially
where the fire hazard is comparatively low. MIL-H-6083
is simply a rust-inhibited version of MIL-H-5606. They
are completely interchangeable. Suppliers generally ship
hydraulic components with MIL-H-6083. Mineral-based
hydraulic fluid (MIL–H-5606) is processed from petroleum.
It has an odor similar to penetrating oil and is dyed red.
Synthetic rubber seals are used with petroleum-based fluids.
MIL-H-83282 is a fire-resistant hydrogenated polyalphaolefinbased fluid developed in the 1960s to overcome the
flammability characteristics of MIL-H-5606. MIL-H-83282
is significantly more flame resistant than MIL-H-5606, but
a disadvantage is the high viscosity at low temperature.
It is generally limited to –40 °F. However, it can be used
in the same system and with the same seals, gaskets, and
hoses as MIL-H-5606. MIL-H-46170 is the rust-inhibited
version of MIL-H-83282. Small aircraft predominantly use
MIL-H-5606, but some have switched to MIL-H-83282 if
they can accommodate the high viscosity at low temperature.
Phosphate Ester-Based Fluid (Skydrol®)
These fluids are used in most commercial transport category
aircraft and are extremely fire-resistant. However, they are not
fireproof and under certain conditions, they burn. The earliest
generation of these fluids was developed after World War II as
a result of the growing number of aircraft hydraulic brake fires
that drew the collective concern of the commercial aviation
industry. Progressive development of these fluids occurred as
a result of performance requirements of newer aircraft designs.
The airframe manufacturers dubbed these new generations of
hydraulic fluid as types based on their performance.
Today, types IV and V fluids are used. Two distinct classes
of type IV fluids exist based on their density: class I fluids
are low density and class II fluids are standard density. The
class I fluids provide weight savings advantages versus
class II. In addition to the type IV fluids that are currently
in use, type V fluids are being developed in response to
industry demands for a more thermally stable fluid at higher
operating temperatures. Type V fluids will be more resistant
to hydrolytic and oxidative degradation at high temperature
than the type IV fluids.
Intermixing of Fluids
Due to the difference in composition, petroleum-based and
phosphate ester-based fluids will not mix; neither are the
seals for any one fluid usable with or tolerant of any of the
other fluids. Should an aircraft hydraulic system be serviced
with the wrong type fluid, immediately drain and flush the
system and maintain the seals according to the manufacturer’s
Compatibility with Aircraft Materials
Aircraft hydraulic systems designed around Skydrol®
fluids should be virtually trouble-free if properly serviced.
Skydrol® is a registered trademark of Monsanto Company.
Skydrol® does not appreciably affect common aircraft
metals—aluminum, silver, zinc, magnesium, cadmium, iron,
stainless steel, bronze, chromium, and others—as long as the
fluids are kept free of contamination. Due to the phosphate
ester base of Skydrol® fluids, thermoplastic resins, including
vinyl compositions, nitrocellulose lacquers, oil-based
paints, linoleum, and asphalt may be softened chemically
by Skydrol® fluids. However, this chemical action usually
requires longer than just momentary exposure, and spills that
are wiped up with soap and water do not harm most of these
materials. Paints that are Skydrol® resistant include epoxies
and polyurethanes. Today, polyurethanes are the standard of
the aircraft industry because of their ability to keep a bright,
shiny finish for long periods of time and for the ease with
which they can be removed.
a systematic process of elimination. Fluid returned to the
reservoir may contain impurities from any part of the system.
To determine which component is defective, liquid samples
should be taken from the reservoir and various other locations
in the system. Samples should be taken in accordance with
the applicable manufacturer’s instructions for a particular
hydraulic system. Some hydraulic systems are equipped with
permanently installed bleed valves for taking liquid samples,
whereas on other systems, lines must be disconnected to
provide a place to take a sample.
Hydraulic Sampling Schedule
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. Skydrol® type V fluid is compatible
with natural fibers and with a number of synthetics, including
nylon and polyester, which are used extensively in most
aircraft. Petroleum oil hydraulic system seals of neoprene or
Buna-N are not compatible with Skydrol® and must be replaced
with seals of butyl rubber or ethylene-propylene elastoiners.
Hydraulic Fluid Contamination
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.
Nonabrasives, including those resulting from oil
oxidation and soft particles worn or shredded from
seals and other organic components.
Routine sampling—each system should be sampled
at least once a year, or every 3,000 flight hours, or
whenever the airframe manufacturer suggests.
Unscheduled maintenance—when malfunctions may
have a fluid related cause, samples should be taken.
Suspicion of contamination—if contamination is
suspected, fluids should be drained and replaced,
with samples taken before and after the maintenance
Pressurize and operate hydraulic system for 10–15
minutes. During this period, operate various flight
controls to activate valves and thoroughly mix
Shut down and depressurize the system.
Before taking samples, always be sure to wear the
proper personal protective equipment that should
include, at the minimum, safety glasses and gloves.
Wipe off sampling port or tube with a lint-free cloth.
Do not use shop towels or paper products that could
produce lint. Generally speaking, the human eye can
see particles down to about 40 microns in size. Since
we are concerned with particles down to 5 microns in
size, it is easy to contaminate a sample without ever
Place a waste container under the reservoir drain
valve and open valve so that a steady, but not forceful,
stream is running.
Allow approximately 1 pint (250 ml) of fluid to drain.
This purges any settled particles from the sampling
Insert a precleaned sample bottle under the fluid stream
and fill, leaving an air space at the top. Withdraw the
bottle and cap immediately.
Close drain valve.
Whenever it is suspected that a hydraulic system has become
contaminated or the system has been operated at temperatures
in excess of the specified maximum, a check of the system
should be made. The filters in most hydraulic systems are
designed to remove most foreign particles that are visible
to the naked eye. Hydraulic liquid that appears clean to the
naked eye may be contaminated to the point that it is unfit for
use. Thus, visual inspection of the hydraulic liquid does not
determine the total amount of contamination in the system.
Large particles of impurities in the hydraulic system are
indications that one or more components are being subjected
to excessive wear. Isolating the defective component requires
Fill out sample identification label supplied in sample
kit, making sure to include customer name, aircraft
type, aircraft tail number, hydraulic system sampled,
and date sampled. Indicate on the sample label under
remarks if this is a routine sample or if it is being taken
due to a suspected problem.
Service system reservoirs to replace the fluid that was
Submit samples for analysis to laboratory.
Filters provide adequate control of the contamination
problem during all normal hydraulic system operations.
Control of the size and amount of contamination entering
the system from any other source is the responsibility of the
people who service and maintain the equipment. Therefore,
precautions should be taken to minimize contamination
during maintenance, repair, and service operations. If the
system becomes contaminated, the filter element should be
removed and cleaned or replaced. As an aid in controlling
contamination, the following maintenance and servicing
procedures should be followed at all times:
Maintain all tools and the work area (workbenches
and test equipment) in a clean, dirt-free condition.
A suitable container should always be provided to
receive the hydraulic liquid that is spilled during
component removal or disassembly procedures.
Before disconnecting hydraulic lines or fittings, clean
the affected area with dry cleaning solvent.
All hydraulic lines and fittings should be capped or
plugged immediately after disconnecting.
Before assembly of any hydraulic components, wash
all parts in an approved dry cleaning solvent.
After cleaning the parts in the dry cleaning solution,
dry the parts thoroughly and lubricate them with the
recommended preservative or hydraulic liquid before
assembly. Use only clean, lint-free cloths to wipe or
dry the component parts.
All seals and gaskets should be replaced during the
reassembly procedure. Use only those seals and
gaskets recommended by the manufacturer.
All parts should be connected with care to avoid
stripping metal slivers from threaded areas. All fittings
and lines should be installed and torqued in accordance
with applicable technical instructions.
All hydraulic servicing equipment should be kept clean
and in good operating condition.
Contamination, both particulate and chemical, is detrimental
to the performance and life of components in the aircraft
hydraulic system. Contamination enters the system through
normal wear of components by ingestion through external
seals during servicing, or maintenance, when the system is
opened to replace/repair components, etc. 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
Hydraulic System Flushing
When inspection of hydraulic filters or hydraulic fluid
evaluation indicates that the fluid is contaminated, flushing
the system may be necessary. This must be done according to
the manufacturer’s instructions; however, a typical procedure
for flushing is as follows:
1. Connect a ground hydraulic test stand to the inlet and
outlet test ports of the system. Verify that the ground
unit fluid is clean and contains the same fluid as
2. Change the system filters.
3. Pump clean, filtered fluid through the system, and
operate all subsystems until no obvious signs of
contamination are found during inspection of the filters.
Dispose of contaminated fluid and filter. Note: A visual
inspection of hydraulic filters is not always effective.
4. Disconnect the test stand and cap the ports.
5. Ensure that the reservoir is filled to the full line or
proper service level.
It is very important to check if the fluid in the hydraulic test
stand, or mule, is clean before the flushing operation starts.
A contaminated hydraulic test stand can quickly contaminate
other aircraft if used for ground maintenance operations.
Health and Handling
Skydrol® fluids are phosphate ester-based fluids blended with
performance additives. Phosphate esters are good solvents
and dissolve away some of the fatty materials of the skin.
Repeated or prolonged exposure may cause drying of the
skin, which if unattended, could result in complications,
such as dermatitis or even secondary infection from bacteria.
Skydrol® fluids could cause itching of the skin but have not
been known to cause allergic-type skin rashes. Always use the
proper gloves and eye protection when handling any type of
hydraulic fluid. When Skydrol®/Hyjet mist or vapor exposure
is possible, a respirator capable of removing organic vapors
and mists must be worn. Ingestion of any hydraulic fluid
should be avoided. Although small amounts do not appear to
be highly hazardous, any significant amount should be tested
in accordance with manufacturer’s direction, followed with
hospital supervised stomach treatment.
Basic Hydraulic Systems
Regardless of its function and design, every hydraulic system
has a minimum number of basic components in addition to a
means through which the fluid is transmitted. A basic system
consists of a pump, reservoir, directional valve, check valve,
pressure relieve valve, selector valve, actuator, and filter.
Figure 12-3. Open center hydraulic system.
Figure 12-2. Basic hydraulic system.
Open Center Hydraulic Systems
An open center system is one having fluid flow, but no
pressure in the system when the actuating mechanisms are
idle. The pump circulates the fluid from the reservoir, through
the selector valves, and back to the reservoir. [Figure 12-3]
The open center system may employ any number of
subsystems, with a selector valve for each subsystem. Unlike
the closed center system, the selector valves of the open
center system are always connected in series with each other.
In this arrangement, the system pressure line goes through
each selector valve. Fluid is always allowed free passage
through each selector valve and back to the reservoir until one
of the selector valves is positioned to operate a mechanism.
When one of the selector valves is positioned to operate an
actuating device, fluid is directed from the pump through one
of the working lines to the actuator. [Figure 12-3B] With the
selector valve in this position, the flow of fluid through the
valve to the reservoir is blocked. The pressure builds up in
the system to overcome the resistance and moves the piston
of the actuating cylinder; fluid from the opposite end of the
actuator returns to the selector valve and flows back to the
reservoir. Operation of the system following actuation of the
component depends on the type of selector valve being used.
Several types of selector valves are used in conjunction with
the open center system. One type is both manually engaged
and manually disengaged. First, the valve is manually moved
to an operating position. Then, the actuating mechanism
reaches the end of its operating cycle, and the pump output
continues until the system relief valve relieves the pressure.
The relief valve unseats and allows the fluid to flow back to
the reservoir. The system pressure remains at the relief valve
set pressure until the selector valve is manually returned to the
neutral position. This action reopens the open center flow and
allows the system pressure to drop to line resistance pressure.
The manually engaged and pressure disengaged type of
selector valve is similar to the valve previously discussed.
When the actuating mechanism reaches the end of its cycle,
the pressure continues to rise to a predetermined pressure.
The valve automatically returns to the neutral position and
to open center flow.
Closed-Center Hydraulic Systems
In the closed-center system, the fluid is under pressure
whenever the power pump is operating. The three actuators are
arranged in parallel and actuating units B and C are operating
at the same time, while actuating unit A is not operating. This
system differs from the open-center system in that the selector
or directional control valves are arranged in parallel and not in
series. The means of controlling pump pressure varies in the
closed-center system. If a constant delivery pump is used, the
system pressure is regulated by a pressure regulator. A relief
valve acts as a backup safety device in case the regulator fails.
If a variable displacement pump is used, system pressure
is controlled by the pump’s integral pressure mechanism
compensator. The compensator automatically varies the
volume output. When pressure approaches normal system
pressure, the compensator begins to reduce the flow output
of the pump. The pump is fully compensated (near zero flow)
when normal system pressure is attained. When the pump
is in this fully compensated condition, its internal bypass
mechanism provides fluid circulation through the pump for
cooling and lubrication. A relief valve is installed in the
system as a safety backup. [Figure 12-4] An advantage of the
open-center system over the closed-center system is that the
continuous pressurization of the system is eliminated. Since
the pressure is built up gradually after the selector valve is
moved to an operating position, there is very little shock from
pressure surges. This action provides a smoother operation of
the actuating mechanisms. The operation is slower than the
closed-center system, in which the pressure is available the
moment the selector valve is positioned. Since most aircraft
applications require instantaneous operation, closed-center
systems are the most widely used.
Hydraulic systems were utilized for brake systems on early
aircraft. When aircraft started to fly faster and got larger in
size, the pilot was not able to move the control surfaces by
hand anymore, and hydraulic power boost systems were
introduced. Power boost systems assist the pilot in overcoming
high control forces, but the pilot still actuates the flight
controls by cable or push rod.
Many modern aircraft use a power supply system and fly-bywire flight control. The pilot input is electronically sent to the
flight control servos. Cables or push rods are not used. Small
power packs are the latest evolution of the hydraulic system.
They reduce weight by eliminating hydraulic lines and large
quantities of hydraulic fluid. Some manufacturers are reducing
hydraulic systems in their aircraft in favor of electrically
controlled systems. The Boeing 787 is the first aircraft designed
with more electrical systems than hydraulic systems.
Hydraulic Power Pack System
A hydraulic power pack is a small unit that consists of an
electric pump, filters, reservoir, valves, and pressure relief
valve. [Figure 12-5] The advantage of the power pack is that
Hydraulic Power Systems
Evolution of Hydraulic Systems
Smaller aircraft have relatively low flight control surface
loads, and the pilot can operate the flight controls by hand.
Figure 12-5. Hydraulic power pack.
Figure 12-4. A basic closed-center hydraulic system with a variable displacement pump.
there is no need for a centralized hydraulic power supply
system and long stretches of hydraulic lines, which reduces
weight. Power packs could be driven by either an engine
gearbox or electric motor. Integration of essential valves,
filters, sensors, and transducers reduces system weight,
virtually eliminates any opportunity for external leakage, and
simplifies troubleshooting. Some power pack systems have
an integrated actuator. These systems are used to control the
stabilizer trim, landing gear, or flight control surfaces directly,
thus eliminating the need for a centralized hydraulic system.
lost through leakage. Furthermore, the reservoir serves as an
overflow basin for excess fluid forced out of the system by
thermal expansion (the increase of fluid volume caused by
temperature changes), the accumulators, and by piston and
The reservoir also furnishes a place for the fluid to purge
itself of air bubbles that may enter the system. Foreign matter
picked up in the system may also be separated from the fluid
in the reservoir or as it flows through line filters. Reservoirs
are either pressurized or nonpressurized.
Hydraulic System Components
Figure 12-6 is a typical example of a hydraulic system in a
large commercial aircraft. The following sections discuss the
components of such system in more detail.
Baffles and/or fins are incorporated in most reservoirs to keep
the fluid within the reservoir from having random movement,
such as vortexing (swirling) and surging. These conditions
can cause fluid to foam and air to enter the pump along with
the fluid. Many reservoirs incorporate strainers in the filler
neck to prevent the entry of foreign matter during servicing.
These strainers are made of fine mesh screening and are
usually referred to as finger strainers because of their shape.
Finger strainers should never be removed or punctured as a
means of speeding up the pouring of fluid into the reservoir.
The reservoir is a tank in which an adequate supply of fluid
for the system is stored. Fluid flows from the reservoir to the
pump, where it is forced through the system and eventually
returned to the reservoir. The reservoir not only supplies the
operating needs of the system, but it also replenishes fluid
RSVR fill bel MLV Hand pump
Right system return
Figure 12-6. Large commercial aircraft hydraulic system.
Reservoirs could have an internal trap to make sure fluid goes
to the pumps during negative-G conditions.
metal. Filter elements are normally installed within the
reservoir to clean returning system hydraulic fluid.
Most aircraft have emergency hydraulic systems that take
over if main systems fail. In many such systems, the pumps of
both systems obtain fluid from a single reservoir. Under such
circumstances, a supply of fluid for the emergency pump is
ensured by drawing the hydraulic fluid from the bottom of the
reservoir. The main system draws its fluid through a standpipe
located at a higher level. With this arrangement, should the
main system’s fluid supply become depleted, adequate fluid
is left for operation of the emergency system. Figure 12-7
illustrates that the engine-driven pump (EDP) is not able
to draw fluid any more if the reservoir gets depleted below
the standpipe. The alternating current motor-driven pump
(ACMP) still has a supply of fluid for emergency operations.
In some of the older aircraft, a filter bypass valve is
incorporated to allow fluid to bypass the filter in the event the
filter becomes clogged. Reservoirs can be serviced by pouring
fluid directly into the reservoir through a filler strainer (finger
strainer) assembly incorporated within the filler well to strain
out impurities as the fluid enters the reservoir. Generally,
nonpressurized reservoirs use a visual gauge to indicate the
fluid quantity. Gauges incorporated on or in the reservoir may
be a direct reading glass tube-type or a float-type rod that is
visible through a transparent dome. In some cases, the fluid
quantity may also be read in the cockpit through the use of
quantity transmitters. A typical nonpressurized reservoir is
shown in Figure 12-8. This reservoir consists of a welded
body and cover assembly clamped together. Gaskets are
incorporated to seal against leakage between assemblies.
Nonpressurized reservoirs are used in aircraft that are not
designed for violent maneuvers, do not fly at high altitudes, or
in which the reservoir is located in the pressurized area of the
aircraft. High altitude in this situation means an altitude where
atmospheric pressure is inadequate to maintain sufficient
flow of fluid to the hydraulic pumps. Most nonpressurized
reservoirs are constructed in a cylindrical shape. The outer
housing is manufactured from a strong corrosion-resistant
Nonpressurized reservoirs are slightly pressurized due to
thermal expansion of fluid and the return of fluid to the
reservoir from the main system. This pressure ensures that
there is a positive flow of fluids to the inlet ports of the
hydraulic pumps. Most reservoirs of this type are vented
directly to the atmosphere or cabin with only a check valve
and filter to control the outside air source. The reservoir system
RSVR pressurized shutoff valve
RSVR pressurized module
RSVR pressurized relief valve
EDP supply shutoff valve
Return filter module
Depress solenoid valve
SYS pressurized XDCR
From RSVR servicing
Figure 12-7. Hydraulic reservoir standpipe for emergency operations.
Connection for vent line or pressurizing line
Filler neck, cap, and fastener
Normal fluid level
Glass sight gauge
Connection for return line
Connection for main system pump
Connection for emergency system pump
Figure 12-8. Nonpressurized reservoir.
Figure 12-9. Air-pressurized reservoir.
includes a pressure and vacuum relief valve. The purpose of
the valve is to maintain a differential pressure range between
the reservoir and cabin. A manual air bleed valve is installed
on top of the reservoir to vent the reservoir. The valve is
connected to the reservoir vent line to allow depressurization
of the reservoir. The valve is actuated prior to servicing the
reservoir to prevent fluid from being blown out of the filler as
the cap is being removed. The manual bleed valve also needs
to be actuated if hydraulic components need to be replaced.
Reservoirs on aircraft designed for high-altitude flight are
usually pressurized. Pressurizing assures a positive flow of
fluid to the pump at high altitudes when low atmospheric
pressures are encountered. On some aircraft, the reservoir is
pressurized by bleed air taken from the compressor section
of the engine. On others, the reservoir may be pressurized
by hydraulic system pressure.
Air-pressurized reservoirs are used in many commercial
transport-type aircraft. [Figures 12-9 and 12-10]
Pressurization of the reservoir is required because the
reservoirs are often located in wheel wells or other nonpressurized areas of the aircraft and at high altitude there
Reservoir pressure relief valve
Return and fill line
EDP supply line
ACMP supply line
Figure 12-10. Components of an air-pressurized reservoir.
is not enough atmospheric pressure to move the fluid to
the pump inlet. Engine bleed air is used to pressurize the
reservoir. The reservoirs are typically cylindrical in shape.
The following components are installed on a typical reservoir:
Reservoir pressure relief valve—prevents over
pressurization of the reservoir. Valve opens at a preset
Sight glasses (low and overfull)—provides visual
indication for flight crews and maintenance personnel
that the reservoir needs to be serviced.
Reservoir sample valve—used to draw a sample of
hydraulic fluid for testing.
Reservoir drain valve—used to drain the fluids out of
the reservoir for maintenance operation.
Reservoir temperature transducer—provides hydraulic
fluid temperature information for the flight deck.
Manual bleed valve
A manual bleeder valve is incorporated into the module.
During hydraulic system maintenance, it is necessary to
relieve reservoir air pressure to assist in the installation and
removal of components, lines, etc. This type of valve is small
in size and has a push button installed in the outer case. When
the bleeder valve push button is pushed, pressurized air from
the reservoir flows through the valve to an overboard vent
until the air pressure is depleted or the button is released.
When the button is released, the internal spring causes the
poppet to return to its seat. Some hydraulic fluid can escape
from the manual bleed valve when the button is depressed.
Caution: Put a rag around the air bleed valve on the reservoir
pressurization module to catch hydraulic fluid spray.
Hydraulic fluid spray can cause injuries to persons.
Figure 12-11. Temperature and quantity sensors.
Reservoir quantity transmitter—transmits fluid
quantity to the flight deck so that the flight crew can
monitor fluid quantity during flight. [Figure 12-11]
A reservoir pressurization module is installed close to the
reservoir. [Figure 12-12] The reservoir pressurization
module supplies airplane bleed air to the reservoirs. The
module consists of the following parts:
Check valves (2)
Some aircraft hydraulic system reservoirs are pressurized
by hydraulic system pressure. Regulated hydraulic pump
output pressure is applied to a movable piston inside the
cylindrical reservoir. This small piston is attached to and
moves a larger piston against the reservoir fluid. The reduced
force of the small piston when applied by the larger piston is
adequate to provide head pressure for high altitude operation.
The small piston protrudes out of the body of the reservoir.
The amount exposed is used as a reservoir fluid quantity
indicator. Figure 12-13 illustrates the concept behind the
fluid-pressurized hydraulic reservoir.
The reservoir has five ports: pump suction, return,
pressurizing, overboard drain, and bleed port. Fluid is
supplied to the pump through the pump suction port. Fluid
returns to the reservoir from the system through the return
port. Pressure from the pump enters the pressurizing cylinder
in the top of the reservoir through the pressurizing port. The
Reservoir pressure switch
RSVR pressurized shutoff valve
To HYD RSWR
Manual bleed valve
Figure 12-12. Reservoir pressurization module.
fluid to pump inlet
Hydraulic system pressure (3,000 PSI)
Figure 12-13. Operating principle behind a fluid-pressurized
overboard drain port drains the reservoir, when necessary,
while performing maintenance. The bleed port is used as
an aid in servicing the reservoir. When servicing a system
equipped with this type of reservoir, place a container under
the bleed drain port. The fluid should then be pumped into the
reservoir until air-free fluid flows through the bleed drain port.
The reservoir fluid level is indicated by the markings on
the part of the pressurizing cylinder that moves through
the reservoir dust cover assembly. There are three fluid
level markings indicated on the cover: full at zero system
pressure (FULL ZERO PRESS), full when system is
pressurized (FULL SYS PRESS), and REFILL. When the
system is unpressurized and the pointer on the reservoir lies
between the two full marks, a marginal reservoir fluid level
is indicated. When the system is pressurized and the pointer
lies between REFILL and FULL SYS PRESS, a marginal
reservoir fluid level is also indicated.
Nonpressurized reservoirs can be serviced by pouring fluid
directly into the reservoir through a filler strainer (finger
strainer) assembly incorporated within the filler well to
strain out impurities as the fluid enters the reservoir. Many
reservoirs also have a quick disconnect service port at
the bottom of the reservoir. A hydraulic filler unit can be
connected to the service port to add fluid to the reservoir. This
method reduces the chances of contamination of the reservoir.
Aircraft that use pressurized reservoirs often have a central
filling station in the ground service bay to service all
reservoirs from a single point. [Figure 12-14]
A built-in hand pump is available to draw fluid from
a container through a suction line and pump it into the
reservoirs. Additionally, a pressure fill port is available for
attachment of a hydraulic mule or serving cart, which uses an
external pump to push fluid into the aircraft hydraulic system.
A check valve keeps the hand pump output from exiting the
pressure fill port. A single filter is located downstream of
both the pressure fill port and the hand pump to prevent the
introduction of contaminants during fluid servicing.
It is very important to follow the maintenance instructions
when servicing the reservoir. To get the correct results when
the hydraulic fluid quantities are checked or the reservoirs are
to be filled, the airplane should be in the correct configuration.
Failure to do so could result in overservicing of the reservoir.
This configuration could be different for each aircraft. The
following service instructions are an example of a large
Before servicing always make sure that the:
Spoilers are retracted,
Landing gear is down,
Landing gear doors are closed,
Thrust reversers are retracted, and
Parking brake accumulator pressure reads at least
A filter is a screening or straining device used to clean
the hydraulic fluid, preventing foreign particles and
contaminating substances from remaining in the system.
[Figure 12-15] If such objectionable material were not
removed, the entire hydraulic system of the aircraft could
fail through the breakdown or malfunctioning of a single
unit of the system.
The hydraulic fluid holds in suspension tiny particles of
metal that are deposited during the normal wear of selector
valves, pumps, and other system components. Such minute
particles of metal may damage the units and parts through
which they pass if they are not removed by a filter. Since
tolerances within the hydraulic system components are quite
small, it is apparent that the reliability and efficiency of the
entire system depends upon adequate filtering.
Fluid quantity indicator
To reservoir pressurization system
System A reservoir
System B reservoir
Pressure fill fitting for
hydraulic service cart
Suction line to clean
container of fluid
Figure 12-14. The hydraulic ground serive station on a Boeing 737 provides for hydraulic fluid servicing with a hand pump or via an
external pressure fluid source. All three reservoirs are serviced from the same location.
Differential pressure indicator
Differential pressure indicators
Filter head assembly with bypass valve
System pressure gauge
Filters (bowl outside, filter inside)
Ground service disconnect
Case drain filter
Figure 12-15. Filter module components.
Filters may be located within the reservoir, in the pressure
line, in the return line, or in any other location the designer
of the system decides that they are needed to safeguard the
hydraulic system against impurities. Modern design often
uses a filter module that contains several filters and other
components. [Figure 12-16] There are many models and
styles of filters. Their position in the aircraft and design
requirements determine their shape and size. Most filters
used in modern aircraft are of the inline type. The inline filter
assembly is comprised of three basic units: head assembly,
bowl, and element. The head assembly is secured to the
Figure 12-16. A transport category filter module with two filters.
aircraft structure and connecting lines. Within the head,
there is a bypass valve that routes the hydraulic fluid directly
from the inlet to the outlet port if the filter element becomes
clogged with foreign matter. The bowl is the housing that
holds the element to the filter head and is removed when
element removal is required.
The element may be a micron, porous metal, or magnetic
type. The micron element is made of a specially treated paper
and is normally thrown away when removed. The porous
metal and magnetic filter elements are designed to be cleaned
by various methods and replaced in the system.
A typical micron-type filter assembly utilizes an element
made of specially treated paper that is formed in vertical
convolutions (wrinkles). An internal spring holds the
elements in shape. The micron element is designed to prevent
the passage of solids greater than 10 microns (0.000394 inch)
in size. [Figure 12-17] In the event that the filter element
becomes clogged, the spring-loaded relief valve in the filter
head bypasses the fluid after a differential pressure of 50 psi
has been built up. Hydraulic fluid enters the filter through
the inlet port in the filter body and flows around the element
inside the bowl. Filtering takes place as the fluid passes
through the element into the hollow core, leaving the foreign
material on the outside of the element.
After the filter element has been replaced, the system must
be pressure tested to ensure that the sealing element in the
filter assembly is intact.
In the event of a major component failure, such as a pump,
consideration must be given to replacing the system filter
elements, as well as the failed component.
Filter Bypass Valve
Filter modules are often equipped with a bypass relief valve.
The bypass relief valve opens if the filter clogs, permitting
continued hydraulic flow and operation of aircraft systems.
Dirty oil is preferred over no flow at all. Figure 12-18 shows
the principle of operation of a filter bypass valve. Ball valve
opens when the filter becomes clogged and the pressure over
the filter increases.
Detail A-B thermal lockout
Human hair is about
100 micron in diameter
25,400 microns = 1 inch
Figure 12-17. Size comparison in microns.
Maintenance of Filters
Maintenance of filters is relatively easy. It mainly involves
cleaning the filter and element or cleaning the filter and
replacing the element. Filters using the micron-type element
should have the element replaced periodically according
to applicable instructions. Since reservoir filters are of the
micron type, they must also be periodically changed or
cleaned. For filters using other than the micron-type element,
cleaning the filter and element is usually all that is necessary.
However, the element should be inspected very closely to
ensure that it is completely undamaged. The methods and
materials used in cleaning all filters are too numerous to be
included in this text. Consult the manufacturer’s instructions
for this information.
When replacing filter elements, be sure that there is no
pressure on the filter bowl. Protective clothing and a face
shield must be used to prevent fluid from contacting the
eye. Replace the element with one that has the proper rating.
Figure 12-18. Filter bypass valve.
Filter Differential Pressure Indicators
The extent to which a filter element is loaded can be
determined by measuring the drop in hydraulic pressure
across the element under rated flow conditions. This drop,
or differential pressure, provides a convenient means of
monitoring the condition of installed filter elements and
is the operating principle used in the differential pressure
or loaded-filter indicators found on many filter assemblies.
Differential pressure indicating devices have many
configurations, including electrical switches, continuousreading visual indicators (gauges), and visual indicators
with memory. Visual indicators with memory usually take
the form of magnetic or mechanically latched buttons or
pins that extend when the differential pressure exceeds that
Double-action hand pumps produce fluid flow and pressure
on each stroke of the handle. [Figure 12-19] The doubleaction hand pump consists essentially of a housing that has
a cylinder bore and two ports, a piston, two spring-loaded
check valves, and an operating handle. An O-ring on the
piston seals against leakage between the two chambers of
the piston cylinder bore. An O-ring in a groove in the end of
the pump housing seals against leakage between the piston
rod and housing.
Differential pressure indicators are a component part of the
filter assembly in which they are installed and are normally
tested and overhauled as part of the complete assembly.
With some model filter assemblies, however, it is possible
to replace the indicator itself without removal of the filter
assembly if it is suspected of being inoperative or out of
calibration. It is important that the external surfaces of buttontype indicators be kept free of dirt or paint to ensure free
movement of the button. Indications of excessive differential
pressure, regardless of the type of indicator employed,
should never be disregarded. All such indications must be
verified and action taken, as required, to replace the loaded
filter element. Failure to replace a loaded element can result
in system starvation, filter element collapse, or the loss of
filtration where bypass assemblies are used. Verification of
loaded filter indications is particularly important with buttontype indicators as they may have been falsely triggered by
mechanical shock, vibration, or cold start of the system.
Verification is usually obtained by manually resetting the
indicator and operating the system to create a maximum flow
demand ensuring that the fluid is at near normal operating
Several types of hand pumps are used: single action, double
action, and rotary. A single action hand pump draws fluid
into the pump on one stroke and pumps that fluid out on the
next stroke. It is rarely used in aircraft due to this inefficiency.
Some button indicators have a thermal lockout device
incorporated in their design that prevents operation of the
indicator below a certain temperature. The lockout prevents
the higher differential pressure generated at cold temperatures
by high fluid viscosity from causing a false indication of a
loaded filter element.
The hydraulic hand pump is used in some older aircraft for the
operation of hydraulic subsystems and in a few newer aircraft
systems as a backup unit. Hand pumps are generally installed
for testing purposes, as well as for use in emergencies. Hand
pumps are also installed to service the reservoirs from a
single refilling station. The single refilling station reduces the
chances for the introduction of fluid contamination.
allowed for a serviceable element. [Figure 12-18, top] When
this increased pressure reaches a specific value, inlet pressure
forces the spring-loaded magnetic piston downward, breaking
the magnetic attachment between the indicator button and
the magnetic piston. This allows the red indicator to pop out,
signifying that the element must be cleaned. The button or
pin, once extended, remains in that position until manually
reset and provides a permanent (until reset) warning of a
loaded element. This feature is particularly useful where it
is impossible for an operator to continuously monitor the
visual indicator, such as in a remote location on the aircraft.
All aircraft hydraulic systems have one or more power-driven
pumps and may have a hand pump as an additional unit 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.
Figure 12-19. Double action hand pump.
When the piston is moved to the right, the pressure in the
chamber left of the piston is lowered. The inlet port ball check
valve opens and hydraulic fluid is drawn into the chamber.
At the same time, the rightward movement of the piston
forces the piston ball check valve against its seat. Fluid in the
chamber to the right of the piston is forced out of the outlet
port into the hydraulic system. When the piston is moved to
the left, the inlet port ball check valve seats. Pressure in the
chamber left of the piston rises, forcing the piston ball check
valve off of its seat. Fluid flows from the left chamber through
the piston to the right chamber. The volume in the chamber
right of the piston is smaller than that of the left chamber due
to the displacement created by the piston rod. As the fluid
from the left chamber flows into the smaller right chamber,
the excess volume of fluid is forced out of the outlet port to
the hydraulic system.
two hydraulic systems, eight engine-driven pumps, and three
electrical driven pumps. The Boeing 777 has three hydraulic
systems with two engine driven pumps, four electrical driven
pumps, two air driven pumps, and a hydraulic pump motor
driven by the RAT. [Figure 12-21 and 12-22]
A rotary hand pump may also be employed. It produces
continuous output while the handle is in motion. Figure 12-20
shows a rotary hand pump in a hydraulic system.
Figure 12-21. Engine-driven pump.
Figure 12-22. Electrically-driven pump.
Classification of Pumps
Figure 12-20. Rotary hand pump.
Many of the power driven hydraulic pumps of current
aircraft are of variable delivery, compensator-controlled
type. Constant delivery pumps are also in use. Principles
of operation are the same for both types of pumps. Modern
aircraft use a combination of engine-driven power pumps,
electrical-driven power pumps, air-driven power pumps,
power transfer units (PTU), and pumps driven by a RAT.
For example, large aircraft, such as the Airbus A380, have
All pumps may be classified as either positive displacement
or nonpositive displacement. Most pumps used in hydraulic
systems are positive displacement. A nonpositivedisplacement pump produces a continuous flow. However,
because it does not provide a positive internal seal against
slippage, its output varies considerably as pressure
varies. Centrifugal and propeller pumps are examples of
nonpositive-displacement pumps. If the output port of a
nonpositive-displacement pump was blocked off, the pressure
would rise and output would decrease to zero. Although
the pumping element would continue moving, flow would
stop because of slippage inside the pump. In a positive-
displacement pump, slippage is negligible compared to
the pump’s volumetric output flow. If the output port were
plugged, pressure would increase instantaneously to the point
that the pump pressure relief valve opens.
A constant-displacement pump, regardless of pump
rotations per minute, forces a fixed or unvarying quantity
of fluid through the outlet port during each revolution of
the pump. Constant-displacement pumps are sometimes
called constant-volume or constant-delivery pumps. They
deliver a fixed quantity of fluid per revolution, regardless
of the pressure demands. Since the constant-delivery pump
provides a fixed quantity of fluid during each revolution
of the pump, the quantity of fluid delivered per minute
depends upon pump rotations per minute. When a constantdisplacement pump is used in a hydraulic system in which
the pressure must be kept at a constant value, a pressure
regulator is required.
Figure 12-23. Gear-type power pump.
When the driving gear turns, as shown in Figure 12-23, it
turns the driven gear. Fluid is captured by the teeth as they
pass the inlet, and it travels around the housing and exits at
Gear-Type Power Pump
A gear-type power pump is a constant-displacement pump.
It consists of two meshed gears that revolve in a housing.
[Figure 12-23] The driving gear is driven by the aircraft
engine or some other power unit. The driven gear meshes
with, and is driven by, the driving gear. Clearance between
the teeth as they mesh and between the teeth and the housing
is very small. The inlet port of the pump is connected to the
reservoir, and the outlet port is connected to the pressure line.
A gerotor-type power pump consists essentially of a housing
containing an eccentric-shaped stationary liner, an internal
gear rotor having seven wide teeth of short height, a spur
driving gear having six narrow teeth, and a pump cover that
contains two crescent-shaped openings. [Figure 12-24] One
opening extends into an inlet port and the other extends into
an outlet port. During the operation of the pump, the gears
turn clockwise together. As the pockets between the gears
Figure 12-24. Gerotor pump.
on the left side of the pump move from a lowermost position
toward a topmost position, the pockets increase in size,
resulting in the production of a partial vacuum within these
pockets. Since the pockets enlarge while over the inlet port
crescent, fluid is drawn into them. As these same pockets
(now full of fluid) rotate over to the right side of the pump,
moving from the topmost position toward the lowermost
position, they decrease in size. This results in the fluid being
expelled from the pockets through the outlet port crescent.
cylinder block, a piston for each bore, and a valve plate with
inlet and outlet slots. The purpose of the valve plate slots is
to let fluid into and out of the bores as the pump operates.
The cylinder bores lie parallel to and symmetrically around
the pump axis. All aircraft axial-piston pumps have an odd
number of pistons. [Figure 12-26]
Piston pumps can be constant-displacement or variabledisplacement pumps. The common features of design and
operation that are applicable to all piston-type hydraulic
pumps are described in the following paragraphs. Pistontype power-driven pumps have flanged mounting bases for
the purpose of mounting the pumps on the accessory drive
cases of aircraft engines. A pump drive shaft, which turns
the mechanism, extends through the pump housing slightly
beyond the mounting base. Torque from the driving unit
is transmitted to the pump drive shaft by a drive coupling.
The drive coupling is a short shaft with a set of male splines
on both ends. The splines on one end engage with female
splines in a driving gear; the splines on the other end engage
with female splines in the pump drive shaft. Pump drive
couplings are designed to serve as safety devices. The shear
section of the drive coupling, located midway between the
two sets of splines, is smaller in diameter than the splines.
If the pump becomes unusually hard to turn or becomes
jammed, this section shears, preventing damage to the
pump or driving unit. [Figure 12-25] The basic pumping
mechanism of piston-type pumps consists of a multiple-bore
Outlet valve plate slot
Figure 12-26. Hydraulic pump shear shaft.
Bent Axis Piston Pump
A typical constant-displacement axial-type pump is shown
in Figure 12-27. The angular housing of the pump causes
a corresponding angle to exist between the cylinder block
and the drive shaft plate to which the pistons are attached.
It is this angular configuration of the pump that causes the
pistons to stroke as the pump shaft is turned. When the pump
operates, all parts within the pump (except the outer races of
the bearings that support the drive shaft, the cylinder bearing
pin on which the cylinder block turns, and the oil seal) turn
together as a rotating group. At one point of rotation of the
rotating group, a minimum distance exists between the top of
The fluid from here flows through the valve plate slot
Inlet valve plate slot
Figure 12-25. Axial inline piston pump.
Drive shaft plate
Cylinder bearing pin
n er block
Figure 12-27. Bent axis piston pump.
the cylinder block and the upper face of the drive shaft plate.
Because of the angled housing at a point of rotation 180°
away, the distance between the top of the cylinder block and
the upper face of the drive shaft plate is at a maximum. At any
given moment of operation, three of the pistons are moving
away from the top face of the cylinder block, producing a
partial vacuum in the bores in which these pistons operate.
This occurs over the inlet port, so fluid is drawn into these
bores at this time. On the opposite side of the cylinder block,
three different pistons are moving toward the top face of the
block. This occurs while the rotating group is passing over
the outlet port causing fluid to be expelled from the pump
by these pistons. The continuous and rapid action of the
pistons is overlapping in nature and results in a practically
nonpulsating pump output.
Inline Piston Pump
The simplest type of axial piston pump is the swash plate
design in which a cylinder block is turned by the drive shaft.
Pistons fitted to bores in the cylinder block are connected
through piston shoes and a retracting ring so that the shoes
bear against an angled swash plate. As the block turns, the
piston shoes follow the swash plate, causing the pistons to
reciprocate. The ports are arranged in the valve plate so
that the pistons pass the inlet as they are pulled out, and
pass the outlet as they are forced back in. In these pumps,
displacement is determined by the size and number of pistons,
as well as their stroke length, which varies with the swash
plate angle. This constant-displacement pump is illustrated
in Figure 12-26.
The vane-type power pump is also a constant-displacement
pump. It consists of a housing containing four vanes (blades),
a hollow steel rotor with slots for the vanes, and a coupling
to turn the rotor. [Figure 12-28] The rotor is positioned off
center within the sleeve. The vanes, which are mounted in
the slots in the rotor, together with the rotor, divide the bore
of the sleeve into four sections. As the rotor turns, each
section passes one point where its volume is at a minimum
and another point where its volume is at a maximum. The
volume gradually increases from minimum to maximum
during the first half of a revolution and gradually decreases
from maximum to minimum during the second half of the
revolution. As the volume of a given section increases, that
section is connected to the pump inlet port through a slot in
Figure 12-28. Vane-type power pump.
the sleeve. Since a partial vacuum is produced by the increase
in volume of the section, fluid is drawn into the section
through the pump inlet port and the slot in the sleeve. As the
rotor turns through the second half of the revolution and the
volume of the given section is decreasing, fluid is displaced
out of the section, through the slot in the sleeve aligned with
the outlet port, and out of the pump.
Case relief valve
A variable-displacement pump has a fluid output that is varied
to meet the pressure demands of the system. The pump output
is changed automatically by a pump compensator within
the pump. The following paragraph discusses a two-stage
Vickers variable displacement pump. The first stage of the
pump consists of a centrifugal pump that boosts the pressure
before the fluid enters the piston pump. [Figure 12-29]
Basic Pumping Operation
The aircraft’s engine rotates the pump drive shaft, cylinder
block, and pistons via a gearbox. Pumping action is generated
by piston shoes that are restrained and slide on the shoe
bearing plate in the yoke assembly. Because the yoke is at
an angle to the drive shaft, the rotary motion of the shaft is
converted to piston reciprocating motion.
As the piston begins to withdraw from the cylinder
block, system inlet pressure forces fluid through a porting
arrangement in the valve plate into the cylinder bore. The
piston shoes are restrained in the yoke by a piston shoe
retaining plate and a shoe plate during the intake stroke.
SCL engine valve
Figure 12-29. Variable displacement pump.
As the drive shaft continues to turn the cylinder block, the
piston shoe continues following the yoke bearing surface.
This begins to return the piston into its bore (i.e., toward the
The fluid contained in the bore is precompressed, then
expelled through the outlet port. Discharge pressure holds
the piston shoe against the yoke bearing surface during
the discharge stroke and also provides the shoe pressure
balance and fluid film through an orifice in the piston and
With each revolution of the drive shaft and cylinder block,
each piston goes through the pumping cycle described
above, completing one intake and one discharge stroke.
High-pressure fluid is ported out through the valve plate, past
the blocking valve, to the pump outlet. The blocking valve
is designed to remain open during normal pump operation.
Internal leakage keeps the pump housing filled with fluid
for lubrication of rotating parts and cooling. The leakage is
returned to the system through a case drain port. The case
valve relief valve protects the pump against excessive case
pressure, relieving it to the pump inlet.
Normal Pumping Mode
The pressure compensator is a spool valve that is held
in the closed position by an adjustable spring load.
[Figure 12-30] When pump outlet pressure (system pressure)
exceeds the pressure setting (2,850 psi for full flow), the spool
moves to admit fluid from the pump outlet against the yoke
actuator piston. In Figure 12-30, the pressure compensator
is shown at cracking pressure; the pump outlet pressure is
just high enough to move the spool to begin admitting fluid
to the actuator piston.
The yoke is supported inside the pump housing on two
bearings. At pump outlet pressures below 2,850 psi, the
yoke is held at its maximum angle relative to the drive shaft
centerline by the force of the yoke return spring. Decreasing
system flow demand causes outlet pressure to become high
enough to crack the compensator valve open and admit fluid
to the actuator piston.
Compensator valve spring
Control pressure (Pc)
(Pi) Inlet pressure
Yoke actuating piston
(Ps) Outlet pressure
(Pc) Control pressure
(Pcase) Case pressure
Figure 12-30. Normal pumping mode.
This control pressure overcomes the yoke return spring force
and strokes the pump yoke to a reduced angle. The reduced
angle of the yoke results in a shorter stroke for the pistons
and reduced displacement. [Figure 12-31]
Maximum pumping angle
The lower displacement results in a corresponding reduction
in pump flow. The pump delivers only that flow required to
maintain the desired pressure in the system. When there is no
demand for flow from the system, the yoke angle decreases
to nearly zero degrees stroke angle. In this mode, the unit
pumps only its own internal leakage. Thus, at pump outlet
pressures above 2,850 psi, pump displacement decreases
as outlet pressure rises. At system pressures below this
level, no fluid is admitted through the pressure compensator
valve to the actuator piston and the pump remains at
full displacement, delivering full flow. Pressure is then
determined by the system demand. The unit maintains zero
flow at system pressure of 3,025 psi.
When the solenoid valve is energized, the EDV solenoid
valve moves up against the spring force and the outlet
fluid is ported to the EDV control piston on the top of the
compensator (depressurizing piston). [Figure 12-32] The
high-pressure fluid pushes the compensator spool beyond
Minimum stroke position
Figure 12-31. Yoke angle.
Control pressure (Pc)
EDV solenoid (deenergized)
(Pi) Inlet pressure
Figure 12-32. Depressurized mode.
(Ps) Outlet pressure
Yoke actuating piston
(Pc) Control pressure
(Pcase) Case pressure
its normal metering position. This removes the compensator
valve from the circuit and connects the actuator piston
directly to the pump outlet. Outlet fluid is also ported to the
blocking valve spring chamber, which equalizes pressure
on both sides of its plunger. The blocking valve closes due
to the force of the blocking valve spring and isolates the
pump from the external hydraulic system. The pump strokes
itself to zero delivery at an outlet pressure that is equal to
the pressure required on the actuator piston to reduce the
yoke angle to nearly zero, approximately 1,100 psi. This
depressurization and blocking feature can be used to reduce
the load on the engine during startup and, in a multiple pump
system, to isolate one pump at a time and check for proper
system pressure output.
with the selector valve to operate the unit in either direction
and a corresponding return path for the fluid to the reservoir
is provided. There are two main types of selector valves:
open-center and closed-center. An open center valve allows
a continuous flow of system hydraulic fluid through the
valve even when the selector is not in a position to actuate
a unit. A closed-center selector valve blocks the flow of
fluid through the valve when it is in the NEUTRAL or OFF
position. [Figure 12-33A]
Selector valves may be poppet-type, spool-type, piston-type,
rotary-type, or plug-type. [Figure 12-34] Regardless, each
selector valve has a unique number of ports. The number
of ports is determined by the particular requirements of
the system in which the valve is used. Closed-centered
selector valves with four ports are most common in aircraft
hydraulic systems. These are known as four-way valves.
Figure 12-33 illustrates how this valve connects to the
pressure and return lines of the hydraulic system, as well as to
the two ports on a common actuator. Most selector valves are
mechanically controlled by a lever or electrically controlled
by solenoid or servo. [Figure 12-35]
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,
priority valves, shuttle valves, quick disconnect valves, and
The four ports on a four-way selector valve always have the
same function. One port receives pressurized fluid from the
system hydraulic pump. A second port always returns fluid
to the reservoir. The third and forth ports are used to connect
the selector valve to the actuating unit. There are two ports
on the actuating unit. When the selector valve is positioned
to connect pressure to one port on the actuator, the other
A selector valve is used to control the direction of movement
of a hydraulic actuating cylinder or similar device. It provides
for the simultaneous flow of hydraulic fluid both into and
out of the unit. Hydraulic system pressure can be routed
Cyl 2 port
Cyl 1 port
Thermal relief valve
Figure 12-33. Operation of a closed-center four-way selector valve, which controls an actuator.
Left actuator line
Right actuator line
Figure 12-36. Servo control valve solenoids not energized.
Figure 12-34. A poppet-type four-way selector valve.
When selected via a switch in the cockpit, the right solenoid is
energized. The right pilot valve plug shifts left, which blocks
reaching the right end of the main
spool. The spool slides to the right due to greater pressure
applied on the left end of the spool. The center lobe of the
spool no longer blocks system pressurized fluid, which flows
to the actuator through the left actuator line. At the same time,
return flow is blocked from the main spool left chamber so
the actuator (not shown) moves in the selected direction.
Return fluid from the moving actuator flows through the
right actuator line past the spool and into the return line.
Figure 12-35. Four-way servo control valve.
actuator port is simultaneously connected to the reservoir
return line through selector valve. [Figure 12-33B] Thus,
the unit operates in a certain direction. When the selector
valve is positioned to connect pressure to the other port
on the actuating unit, the original port is simultaneously
connected to the return line through the selector valve and
the unit operates in the opposite direction. [Figure 12-33C]
Figure 12-36 illustrates the internal flow paths of a solenoid
operated selector valve. The closed center valve is shown
in the NEUTRAL or OFF position. Neither solenoid is
energized. The pressure port routes fluid to the center lobe
on the spool, which blocks the flow. Fluid pressure flows
through the pilot valves and applies equal pressure on both
ends of the spool. The actuator lines are connected around
the spool to the return line.
Figure 12-37. Servo control valve right solenoid energized.
Typically, the actuator or moving device contacts a limit
switch when the desired motion is complete. The switch
causes the right solenoid to de-energize and the right pilot
valve reopens. Pressurized fluid can once again flow through
the pilot valve and into the main spool right end chamber.
There, the spring and fluid pressure shift the spool back
to the left into the NEUTRAL or OFF position shown in
To make the actuator move in the opposite direction, the
cockpit switch is moved in the opposite direction. All motion
inside the selector valve is the same as described above but in
the opposite direction. The left solenoid is energized. Pressure
is applied to the actuator through the right port and return
fluid from the left actuator line is connected to the return port
through the motion of the spool to the left.
Another common flow control valve in aircraft hydraulic
systems is the check valve. A check valve allows fluid to
flow unimpeded in one direction, but prevents or restricts
fluid flow in the opposite direction. A check valve may be
an independent component situated in-line somewhere in
the hydraulic system or it may be built-in to a component.
When part of a component, the check valve is said to be an
integral check valve.
A typical check valve consists of a spring loaded ball and seat
inside a housing. The spring compresses to allow fluid flow
in the designed direction. When flow stops, the spring pushes
the ball against the seat which prevents fluid from flowing
in the opposite direction through the valve. An arrow on the
outside of the housing indicated the direction in which fluid
flow is permitted. [Figure 12-38] A check valve may also
be constructed with spring loaded flapper or coned shape
piston instead of a ball.
Orifice-Type Check Valve
Some check valves allow full fluid flow in one direction
and restricted flow in the opposite direction. These are
known as orifice-type check valves, or damping valves. The
valve contains the same spring, ball, and seat combination
as a normal check valve but the seat area has a calibrated
orifice machined into it. Thus fluid flow is unrestricted in the
designed direction while the ball is pushed off of its seat. The
downstream actuator operates at full speed. When fluid back
flows into the valve, the spring forces the ball against the seat
which limits fluid flow to the amount that can pass through the
orifice. The reduced flow in this opposite direction slows the
motion, or dampens, the actuator associated with the check
valve. [Figure 12-38]
An orifice check valve may be included in a hydraulic
landing gear actuator system. When the gear is raised, the
check valve allows full fluid flow to lift the heavy gear at
maximum speed. When lowering the gear, the orifice in the
check valve prevents the gear from violently dropping by
restricting fluid flow out of the actuating cylinder.
Sequence valves control the sequence of operation between
two branches in a circuit; they enable one unit to automatically
set another unit into motion. An example of the use of a
sequence valve is in an aircraft landing gear actuating system.
In a landing gear actuating system, the landing gear doors must
open before the landing gear starts to extend. Conversely,
the landing gear must be completely retracted before the
doors close. A sequence valve installed in each landing gear
actuating line performs this function. A sequence valve is
somewhat similar to a relief valve except that, after the set
pressure has been reached, the sequence valve diverts the fluid
to a second actuator or motor to do work in another part of
the system. There are various types of sequence valves. Some
are controlled by pressure, some are controlled mechanically,
and some are controlled by electric switches.
Simple-type in-line check valve (ball-type)
Orifice-type in-line check valve (ball-type)
Flow direction marking on simple-type in-line check valve
Flow direction marking on orifice-type in-line check valve
Figure 12-38. An in-line check valve and orifice type in-line check valve.
Pressure-Controlled Sequence Valve
The operation of a typical pressure-controlled sequence
valve is illustrated in Figure 12-36. The opening pressure is
obtained by adjusting the tension of the spring that normally
holds the piston in the closed position. (Note that the top part
of the piston has a larger diameter than the lower part.) Fluid
enters the valve through the inlet port, flows around the lower
part of the piston and exits the outlet port, where it flows to
the primary (first) unit to be operated. [Figure 12-39A] This
fluid pressure also acts against the lower surface of the piston.
When the primary actuating unit completes its operation,
pressure in the line to the actuating unit increases sufficiently
to overcome the force of the spring, and the piston rises.
The valve is then in the open position. [Figure 12-39B] The
fluid entering the valve takes the path of least resistance and
flows to the secondary unit. A drain passage is provided to
allow any fluid leaking past the piston to flow from the top
of the valve. In hydraulic systems, this drain line is usually
connected to the main return line.
Figure 12-40. Mechanically operated sequence valve.
primary unit moves the plunger as it completes its operation.
The plunger unseats the check valve and allows the fluid to
flow through the valve, out port B, and to the secondary unit.
A priority valve gives priority to the critical hydraulic
subsystems over noncritical systems when system pressure
is low. For instance, if the pressure of the priority valve is set
for 2,200 psi, all systems receive pressure when the pressure
is above 2,200 psi. If the pressure drops below 2,200 psi,
the priority valve closes and no fluid pressure flows to the
noncritical systems. [Figure 12-41] Some hydraulic designs
use pressure switches and electrical shutoff valves to assure
that the critical systems have priority over noncritical systems
when system pressure is low.
Quick Disconnect Valves
Quick disconnect valves are installed in hydraulic lines to
prevent loss of fluid when units are removed. Such valves
Outlet to primary unit
Outlet to secondary unit
Figure 12-39. A pressure-controlled sequence valve.
Mechanically Operated Sequence Valve
The mechanically operated sequence valve is operated
by a plunger that extends through the body of the valve.
[Figure 12-40] The valve is mounted so that the plunger is
operated by the primary unit. A check valve, either a ball or
a poppet, is installed between the fluid ports in the body. It
can be unseated by either the plunger or fluid pressure. Port
A and the actuator of the primary unit are connected by a
common line. Port B is connected by a line to the actuator of
the secondary unit. When fluid under pressure flows to the
primary unit, it also flows into the sequence valve through
port A to the seated check valve in the sequence valve. In
order to operate the secondary unit, the fluid must flow
through the sequence valve. The valve is located so that the
A hydraulic fuse is a safety device. Fuses may be installed
at strategic locations throughout a hydraulic system. They
detect a sudden increase in flow, such as a burst downstream,
and shut off the fluid flow. By closing, a fuse preserves
hydraulic fluid for the rest of the system. Hydraulic fuses
are fitted to the brake system, leading edge flap and slat
extend and retract lines, nose landing gear up and down
lines, and the thrust reverser pressure and return lines. One
type of fuse, referred to as the automatic resetting type, is
designed to allow a certain volume of fluid per minute to pass
through it. If the volume passing through the fuse becomes
excessive, the fuse closes and shuts off the flow. When the
pressure is removed from the pressure supply side of the fuse,
it automatically resets itself to the open position. Fuses are
usually cylindrical in shape, with an inlet and outlet port at
opposite ends. [Figure 12-43]
Figure 12-41. Priority valve.
are installed in the pressure and suction lines of the system
immediately upstream and downstream of the power pump. In
addition to pump removal, a power pump can be disconnected
from the system and a hydraulic test stand connected in
its place. These valve units consist of two interconnecting
sections coupled together by a nut when installed in the
system. Each valve section has a piston and poppet assembly.
These are spring loaded to the closed position when the unit
is disconnected. [Figure 12-42]
Figure 12-43. Hydraulic fuse.
Pressure Control Valves
Figure 12-42. A hydraulic quick-disconnect valve.
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. 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.
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.
The design of pressure relief valves incorporates adjustable
spring-loaded valves. They are installed in such a manner as
to discharge fluid from the pressure line into a reservoir return
line when the pressure exceeds the predetermined maximum
for which the valve is adjusted. Various makes and designs
of pressure relief valves are in use, but, in general, they all
employ a spring-loaded valving device operated by hydraulic
pressure and spring tension. [Figure 12-44] Pressure relief
valves are adjusted by increasing or decreasing the tension
on the spring to determine the pressure required to open the
valve. They may be classified by type of construction or uses
in the system. The most common types of valve are:
Adjusting screw cap
Figure 12-44. Pressure relief valve.
1. Ball type—in pressure relief valves with a ball-type
valving device, the ball rests on a contoured seat.
Pressure acting on the bottom of the ball pushes it off
its seat, allowing the fluid to bypass.
2. Sleeve type—in pressure relief valves with a sleevetype valving device, the ball remains stationary and
a sleeve-type seat is moved up by the fluid pressure.
This allows the fluid to bypass between the ball and
the sliding sleeve-type seat.
3. Poppet type—in pressure relief valves with a poppettype valving device, a cone-shaped poppet may have
any of several design configurations; however, it
is basically a cone and seat machined at matched
angles to prevent leakage. As the pressure rises to its
predetermined setting, the poppet is lifted off its seat,
as in the ball-type device. This allows the fluid to pass
through the opening created and out the return port.
Pressure relief valves cannot be used as pressure regulators
in large hydraulic systems that depend on engine-driven
pumps for the primary source of pressure because the pump
is constantly under load and the energy expended in holding
the pressure relief valve off its seat is changed into heat. This
heat is transferred to the fluid and, in turn, to the packing
rings, causing them to deteriorate rapidly. Pressure relief
valves, however, may be used as pressure regulators in small,
low-pressure systems or when the pump is electrically driven
and is used intermittently.
Pressure relief valves may be used as:
1. System relief valve—the most common use of the
pressure relief valve is as a safety device against
the possible failure of a pump compensator or other
pressure regulating device. All hydraulic systems
that have hydraulic pumps incorporate pressure relief
valves as safety devices.
2. Thermal relief valve—the pressure relief valve is used
to relieve excessive pressures that may exist due to
thermal expansion of the fluid. They are used where a
check valve or selector valve prevents pressure from
being relieved through the main system relief valve.
Thermal relief valves are usually smaller than system
relief valves. As pressurized fluid in the line in which
it is installed builds to an excessive amount, the valve
poppet is forced off its seat. This allows excessive
pressurized fluid to flow through the relief valve to the
reservoir return line. When system pressure decreases
to a predetermined pressure, spring tension overcomes
system pressure and forces the valve poppet to the
The term pressure regulator is applied to a device used in
hydraulic systems that are pressurized by constant-deliverytype pumps. One purpose of the pressure regulator is to
manage the output of the pump to maintain system operating
pressure within a predetermined range. The other purpose
is to permit the pump to turn without resistance (termed
unloading the pump) at times when pressure in the system
is within normal operating range. The pressure regulator is
located in the system so that pump output can get into the
system pressure circuit only by passing through the regulator.
The combination of a constant-delivery-type pump and the
pressure regulator is virtually the equivalent of a compensatorcontrolled, variable-delivery-type pump. [Figure 12-45]
Pressure reducing valves are used in hydraulic systems where
it is necessary to lower the normal system operating pressure
by a specified amount. Pressure reducing valves provide a
steady pressure into a system that operates at a lower pressure
Constant delivery pump
Emergency hand pump
Figure 12-45. The location of a pressure regulator in a basic hydraulic system. The regulator unloads the constant delivery pump by
bypassing fluid to the return line when the predetermined system pressure is reached.
than the supply system. A reducing valve can normally be set
for any desired downstream pressure within the design limits
of the valve. Once the valve is set, the reduced pressure is
maintained regardless of changes in supply pressure (as long
as the supply pressure is at least as high as the reduced pressure
desired) and regardless of the system load, if the load does not
exceed the designed capacity of the reducer. [Figure 12-46]
In certain fluid power systems, the supply of fluid to a
subsystem must be from more than one source to meet
system requirements. In some systems, an emergency system
is provided as a source of pressure in the event of normal
system failure. The emergency system usually actuates only
essential components. The main purpose of the shuttle valve
is to isolate the normal system from an alternate or emergency
system. It is small and simple; yet, it is a very important
component. [Figure 12-47] The housing contains three
ports—normal system inlet, alternate or emergency system
inlet, and outlet. A shuttle valve used to operate more than
one actuating unit may contain additional unit outlet ports.
100 pound spring
Relief valve 750 psi
5 square inch shouder
To reduced pressure operated sub-system
Figure 12-46. Operating mechanism of a pressure reducing valve.
Normal system inlet
Figure 12-47. A spring-loaded piston-type shuttle valve in normal
configuration (A) and with alternate/emergency supply (B).
Enclosed in the housing is a sliding part called the shuttle. Its
purpose is to seal off one of the inlet ports. There is a shuttle
seat at each inlet port. When a shuttle valve is in the normal
operation position, fluid has a free flow from the normal
system inlet port, through the valve, and out through the outlet
port to the actuating unit. The shuttle is seated against the
alternate system inlet port and held there by normal system
pressure and by the shuttle valve spring. The shuttle remains
in this position until the alternate system is activated. This
action directs fluid under pressure from the alternate system
to the shuttle valve and forces the shuttle from the alternate
system inlet port to the normal system inlet port. Fluid from
the alternate system then has a free flow to the outlet port, but
is prevented from entering the normal system by the shuttle,
which seals off the normal system port.
Figure 12-48. Shutoff valves.
with nitrogen or air. Cylindrical types are also used in highpressure hydraulic systems. Many aircraft have several
accumulators in the hydraulic system. There may be a main
system accumulator and an emergency system accumulator.
There may also be auxiliary accumulators located in various
The function of an accumulator is to:
Dampen pressure surges in the hydraulic system
caused by actuation of a unit and the effort of the
pump to maintain pressure at a preset level.
Aid or supplement the power pump when several units
are operating at once by supplying extra power from
its accumulated, or stored, power.
Store power for the limited operation of a hydraulic
unit when the pump is not operating.
Supply fluid under pressure to compensate for small
internal or external (not desired) leaks that would
cause the system to cycle continuously by action of
the pressure switches continually kicking in.
The shuttle may be one of four types:
1. Sliding plunge
2. Spring-loaded piston
3. Spring-loaded ball
4. Spring-loaded poppet
In shuttle valves that are designed with a spring, the shuttle
is normally held against the alternate system inlet port by
Shutoff valves are used to shutoff the flow of fluid to a
particular system or component. In general, these types of
valves are electrically powered. Shutoff valves are also used
to create a priority in a hydraulic system and are controlled
by pressure switches. [Figure 12-48]
The accumulator is a steel sphere divided into two chambers
by a synthetic rubber diaphragm. The upper chamber contains
fluid at system pressure, while the lower chamber is charged
Types of Accumulators
There are two general types of accumulators used in aircraft
hydraulic systems: spherical and cylindrical.
The spherical-type accumulator is constructed in two
halves that are fastened and threaded, or welded, together.
Two threaded openings exist. The top port accepts fittings
to connect to the pressurized hydraulic system to the
accumulator. The bottom port is fitted with a gas servicing
valve, such as a Schrader valve. A synthetic rubber
diaphragm, or bladder, is installed in the sphere to create
two chambers. Pressurized hydraulic fluid occupies the upper
chamber and nitrogen or air charges the lower chamber. A
screen at the fluid pressure port keeps the diaphragm, or
bladder, from extruding through the port when the lower
chamber is charged and hydraulic fluid pressure is zero. A
rigid button or disc may also be attached to the diaphragm,
or bladder, for this purpose. [Figure 12-49] The bladder is
installed through a large opening in the bottom of the sphere
and is secured with a threaded retainer plug. The gas servicing
valve mounts into the retainer plug.
Cylindrical accumulators consist of a cylinder and piston
assembly. End caps are attached to both ends of the cylinder.
The internal piston separates the fluid and air/nitrogen
chambers. The end caps and piston are sealed with gaskets
and packings to prevent external leakage around the end
caps and internal leakage between the chambers. In one end
cap, a hydraulic fitting is used to attach the fluid chamber
to the hydraulic system. In the other end cap, a filler valve
is installed to perform the same function as the filler valve
installed in the spherical accumulator. [Figure 12-50]
In operation, the compressed-air chamber is charged to a
predetermined pressure that is somewhat lower than the
system operating pressure. This initial charge is referred to
as the accumulator preload. As an example of accumulator
operation, let us assume that the cylindrical accumulator is
designed for a preload of 1,300 psi in a 3,000-psi system.
Figure 12-50. Cylindrical accumulator.
When the initial charge of 1,300 psi is introduced into the
unit, hydraulic system pressure is zero. As air pressure is
applied through a gas servicing valve, it moves the piston
toward the opposite end until it bottoms. If the air behind
the piston has a pressure of 1,300 psi, the hydraulic system
pump has to create a pressure within the system greater than
1,300 psi before the hydraulic fluid can actuate the piston.
At 1,301 psi the piston starts to move within the cylinder,
compressing the air as it moves. At 2,000 psi, it has backed
up several inches. At 3,000 psi, the piston has backed up to
its normal operating position, compressing the air until it
Hydraulic system pressure
Hydraulic system pressure
Screen to prevent extrusion
Rigid button or
disc to prevent
Nitrogen or air
Gas servicing valve
Figure 12-49. A spherical accumulator with diaphragm (left) and bladder (right). The dotted lines in the right drawing depict the bladder
when the accumulator is charged with both hydraulic system fluid and nitrogen preload.
occupies a space less than one-half the length of the cylinder.
When actuation of hydraulic units lowers the system pressure,
the compressed air expands against the piston, forcing fluid
from the accumulator. This supplies an instantaneous supply
of fluid to the hydraulic system component. The charged
accumulator may also supply fluid pressure to actuate a
component(s) briefly in case of pump failure.
Maintenance of Accumulators
Maintenance consists of inspections, minor repairs,
replacement of component parts, and testing. There is an
element of danger in maintaining accumulators. Therefore,
proper precautions must be strictly observed to prevent injury
Center system heat exchanger
Right system heat exchanger
Before disassembling any accumulator, ensure that all
preload air (or nitrogen) pressure has been discharged. Failure
to release the preload could result in serious injury to the
technician. Before making this check, be certain you know
the type of high-pressure air valve used. When you know
that all air pressure has been removed, you can take the unit
apart. Be sure to follow manufacturer’s instructions for the
specific unit you have.
Transport-type aircraft use heat exchangers in their hydraulic
power supply system to cool the hydraulic fluid from the
hydraulic pumps. This extends the service life of the fluid
and the hydraulic pumps. They are located in the fuel tanks
of the aircraft. The heat exchangers use aluminum finned
tubes to transfer heat from the fluid to the fuel. The fuel in
the tanks that contain the heat exchangers must be maintained
at a specific level to ensure adequate cooling of the fluid.
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
Figure 12-51. Hydraulic heat exchanger.
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.
Actuating cylinders are of two major types: single action
and double action. The single-action (single port) actuating
cylinder is capable of producing powered movement in
one direction only. The double-action (two ports) actuating
cylinder is capable of producing powered movement in two
A single-action actuating cylinder is illustrated in
Figure 12-52A. Fluid under pressure enters the port at the
left and pushes against the face of the piston, forcing the
piston to the right. As the piston moves, air is forced out of
the spring chamber through the vent hole, compressing the
spring. When pressure on the fluid is released to the point
it exerts less force than is present in the compressed spring,
the spring pushes the piston toward the left. As the piston
moves to the left, fluid is forced out of the fluid port. At
the same time, the moving piston pulls air into the spring
chamber through the vent hole. A three-way control valve is
normally used for controlling the operation of a single-action
Rod wiper seal
Fluid extension/sping return
Hydraulic fluid pressure
Fluid return to reservoir
Fluid return to reservoir
Hydraulic fluid pressure
Figure 12-52. Linear actuator.
A double-action (two ports) actuating cylinder is illustrated
in Figure 12-52B. The operation of a double-action actuating
cylinder is usually controlled by a four-way selector valve.
Figure 12-53 shows an actuating cylinder interconnected
with a selector valve. Operation of the selector valve and
actuating cylinder is discussed below.
Besides having the ability to move a load into position, a
double-acting cylinder also has the ability to hold a load in
position. This capability exists because when the selector
valve used to control operation of the actuating cylinder is
placed in the off position, fluid is trapped in the chambers on
both sides of the actuating cylinder piston. Internal locking
actuators also are used in some applications.
Figure 12-53. Linear actuator operation.
When the selector valve is placed in the ON or EXTEND
position, fluid is admitted under pressure to the left-hand
chamber of the actuating cylinder. [Figure 12-53] This
results in the piston being forced toward the right. As the
piston moves toward the right, it pushes return fluid out of
the right-hand chamber and through the selector valve to the
reservoir. When the selector valve is placed in its RETRACT
position, as illustrated in Figure 12-50, fluid pressure enters
the right chamber, forcing the piston toward the left. As the
piston moves toward the left, it pushes return fluid out of the
left chamber and through the selector valve to the reservoir.
Rotary actuators can mount right at the part without taking up
the long stroke lengths required of cylinders. Rotary actuators
are not limited to the 90° pivot arc typical of cylinders; they
can achieve arc lengths of 180°, 360°, or even 720° or more,
depending on the configuration. An often used type of rotary
actuator is the rack and pinion actuator used for many nose
wheel steering mechanisms. In a rack-and-pinion actuator,
a long piston with one side machined into a rack engages a
pinion to turn the output shaft. [Figure 12-54] One side of the
piston receive fluid pressure while the other side is connected
to the return. When the piston moves, it rotates the pinion.
Rack and pinion
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. 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.
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
Rapid, smooth acceleration or deceleration
Control over maximum torque and power
Cushioning effect to reduce shock loads
Smoother reversal of motion
Fluid pressure and return
Figure 12-54. Rack and pinion gear.
Piston-type motors are the most commonly used in hydraulic
systems. [Figure 12-55] They are basically the same as
Fluid is carried in cylinder to outlet and forced
out as piston is pulled back in by shaft flange
Piston thrust on shaft flange
generates torque on output shaft
Fluid pressure at inlet generates thrust on pistons
Figure 12-55. Bent axis pistion motor.
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.
aircraft systems has created a need for packings and gaskets
of varying characteristics and design to meet the many
variations of operating speeds and temperatures to which they
are subjected. No one style or type of seal is satisfactory for
all installations. Some of the reasons for this are:
Pressure at which the system operates.
The type fluid used in the system.
The metal finish and the clearance between adjacent
The type motion (rotary or reciprocating), if any.
Seals are divided into three main classes: packings, gaskets,
and wipers. A seal may consist of more than one component,
such as an O-ring and a backup ring, or possibly an O-ring
and two backup rings. Hydraulic seals used internally on a
sliding or moving assembly are normally called packings.
[Figure 12-58] Hydraulic seals used between nonmoving
fittings and bosses are normally called gaskets.
Figure 12-56. 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. [Figure 12-57]
Hydraulic Motor-Driven Generator (HMDG)
The HMDG is a servo-controlled variable displacement
motor integrated with an AC generator. The HMDG is
designed to maintain a desired output frequency of 400 Hz.
In case of an electrical failure, the HMDG could provide an
alternative source of electrical power.
Seals are used to prevent fluid from passing a certain point,
and to keep air and dirt out of the system in which they are
used. The increased use of hydraulics and pneumatics in
V-ring packings (AN6225) are one-way seals and are always
installed with the open end of the V facing the pressure.
V-ring packings must have a male and female adapter to
hold them in the proper position after installation. It is also
necessary to torque the seal retainer to the value specified
by the manufacturer of the component being serviced, or the
seal may not give satisfactory service.
U-ring packings (AN6226) and U-cup packings are used in
brake assemblies and brake master cylinders. The U-ring
and U-cup seals pressure in only one direction; therefore,
the lip of the packings must face toward the pressure. U-ring
packings are primarily low pressure packings to be used with
pressures of less than 1,000 psi.
Most packings and gaskets used in aircraft are manufactured
in the form of O-rings. An O-ring is circular in shape,
and its cross-section is small in relation to its diameter.
The cross-section is truly round and has been molded and
trimmed to extremely close tolerances. The O-ring packing
seals effectively in both directions. This sealing is done by
distortion of its elastic compound.
Advances in aircraft design have made new O-ring
composition necessary to meet changing conditions.
Hydraulic O-rings were originally established under Air
Force-Navy (AN) specification numbers 6227, 6230, and
6290 for use in fluid at operating temperatures ranging from
Control flow filter
Shaft seal S.A.
Bent axis housing
Impeller boost pressure
C1 control pressure
C2 control pressure
Green System Pump
Yellow System Pump
Figure 12-57. Power transfer unit.
–65 °F to +160 °F. When new designs raised operating
temperatures to a possible +275 °F, more compounds were
developed and perfected.
Figure 12-58. Packings.
Recently, newer compounds were developed under Military
Standard (MS) specifications that offered improved
low-temperature performance without sacrificing hightemperature performance. These superior materials were
adopted in the MS28775 O-ring, which is replacing AN6227
and AN6230 O-rings, and the MS28778 O-ring, which
is replacing the AN6290 O-ring. These O-rings are now
standard for systems where the operating temperatures may
vary from –65 °F to +275 °F.
O-Ring Color Coding
Manufacturers provide color coding on some O-rings, but this
is not a reliable or complete means of identification. The color
coding system does not identify sizes, but only system fluid
or vapor compatibility and, in some cases, the manufacturer.
Color codes on O-rings that are compatible with MIL-H-5606
fluid always contains blue, but may also contain red or other
colors. Packings and gaskets suitable for use with Skydrol®
fluid are always coded with a green stripe, but may also
have a blue, grey, red, green, or yellow dot as a part of the
color code. Color codes on O-rings that are compatible with
hydrocarbon fluid always contain red, but never contain blue.
A colored stripe around the circumference indicates that the
O-ring is a boss gasket seal. The color of the stripe indicates
fluid compatibility: red for fuel, blue for hydraulic fluid. The
coding on some rings is not permanent. On others, it may be
omitted due to manufacturing difficulties or interference with
operation. Furthermore, the color coding system provides no
means to establish the age of the O-ring or its temperature
limitations. Because of the difficulties with color coding,
O-rings are available in individual hermetically sealed
envelopes labeled with all pertinent data. When selecting an
O-ring for installation, the basic part number on the sealed
envelope provides the most reliable compound identification.
Backup rings (MS28782) made of Teflon™ do not deteriorate
with age, are unaffected by any system fluid or vapor, and can
tolerate temperature extremes in excess of those encountered
in high pressure hydraulic systems. Their dash numbers
indicate not only their size but also relate directly to the dash
number of the O-ring for which they are dimensionally suited.
They are procurable under a number of basic part numbers,
but they are interchangeable; any Teflon™ backup ring may be
used to replace any other Teflon™ backup ring if it is of proper
overall dimension to support the applicable O-ring. Backup
rings are not color coded or otherwise marked and must be
identified from package labels. The inspection of backup
rings should include a check to ensure that surfaces are free
from irregularities, that the edges are clean cut and sharp,
and that scarf cuts are parallel. When checking Teflon™
spiral backup rings, make sure that the coils do not separate
more than ¼ inch when unrestrained. Be certain that backup
rings are installed downstream of the O-ring. [Figure 12-59]
Gaskets are used as static (stationary) seals between two
flat surfaces. Some of the more common gasket materials
are asbestos, copper, cork, and rubber. Asbestos sheeting
is used wherever a heat resistant gasket is needed. It is
used extensively for exhaust system gaskets. Most asbestos
exhaust gaskets have a thin sheet of copper edging to prolong
A solid copper washer is used for spark plug gaskets where
it is essential to have a noncompressible, yet semisoft gasket.
Cork gaskets can be used as an oil seal between the engine
crankcase and accessories, and where a gasket is required
Single turn (scarf-cut)
No cut solid
O-ring without back up ring extrudes into gap
Backup rings are installed downstream of
some O-rings to prevent extrusion
Correct after installation
Orientation of a spiral cut teflon backup ring
Figure 12-59. Backup O-rings installed downstream.
that is capable of occupying an uneven or varying space
caused by a rough surface or expansion and contraction.
Rubber sheeting can be used where there is a need for a
compressible gasket. It should not be used in any place
where it may come in contact with gasoline or oil because
the rubber deteriorates very rapidly when exposed to these
substances. Gaskets are used in fluid systems around the
end caps of actuating cylinders, valves, and other units. The
gasket generally used for this purpose is in the shape of an
O-ring, similar to O-ring packings.
Most seals are made from synthetic materials that are
compatible with the hydraulic fluid used. Seals used for
MIL-H-5606 hydraulic fluid are not compatible with
Skydrol® and servicing the hydraulic system with the wrong
fluid could result in leaks and system malfunctions. Seals
for systems that use MIl-H-5606 are made of neoprene or
Buna-N. Seals for Skydrol® are made from butyl rubber or
When removing or installing O-rings, avoid using pointed
or sharp-edged tools that might cause scratching or marring
of hydraulic component surfaces or cause damage to the
O-rings. Special tooling for the installation of O-rings is
available. While using the seal removal and the installation
tools, contact with cylinder walls, piston heads, and related
precision components is not desirable.
After the removal of all O-rings, the parts that receive new
O-rings have to be cleaned and inspected to make sure that
they are free from all contamination. Each replacement
O-ring should be removed from its sealed package and
inspected for defects, such as blemishes, abrasions, cuts,
or punctures. Although an O-ring may appear perfect at
first glance, slight surface flaws may exist. These flaws are
often capable of preventing satisfactory O-ring performance
under the variable operating pressures of aircraft systems;
therefore, O-rings should be rejected for flaws that affect
their performance. Such flaws are difficult to detect, and
one aircraft manufacturer recommends using a four-power
magnifying glass with adequate lighting to inspect each ring
before it is installed. By rolling the ring on an inspection
cone or dowel, the inner diameter surface can also be
checked for small cracks, particles of foreign material, or
other irregularities that cause leakage or shorten the life of
the O-ring. The slight stretching of the ring when it is rolled
inside out helps to reveal some defects not otherwise visible.
After inspection and prior to installation, immerse the
O-ring in clean hydraulic fluid. During the installation, avoid
rolling and twisting the O-ring to maneuver it into place. If
possible, keep the position of the O-ring’s mold line constant.
When the O-ring installation requires spanning or inserting
through sharply threaded areas, ridges, slots, and edges, use
protective measures, such as O-ring entering sleeves, as
shown in Figure 12-60A. After the O-ring is placed in the
cavity provided, gently roll the O-ring with the fingers to
remove any twist that might have occurred during installation.
Wipers are used to clean and lubricate the exposed portions
of piston shafts. They prevent dirt from entering the system
and help protect the piston shaft against scoring. Wipers may
be either metallic or felt. They are sometimes used together,
a felt wiper installed behind a metallic wiper.
Large Aircraft Hydraulic Systems
Figure 12-62 provides an overview of hydraulic components
in large aircraft.
Boeing 737 Next Generation Hydraulic System
The Boeing 737 Next Generation has three 3,000 psi
hydraulic systems: system A, system B, and standby. The
standby system is used if system A and/or B pressure is lost.
The hydraulic systems power the following aircraft systems:
Leading edge flaps and slats
Trailing edge flaps
Nose wheel steering
The system A, B, and standby reservoirs are located in the
wheel well area. The 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. [Figure 12-63]
Refer to Figure 12-64 for the following description. Both A
and B hydraulic systems have an engine-driven pump (EDP)
and an ACMP. The system A engine-driven pump is installed
on the number 1 engine and the system B engine-driven
pump is installed on the number 2 engine. The AC pumps
are controlled by a switch on the flight deck. The hydraulic
case drain fluid that lubricates and cools the pumps return
to the reservoir through a heat exchanger. [Figure 12-65]
The heat exchanger for the A system is installed in the main
fuel tank No. 1, and the heat exchanger for the B system
is installed in the main fuel tank No. 2. Minimum fuel for
ground operation of electric motor-driven pumps is 1,675
pounds in the related main tank. Pressure switches, located
in the EDP and ACMP pump output lines, send signals to
illuminate the related LOW PRESSURE light if pump output
pressure is low. The related system pressure transmitter sends
the combined pressure of the EDP and ACMP to the related
hydraulic system pressure indicator.
Removal tool (hook type)
Extractor tool (pull type)
Internal O-ring removal (using pull-type
extractor and hook-type removal tools)
Internal O-ring removal (using push-type
extractor and hook-type removal tools)
Extractor tool (push type)
Removal tool (hook type)
O-ring extractor tool
Removal tool (hook type)
O-ring removal tool
Extractor tool (push type)
Removal tool (hook type)
Dual internal O-ring removal (using push-type
extractor and hook-type removal tools)
Extractor tool (hook type)
Internal O-ring removal (using wedge-type
extractor and hook-type removal tools)
External O-ring removal (using spoon-type
extractor removal tools)
Removal tool (hook type)
Extractor tool (wedge type)
External O-ring removal (using wedge-type
extractor and hook-type removal tools)
Figure 12-60. O-ring installation techniques.
O-ring receiving groove
Sharp edges corners and threads
O-ring receiving grooves
Paper entering sleeve
Sharp edges and threads
Soft thin-wall metallic sleeve
Installation tool push type
Internal O-Ring Installation
(using metallic sleeve to avoid O-ring damage from
sharp edges or threads and push-type installation tool)
Internal O-Ring Installation
(using paper entering sleeve to avoid O-ring damage from
sharp edges or threads and push-type installation tool)
Sharp edges and corners
O-ring receiving grooves
External O-Ring Installation
(using paper cover to avoid O-ring damage from sharp edges or threads)
O-ring receiving grooves
Figure 12-61. More O-ring installation techniques.
Filter modules are installed in the pressure, case drain,
and return lines to clean the hydraulic fluid. Filters have a
differential pressure indicator that pops out when the filter
is dirty and needs to be replaced.
Power Transfer Unit (PTU)
The purpose of the PTU is to supply the additional volume
of hydraulic fluid needed to operate the autoslats and leading
edge flaps and slats at the normal rate when system B EDP
malfunctions. The PTU unit consists of a hydraulic motor
and hydraulic pump that are connected through a shaft. The
PTU uses system A pressure to drive a hydraulic motor. The
hydraulic motor of the PTU unit is connected through a shaft
with a hydraulic pump that can draw fluid from the system
B reservoir. The PTU can only transfer power and cannot
transfer fluid. The PTU operates automatically when all of
the following conditions are met:
System B EDP pressure drops below limits.
Flaps are less than 15° but not up.
Hydraulic MotorDriven Generator
Stabilizer Trim Motor
Stabilizer trim actuation on the
aircraft is provided by two 3,000psi, constant-displacement,
nine-piston, bent-axis hydraulic
motors. Each motor produces
77.3 in-lb torque at 2,250 psid
with a rated speed of 2,700 rpm
and an intermittent speed of
4,050 rpm. Displacement is
v; weight is 3.9 lb.
The hydraulic motor-driven
generator (HMDG) is a servocontrolled, variable
displacement, inline axis-piston
hydraulic motor integrated
with a three-stage, brushless
generator. The HMDG is
designed to maintain a
steady state generator
output frequency of 400 ±2V
(at the point of regulation)
over a rated electrical output
range of 10kVA.
AC Motor Pump
Leading Edge Slat Drive Motor
Auxiliary power is provided
by a 3,110-psi, 12-gpm,
motor pump. Some AC
motor pumps feature a
design. This protects the
electrical wiring from being
exposed to the caustic
hydrauic fluid environment.
Leading edge slat actuation on the
aircraft provided by one constantdisplacement, nine-piston, bent-axis
hydraulic motor. The motor produces
544.3 in-lb torque at 2,250 psid with
a rated speed of 3,170 rpm and an
intermittent speed of 4,755 rpm.
Displacement is 1.52 in2/rev; weight
is 13.21 lb.
Power Transfer Unit
transfer of hydraulic power
(but not fluid) between the left
right independent hydraulic
system is accomplished with a
nonreversible power transfer
(PTU) that provides an
alternate power source for the
leading and trailing edge flaps
and the landing gear, including
ge steering, which are
normally driven by the left
hydraulic system. The PTU
consists of a bent-axis hydraulic
driving a fixed displacement,
Rated speed is 3,900
Displacement of the pump is
1.39 in /rev and displacement of
motor is 1.52 in2/rev. The unit
weight is 35 lb.
Plays a critical
cal role in
nds to open
door when actuated by nitrogen
reaches full extension in 2.75
onds with output force
to 4.16 seconds
of 2,507 to 2,830 lb.
Trailing Edge Flap Drive Motor
e flap actuation is provided by
psi, constant-displacement, ninet-axis hydraulic motor. The motor
1.4 in-lb torque at 2,250 psid with
ed of 3,750 rpm and an intera rated speed
ed of 5,660 rpm. Displacement is
0.596 in2/rev; weight is 6.5 lb.
Ram Air Turbine Pump
Hydraulic power for the left and right systems is
supplied by two 48-gpm variable-displacement,
3,000-psi pressure compensated in-line pumps.
Displacements 3.0 in2/rev; weight is 40.1 lb.
A 3,025 psi in-line piston pump
provides 20 gpm at 3,920 rpm,
delivering hydraulic power for
the priority flight control surfaces
in the event both engines are
lost or a total electrical power
failure occurs. Displacement
is 1.25 in2/rev; weight is 15 lb.
Nose Wheel Steering System
Consists of a digital electronic controller, hydromecharical power unit, mounting collar, tiller,
and rudder pedal positon sensors. The hydromecharical power unit (an integrated assembly)
includes all the hydraulic valving, power
amplification, actuation, and damping
Figure 12-62. Large aircraft hydraulic systems.
System A reservoir
System B reservoir
Landing Gear Transfer Unit
The purpose of the landing gear transfer unit is to supply the
volume of hydraulic fluid needed to raise the landing gear at
the normal rate when system A EDP is lost. The system B
EDP supplies the volume of hydraulic fluid needed to operate
the landing gear transfer unit when all of the following
conditions are met:
No. 1 engine rpm drops below a limit value.
Landing gear lever is up.
Either or both main landing gear not up and locked.
Figure 12-63. Hydraulic reservoirs on a Boeing 737.
flaps & slats
No. 1 thrust
No. 2 thrust
+ elevator feel
Figure 12-64. Boeing 737 hydraulic system (simplified).
Standby Hydraulic System
The standby hydraulic system is provided as a backup if
system A and/or B pressure is lost. The standby system can
be activated manually or automatically and uses a single
electric ACMP to power:
Leading edge flaps and slats (extend only)
Standby yaw damper
A master caution light illuminates if an overheat or low
pressure is detected in the hydraulic system. An overheat
light on the flight deck illuminates if an overheat is detected
in either system A or B and a low-pressure light illuminates
if a low pressure is detected in system A and B.
Boeing 777 Hydraulic System
The Boeing 777 is equipped with three hydraulic systems.
The left, center, and right systems deliver hydraulic fluid at a
rated pressure of 3,000 psi (207 bar) to operate flight controls,
flap systems, actuators, landing gear, and brakes. Primary
hydraulic power for the left and right systems is provided
by two EDPs and supplemented by two on-demand ACMPs.
Primary hydraulic power for the center system is provided
by two electric motor pumps (ACMP) and supplemented
by two on-demand air turbine-driven pumps (ADP). The
center system provides hydraulic power for the engine
thrust reversers, primary flight controls, landing gear, and
flaps/slats. Under emergency conditions, hydraulic power is
generated by the ram air turbine (RAT), which is deployed
automatically and drives a variable displacement inline pump.
The RAT pump provides flow to the center system flight
controls. [Figure 12-66]
To No. 1 hydraulic pump
No. 1 pump pressure
From case drain filter
Hydraulic system A heat exchanger
Hydraulic supply (fire) shutoff valve
No. 1 Engine pump supply
No. 1 Pump pressure
Case drain return line
Hydraulic line connectors
Mounting clamp (3 places)
Figure 12-65. Boeing 737 hydraulic case drain fluid heat exchanger installed in the fuel tank.
Left and Right System Description
The left and right hydraulic systems are functionally the same.
The left hydraulic system supplies pressurized hydraulic
fluid to operate the left thrust reverser and the flight control
systems. The right hydraulic system supplies pressurized
hydraulic fluid to operate the right thrust reverser, flight
control systems, and the normal brake system. [Figure 12-67]
DC HYD main tank)
EDP = Engine driven pump
ACMP = AC motor pump
ADP = Air driven pump
Figure 12-66 A Boeing 777 hydraulic system.
RSVR pressurized shutoff valve
RSVR pressurized module
RSVR pressurized SW
EDP supply shutoff valve
Return filter module
Depress solenoid valve
SYS pressurized XDCR
From RSVR servicing
Figure 12-67. Right hydraulic system of a Boeing 777. A left system is similar.
The hydraulic system reservoirs of the left and right system
contain the hydraulic fluid supply for the hydraulic pumps.
The reservoir is pressurized by bleed air through a reservoir
pressurization module. The EDP draws fluid through a
standpipe. The ACMP draws fluid from the bottom of
the reservoir. If the fluid level in the reservoir gets below
the standpipe, the EDP cannot draw any fluid any longer,
and the ACMP is the only source of hydraulic power. The
reservoir can be serviced through a center servicing point
in the fuselage of the aircraft. The reservoir has a sample
valve for contamination testing purposes, a temperature
transmitter for temperature indication on the flight deck, a
pressure transducer for reservoir pressure, and a drain valve
for reservoir draining.
The EDPs are the primary pumps for the left and right
hydraulic systems. The EDPs get reservoir fluid through the
EDP supply shutoff valves. The EDPs operates whenever the
engines operate. A solenoid valve in each EDP controls the
pressurization and depressurization of the pump. The pumps
are variable displacement inline piston pumps consisting of
a first stage impeller pump and a second stage piston pump.
The impeller pump delivers fluid under pressure to the piston
pump. The ACMPs are the demand pumps for the left and
right hydraulic systems. The ACMPs normally operate only
when there is high hydraulic system demand.
Pressure and case drain filter modules clean the pressure flows
and the case drain flows of the hydraulic pumps. A return
filter module cleans the return flow of hydraulic fluid from
the user systems. The module can be bypassed if the filter
clogs, and a visible indicator pops to indicate a clogged filter.
The heat exchanger, which is installed in the wing fuel tanks,
cools the hydraulic fluid from ACMP and EDP case drain
lines before the fluid goes back to the reservoir.
The hydraulic system sensors send pressure, temperature,
and quantity signals to the flight deck. A reservoir quantity
transmitter and temperature transducer are installed on each
of the reservoirs, and a hydraulic reservoir pressure switch
is located on the pneumatic line between the reservoir
pressurization module and the reservoir. The ACMP and EDP
filter modules each have a pressure transducer to measure
pump output pressure. A temperature transducer is installed in
the case drain line of each filter module and measures pump
case drain fluid temperature. A system pressure transducer
measures hydraulic system pressure. A pressure relief
valve on the EDP filter module protects the system against
overpressurization. [Figure 12-67]
Center Hydraulic System
The center hydraulic system supplies pressurized hydraulic
fluid to operate these systems [Figure 12-68]:
Nose landing gear actuation
Nose landing gear steering
Main landing gear actuation
Main landing gear steering
Trailing edge flaps
Leading edge slat
The hydraulic system reservoir of the center system contains
the hydraulic fluid supply for the hydraulic pumps. The
reservoir is pressurized by bleed air through a reservoir
pressurization module. The reservoir supplies fluid to the
ADPs, the RAT, and one of the ACMPs through a standpipe.
The other ACMP gets fluid from the bottom of the reservoir.
The reservoir also supplies hydraulic fluid to the landing gear
alternate extension system.
The ACMPs are the primary pumps in the center hydraulic
system and are normally turned on. The ADPs are the
demand pumps in the center system. They normally operate
only when the center system needs more hydraulic flow
capacity. The RAT system supplies an emergency source of
hydraulic power to the center hydraulic system flight controls.
A reservoir quantity transmitter and temperature transducer
are installed on the reservoir. A hydraulic reservoir pressure
switch is installed on the pneumatic line between the reservoir
and the reservoir pressurization module.
Filter modules clean the pressure and case drain output of
the hydraulic pumps. A return filter module cleans the return
flow of hydraulic fluid from the user systems. The module
can be bypassed. The heat exchanger cools the hydraulic
fluid from the ACMP case drains before the fluid goes back
to the reservoir. ADP case drain fluid does not go through
the heat exchangers.
The ACMP and ADP filter modules each have a pressure
transducer to measure pump output pressure. A temperature
transducer in each filter module measures the pump case
drain temperature. A system pressure transducer measures
hydraulic system pressure.
Center SYS RTN
GND SVC disk RTN
NG ISLN valve
RSV ISLN valve
Figure 12-68. Center hydraulic system.
Pressure relief valves in each ADP filter module prevent
system overpressurization. A pressure relief valve near
ACMP C1 supplies overpressure protection for the center
hydraulic isolation system (CHIS).
Center Hydraulic Isolation System (CHIS)
The CHIS supplies engine burst protection and a reserve
brakes and steering function. CHIS operation is fully
automatic. Relays control the electric motors in the reserve
and nose gear isolation valves. When the CHIS system is
operational, it prevents hydraulic operation of the leading
ACMP C1 gets hydraulic fluid from the bottom of the center
system reservoir. All other hydraulic pumps in the center
system get fluid through a standpipe in the reservoir. This
gives ACMP C1 a 1.2 gallon (4.5 liter) reserve supply of
The reserve and nose gear isolation valves are normally
open. Both valves close if the quantity in the center system
reservoir is low (less than 0.40) and the airspeed is more than
60 knots for more than one second. When CHIS is active,
this divides the center hydraulic system into different parts.
The NLG actuation and steering and the leading edge slat
hydraulic lines are isolated from center system pressure. The
output of ACMP C1 goes only to the alternate brake system.
The output of the other center hydraulic system pumps goes
to the trailing edge flaps, the MLG actuation and steering, and
the flight controls. If there is a leak in the NLG actuation and
steering or LE slat lines, there is no further loss of hydraulic
fluid. The alternate brakes, the trailing edge flaps, the MLG
actuation and steering, and the PFCS continue to operate
If there is a leak in the trailing edge flaps, the MLG actuation
and steering, or the flight control lines, the reservoir loses
fluid down to the standpipe level (0.00 indication). This
causes a loss of these systems but the alternate brake system
continues to get hydraulic power from ACMP C1. If there is
a leak in the lines between ACMP C1 and the alternate brake
system, all center hydraulic system fluid is lost.
Nose Gear Isolation Valve
The nose gear isolation valve opens for any of these
Airspeed is less than 60 knots.
Pump pressures for ACMP C2, ADP C1, ADP C2,
and the RAT is less than 1,200 psi for 30 seconds.
Left and right engine rpm is above idle, left and right
EDP pressure is more than 2,400 psi, and the NLG is
not up, the NLG doors are not closed, or the landing
gear lever is not up for 30 seconds.
The first condition permits the flight crew to operate the
NLG steering when airspeed is less than 60 knots (decreased
rudder control authority during taxi). The second condition
permits operation of the NLG actuation and steering if the
hydraulic leak is in the part of the center hydraulic system
isolated by the reserve isolation valve. The third condition
permits operation of the NLG actuation and steering if there
has not been an engine burst and the other hydraulic systems
are pressurized. The nose gear isolation valve opens when
pressure is necessary at the NLG. If the NLG is not fully
retracted or the NLG doors are not closed, the nose gear
isolation valve opens to let the NLG complete the retraction.
When the landing gear lever is moved to the down position,
the nose gear isolation valve opens to let the NLG extend
with center system pressure.
Central Hydraulic System Reset
Both valves open again automatically when the center system
quantity is more than 0.70 and airspeed is less than 60
knots for 5 seconds. Both valves also reset when the center
system quantity is more than 0.70 and both engines and
both engine-driven pumps operate normally for 30 seconds.
Aircraft Pneumatic Systems
Some aircraft manufacturers have equipped their aircraft with
a high pressure pneumatic system (3,000 psi) in the past. The
last aircraft to utilize this type of system was the Fokker F27.
Such systems operate a great deal like hydraulic systems,
except they employ air instead of a liquid for transmitting
power. Pneumatic systems are sometimes used for:
Opening and closing doors
Driving hydraulic pumps, alternators, starters, water
injection pumps, etc.
Operating emergency devices
RSV and NG ISLN valves close
• Airspeed > 60 kts and C SYS
qty < 0.4 (1 second delay)
Nose gear ISLN valve open
• Airspeed < 60 kts
• ACMP C2, ADP C1, ADP C2, and
RAT press < 1,200 psi
(30 second delay)
• L & R eng rpm > idle, L & R EDP
press > 2,400 psi, and (NLG is not
up or NLG doors are not closed,
or the LG CTRL lever is DN) (30
• Qty > 0.7 and airspeed < 60 kts
(5 second delay)
• Qty > 0.7 and L & R eng rpm >
idle and L & R EDP press >
2,400 psi (30 second delay)
P110 left PWR
Lines to slat hydraulic motor
P310 stby PWR
LE slat lines
Nose gear ISLN valve
Main hydraulic systems - center hydraulic isolation system - functional description
Figure 12-69. Center hydraulic isolation system.
Both pneumatic and hydraulic systems are similar units and
use confined fluids. The word confined means trapped or
completely enclosed. The word fluid implies such liquids
as water, oil, or anything that flows. Since both liquids and
gases flow, they are considered as fluids; however, there is
a great deal of difference in the characteristics of the two.
Liquids are practically incompressible; a quart of water still
occupies about a quart of space regardless of how hard it is
compressed. But gases are highly compressible; a quart of
air can be compressed into a thimbleful of space. In spite of
this difference, gases and liquids are both fluids and can be
confined and made to transmit power. The type of unit used to
provide pressurized air for pneumatic systems is determined
by the system’s air pressure requirements.
For high-pressure systems, air is usually stored in metal
bottles at pressures ranging from 1,000 to 3,000 psi,
depending on the particular system. [Figure 12-70] This
type of air bottle has two valves, one of which is a charging
valve. A ground-operated compressor can be connected to
this valve to add air to the bottle. The other valve is a control
valve. It acts as a shutoff valve, keeping air trapped inside
the bottle until the system is operated. Although the highpressure storage cylinder is light in weight, it has a definite
disadvantage. Since the system cannot be recharged during
flight, operation is limited by the small supply of bottled
air. Such an arrangement cannot be used for the continuous
operation of a system. Instead, the supply of bottled air is
reserved for emergency operation of such systems as the
landing gear or brakes. The usefulness of this type of system
is increased, however, if other air-pressurizing units are added
to the aircraft. [Figure 12-71]
Pneumatic System Components
Pneumatic systems are often compared to hydraulic systems,
but such comparisons can only hold true in general terms.
Pneumatic systems do not utilize reservoirs, hand pumps,
accumulators, regulators, or engine-driven or electrically
driven power pumps for building normal pressure. But
similarities do exist in some components.
Pressure relief valve
Porous metal filter
Pressure reducing valve
Pressure relief valve
To nose wheel steering
Alternate/emergency gear control
Normal gear control
Figure 12-70. High-pressure pneumatic system.
Brake control valve
Brake pressure gauge
Rapid exhaust valve
A. Control valve “off”
Figure 12-71. Pneumatic brake system.
On some aircraft, permanently installed air compressors
have been added to recharge air bottles whenever pressure
is used for operating a unit. Several types of compressors are
used for this purpose. Some have two stages of compression,
while others have three, depending on the maximum desired
Relief valves are used in pneumatic systems to prevent
damage. They act as pressure limiting units and prevent
excessive pressures from bursting lines and blowing out seals.
Control valves are also a necessary part of a typical pneumatic
system. Figure 12-72 illustrates how a valve is used to
control emergency air brakes. The control valve consists of
a three-port housing, two poppet valves, and a control lever
with two lobes.
In Figure 12-72A, the control valve is shown in the off
position. A spring holds the left poppet closed so that
compressed air entering the pressure port cannot flow to the
brakes. In Figure 12-72B, the control valve has been placed
in the on position. One lobe of the lever holds the left poppet
From power source
To emergency brakes
B. Control valve “on”
Figure 12-72. Pneumatic control valve.
open, and a spring closes the right poppet. Compressed air
now flows around the opened left poppet, through a drilled
passage, and into a chamber below the right poppet. Since
the right poppet is closed, the high-pressure air flows out of
the brake port and into the brake line to apply the brakes.
To release the brakes, the control valve is returned to the
off position. [Figure 12-72A] The left poppet now closes,
stopping the flow of high-pressure air to the brakes. At the
same time, the right poppet is opened, allowing compressed
air in the brake line to exhaust through the vent port and into
Check valves are used in both hydraulic and pneumatic
systems. Figure 12-73 illustrates a flap-type pneumatic check
valve. Air enters the left port of the check valve, compresses
a light spring, forcing the check valve open and allowing air
Adjustable needle valve
Figure 12-73. Flap-type pneumatic check valve.
to flow out the right port. But if air enters from the right, air
pressure closes the valve, preventing a flow of air out the
left port. Thus, a pneumatic check valve is a one-direction
flow control valve.
Restrictors are a type of control valve used in pneumatic
systems. Figure 12-74 illustrates an orifice-type restrictor
with a large inlet port and a small outlet port. The small outlet
port reduces the rate of airflow and the speed of operation
of an actuating unit.
Figure 12-74. Pneumatic orifice valve.
Another type of speed-regulating unit is the variable
restrictor. [Figure 12-75] It contains an adjustable needle
valve, which has threads around the top and a point on the
lower end. Depending on the direction turned, the needle
valve moves the sharp point either into or out of a small
opening to decrease or increase the size of the opening.
Since air entering the inlet port must pass through this
opening before reaching the outlet port, this adjustment also
determines the rate of airflow through the restrictor.
Figure 12-75. Variable pneumatic restrictor.
Pneumatic systems are protected against dirt by means of
various types of filters. A micronic filter consists of a housing
with two ports, a replaceable cartridge, and a relief valve.
Normally, air enters the inlet, circulates around the cellulose
cartridge, and flows to the center of the cartridge and out
the outlet port. If the cartridge becomes clogged with dirt,
pressure forces the relief valve open and allows unfiltered
air to flow out the outlet port.
A screen-type filter is similar to the micron filter but contains
a permanent wire screen instead of a replaceable cartridge.
In the screen filter, a handle extends through the top of the
housing and can be used to clean the screen by rotating it
against metal scrapers.
The moisture separator in a pneumatic system is always
located downstream of the compressor. Its purpose is to
remove any moisture caused by the compressor. A complete
moisture separator consists of a reservoir, a pressure switch,
a dump valve, and a check valve. It may also include a
regulator and a relief valve. The dump valve is energized
and deenergized by the pressure switch. When deenergized,
it completely purges the separator reservoir and lines up to
the compressor. The check valve protects the system against
pressure loss during the dumping cycle and prevents reverse
flow through the separator.
Chemical driers are incorporated at various locations in a
pneumatic system. Their purpose is to absorb any moisture
that may collect in the lines and other parts of the system.
Each drier contains a cartridge that should be blue in
color. If otherwise noted, the cartridge is to be considered
contaminated with moisture and should be replaced.
Gear Emergency Extension Cable and Handle
The outlet valve is connected to a cable and handle assembly.
The handle is located on the side of the copilot’s console
and is labeled EMER LDG GEAR. Pulling the handle fully
upward opens the outlet valve, releasing compressed nitrogen
into the landing gear extension system. Pushing the handle
fully downward closes the outlet valve and allows any
nitrogen present in the emergency landing gear extension
system to be vented overboard. The venting process takes
approximately 30 seconds.
Emergency Backup Systems
Many aircraft use a high-pressure pneumatic back-up source
of power to extend the landing gear or actuate the brakes,
if the main hydraulic braking system fails. The nitrogen is
not directly used to actuate the landing gear actuators or
brake units but, instead, it applies the pressurized nitrogen
to move hydraulic fluid to the actuator. This process is
called pneudraulics. The following paragraph discusses the
components and operation of an emergency pneumatic landing
gear extension system used on a business jet. [Figure 12-76]
As compressed nitrogen is released to the landing gear
selector/dump valve during emergency extension, the
pneudraulic pressure actuates the dump valve portion of
the landing gear selector/dump valve to isolate the landing
gear system from the remainder of hydraulic system. When
activated, a blue DUMP legend is illuminated on the LDG
GR DUMP V switch, located on the cockpit overhead panel.
A dump valve reset switch is used to reset the dump valve
after the system has been used and serviced.
Nitrogen used for emergency landing gear extension is
stored in two bottles, one bottle located on each side of
the nose wheel well. Nitrogen from the bottles is released
by actuation of an outlet valve. Once depleted, the bottles
must be recharged by maintenance personnel. Fully serviced
pressure is approximately 3,100 psi at 70 °F/21 °C, enough
for only one extension of the landing gear.
Nose wheel steer
Power WOW inputs
NWS cont valves
N2 CYLS (2)
& vent valve
GRND serv valve
with ASC 87
Power transfer unit
with ASC 87
Figure 12-76. Pneumatic emergency landing gear extension system.
Emergency Extension Sequence:
1. Landing gear handle is placed in the DOWN position.
5. Pneudraulic pressure actuates the dump valve portion
of the landing gear selector/dump valve.
6. Blue DUMP legend is illuminated on the LDG GR
7. Landing gear system is isolated from the remainder
of hydraulic system.
8. Pneudraulic pressure is routed to the OPEN side of
the landing gear door actuators, the UNLOCK side of
the landing gear uplock actuators, and the EXTEND
side of the main landing gear sidebrace actuators and
nose landing gear extend/retract actuator.
9. Landing gear doors open.
10. Uplock actuators unlock.
11. Landing gear extends down and locks.
12. Three green DOWN AND LOCKED lights on the
landing gear control panel are illuminated.
13. Landing gear doors remain open.
A medium-pressure pneumatic system (50–150 psi) usually
does not include an air bottle. Instead, it generally draws air
from the compressor section of a turbine engine. This process
is often called bleed air and is used to provide pneumatic
power for engine starts, engine deicing, wing deicing, and in
some cases, it provides hydraulic power to the aircraft systems
(if the hydraulic system is equipped with an air-driven
hydraulic pump). Engine bleed air is also used to pressurize
the reservoirs of the hydraulic system. Bleed air systems are
discussed in more detail in the powerplant handbook.
Many aircraft equipped with reciprocating engines obtain
a supply of low-pressure air from vane-type pumps. These
pumps are driven by electric motors or by the aircraft engine.
Figure 12-77 shows a schematic view of one of these pumps,
which consists of a housing with two ports, a drive shaft, and
two vanes. The drive shaft and the vanes contain slots so the
vanes can slide back and forth through the drive shaft. The
shaft is eccentrically mounted in the housing, causing the
vanes to form four different sizes of chambers (A, B, C, and
D). In the position shown, B is the largest chamber and is
4. Compressed nitrogen is released to the landing gear
3. EMER LDG GEAR handle is pulled fully outward.
2. Red light in the landing gear control handle is
m b er D
Figure 12-77. Schematic of vane-type air pump.
connected to the supply port. As depicted in Figure 12-77,
outside air can enter chamber B of the pump. When the
pump begins to operate, the drive shaft rotates and changes
positions of the vanes and sizes of the chambers. Vane No.
1 then moves to the right, separating chamber B from the
supply port. Chamber B now contains trapped air.
As the shaft continues to turn, chamber B moves downward
and becomes increasingly smaller, gradually compressing its
air. Near the bottom of the pump, chamber B connects to the
pressure port and sends compressed air into the pressure line.
Then chamber B moves upward again becoming increasingly
larger in area. At the supply port, it receives another supply
of air. There are four such chambers in this pump and each
goes through this same cycle of operation. Thus, the pump
delivers to the pneumatic system a continuous supply of
compressed air from 1 to 10 psi. Low-pressure systems are
used for wing deicing boot systems.
Pneumatic Power System Maintenance
Maintenance of the pneumatic power system consists of
servicing, troubleshooting, removal, and installation of
components, and operational testing.
The air compressor’s lubricating oil level should be checked
daily in accordance with the applicable manufacturer’s
instructions. The oil level is indicated by means of a sight gauge
or dipstick. When refilling the compressor oil tank, the oil (type
specified in the applicable instructions manual) is added until
the specified level. After the oil is added, ensure that the filler
plug is torqued and safety wire is properly installed.
The pneumatic system should be purged periodically
to remove the contamination, moisture, or oil from the
components and lines. Purging the system is accomplished
by pressurizing it and removing the plumbing from
various components throughout the system. Removal of
the pressurized lines causes a high rate of airflow through
the system, causing foreign matter to be exhausted from
the system. If an excessive amount of foreign matter,
particularly oil, is exhausted from any one system, the lines
and components should be removed and cleaned or replaced.
Upon completion of pneumatic system purging and after
reconnecting all the system components, the system air
bottles should be drained to exhaust any moisture or
impurities that may have accumulated there.
After draining the air bottles, service the system with nitrogen
or clean, dry compressed air. The system should then be
given a thorough operational check and an inspection for
leaks and security.