Hydraulic System Components - Hydraulics - Airframes & Aircraft Systems #8

Summary

This lesson delves into the intricacies of an aircraft's hydraulic system, focusing on the reservoir's role in storing fluid and preventing leaks. It discusses the necessity of reservoirs despite the system's sealed nature, due to fluid expansion and contraction, and the presence of small, controlled leaks for actuator lubrication. The lesson also covers accumulators, their types, and functions, including pressure stabilization and emergency fluid supply. Additionally, it touches on the importance of filters for protecting system components from debris, and the automatic cutout valve's role in controlling system pressure and reducing pump wear.

Takeaways

• 💧 A reservoir is essential for an aircraft's hydraulic system, even though the system is designed to be leak-free, as it compensates for fluid expansion and contraction, and lubricates actuators.
• 🔄 Accumulators store variable amounts of fluid in hydraulic systems, which is crucial for the system's operation and is detailed further in the lesson.
• 💨 The hydraulic pump's operation involves both supplying pressure and creating suction, which can lead to cavitation if not managed properly by the reservoir.
• 🛑 Cavitation occurs when bubbles in the fluid implode with force, potentially causing severe pump damage, and it can be identified by specific sounds.
• 📡 Reservoirs help prevent cavitation by maintaining a positive pressure at the pump inlet, often achieved by being located higher than the pump or by pressurization with air.
• 🔗 The reservoir includes various connections for the hydraulic system, such as suction pipes to pumps, return pipes, and a quantity transmitter unit for monitoring.
• 🚫 To prevent debris from entering the reservoir, a strainer is used in the filling inlet, which is crucial for maintaining fluid cleanliness.
• 🛠️ Filters are used throughout the hydraulic system to protect components from damage caused by foreign particles and to ensure system efficiency.
• 🔄 An accumulator's function includes dampening pressure fluctuations and providing an emergency fluid supply in case of pump failure, highlighting its importance in system reliability.
• 🔩 The automatic cutout valve (ACOV) manages system pressure in constant delivery fixed volume pump systems, reducing pump wear by controlling the pump's operation.

Q & A

Why is a reservoir necessary in an aircraft's hydraulic system even though it is a sealed system?

A reservoir is necessary because no system can be completely leak-free. It allows for small amounts of fluid to leak across the seals of actuators for lubrication, accommodates fluid expansion and contraction due to temperature changes, and stores extra fluid needed by actuators depending on their extension.

What is the role of an accumulator in a hydraulic system?

An accumulator stores hydraulic fluid under pressure and serves to dampen pressure fluctuations, provide a small emergency supply of fluid in case of pump failure, and allow for thermal expansion and contraction of fluid.

How does a reservoir help prevent cavitation in a hydraulic system?

A reservoir helps prevent cavitation by providing a head of fluid to the pump, ensuring a positive pressure at the pump inlet. It is often located higher in the system than the pump, and in some installations, it is also pressurized with air from the engine compressor.

What is the purpose of the baffles and fins in a reservoir?

Baffles and fins in a reservoir are fitted to prevent sloshing and foaming of the fluid during in-flight maneuvers, ensuring the stability of the fluid within the reservoir.

What is the function of a suction filter in a hydraulic system?

A suction filter protects the pump and other system components by removing foreign particles from the fluid before it enters the pump, preventing damage that could be caused by debris.

How does the bootstrap method work in pressurizing a reservoir?

In the bootstrap method, a piston sits on top of the fluid in the reservoir and is pushed down by an actuator powered from its own hydraulic system, pressurizing the fluid. When the system is not operating, the pressure is held by a non-return valve.

What is the purpose of a filling inlet and strainer in a reservoir?

A filling inlet, normally sealed with a filler cap, allows for the addition of fluid to the reservoir. The strainer in the filling inlet prevents debris from entering the reservoir, helping to keep the hydraulic fluid clean.

What is the role of an automatic cutout valve (ACOV) in a hydraulic system?

An automatic cutout valve (ACOV) controls system pressure and provides the pump with an idling circuit when no services have been selected. It operates in two modes, cut in and cut out, to regulate the flow of fluid and reduce pump wear.

How does an accumulator provide the initial fluid when a selection is made and the pump is cut out?

When a service is selected and the pump is cut out, the accumulator provides the initial fluid under pressure until the system pressure drops efficiently, signaling the pump to come online.

What can be inferred about the condition of a hydraulic system based on the time between cut out and cut in of the ACOV?

The time between cut out and cut in of the ACOV can indicate the condition of the system. External leakage may cause frequent loading and unloading of the pump, while internal leakage, often caused by piston seal failure, can also lead to increased pump activity and potentially higher fluid temperatures.

Outlines

00:00

🛫 Hydraulic System Components: The Role of the Reservoir

This paragraph delves into the intricacies of an aircraft's hydraulic system, emphasizing the critical role of the reservoir. Despite the system's design for minimal leakage, a reservoir is essential due to the inherent impossibility of a perfectly sealed system. It serves to accommodate fluid expansion and contraction with temperature changes and to provide extra fluid for actuators that require more when extended. The reservoir also integrates accumulators, which store variable fluid amounts. A key function of the reservoir is to prevent cavitation in the hydraulic pump by maintaining a positive pressure at the pump inlet, often aided by pressurization with air from the engine compressor. The reservoir's design includes connections for suction and return pipes, a quantity transmitter for monitoring, and a filling inlet with a strainer to prevent debris entry. Additionally, baffles and fins are installed to mitigate fluid sloshing during flight maneuvers. The importance of keeping hydraulic fluid free from contaminants to avoid damage to system components is highlighted.

05:02

💧 Hydraulic Filters and Accumulators: Ensuring System Integrity

The second paragraph focuses on the importance of filters within the hydraulic system, which are crucial for protecting pumps and other components from damage caused by foreign particles. Filters are installed in both suction and pressure lines to maintain fluid cleanliness. Some filters are equipped with a device that detects pressure differential, signaling when they become clogged. Filters can be made from various materials, and while most are disposable, wire elements can be cleaned, often using ultrasonic processes. The paragraph also introduces accumulators, devices that store hydraulic fluid under pressure. Two common types are described: cylindrical with a floating piston and spherical with a flexible diaphragm. Accumulators help stabilize system pressure, provide emergency fluid supply in case of pump failure, and assist in thermal expansion management. They are charged through a charging valve and can be pressurized with air or nitrogen. The discussion also touches on the operation of an automatic cutout valve (ACOV) and its role in controlling system pressure and pump idling.

10:05

🔧 Functions of the Accumulator and Automatic Cutout Valve

This section further elaborates on the functions of the accumulator in an aircraft's hydraulic system. The accumulator not only stores hydraulic fluid under pressure but also plays a vital role in dampening pressure fluctuations and providing an emergency fluid supply in the event of pump failure. It ensures that there is sufficient fluid available for system operation, even when the pump is not actively supplying fluid. The paragraph also discusses the automatic cutout valve (ACOV), which is used in systems with constant delivery fixed volume pumps. The ACOV controls system pressure and provides an idling circuit for the pump when no services are selected. It operates in two modes: cut in, where it allows the pump to operate the service, and cut out, where it returns the pump's output to the reservoir without load. The time between cut out and cut in indicates the system's condition, with external or internal leakages causing frequent valve operations and potentially leading to increased fluid temperature and pump wear.

15:06

🛢️ Reservoir and Accumulator: System Stability and Fluid Management

The final paragraph summarizes the key points about the reservoir and accumulator in the hydraulic system. The reservoir provides storage for system fluid and air space to accommodate fluid volume variations due to actuator displacement or thermal expansion. It compensates for small leaks and supplies fluid to the pump, often with pressurization to prevent cavitation. Filters are essential for protecting system components from damage caused by foreign particles in the fluid. The cutout valve is fitted to manage system pressure and to give the pump an idling circuit when no services are selected. The accumulator stores hydraulic fluid under pressure to stabilize pressure fluctuations, allow for thermal expansion, and provide an emergency fluid supply in case of pump failure. It also reduces pump wear by controlling the time between cut out and cut in of the ACOV and supplies initial fluid when a selection is made with the pump cut out.

Keywords

💡Reservoir

A reservoir in the context of an aircraft's hydraulic system is a storage container for hydraulic fluid. It serves to accommodate fluid volume changes due to system operation and temperature fluctuations. The script mentions that even though systems are designed to be leak-free, reservoirs are necessary because no system can be completely leak-proof, and fluid must be available to lubricate seals and accommodate thermal expansion. The reservoir also plays a role in preventing cavitation by providing a positive pressure at the pump inlet.

💡Cavitation

Cavitation refers to the formation of vapor bubbles in a liquid when the pressure of the liquid drops below its vapor pressure. In the script, it is explained as a phenomenon that occurs when the hydraulic pump sucks fluid in at its inlet, causing a drop in pressure that can lead to the formation of bubbles. These bubbles, when imploded on the pressure side of the pump, can cause severe damage. The script emphasizes the importance of the reservoir in preventing cavitation by maintaining a positive pressure at the pump inlet.

💡Accumulator

An accumulator is a device that stores hydraulic fluid under pressure. It is used in aircraft hydraulic systems to dampen pressure fluctuations and provide an emergency supply of fluid in case of pump failure. The script describes two types of accumulators: cylindrical with a floating piston and spherical with a flexible diaphragm. Accumulators help maintain system pressure and provide initial fluid when a service is selected, which is crucial for the smooth operation of hydraulic systems.

💡Hydraulic Pump

The hydraulic pump is a key component of the aircraft's hydraulic system, responsible for circulating fluid to provide power to various aircraft systems. The script discusses how the pump operates by sucking fluid in at its inlet and supplying pressure at its outlet. It also mentions the potential for cavitation if the pressure at the inlet is too low, which can be mitigated by the use of a reservoir and accumulator.

💡Seals

Seals in the hydraulic system are used to prevent fluid leakage while allowing movement in mechanical components such as actuators. The script notes that a small amount of fluid is allowed to leak across the seals to lubricate them, which is necessary for the system's operation. However, excessive leakage can lead to a decrease in system efficiency and potential damage to components.

💡Filters

Filters are used in hydraulic systems to remove foreign particles from the fluid, protecting pumps and other components from damage. The script describes various types of filters, including suction and pressure filters, which are installed on both sides of the pump. Filters are essential for maintaining the cleanliness of the hydraulic fluid and ensuring the longevity of the system components.

💡Automatic Cutout Valve (ACOV)

The Automatic Cutout Valve is a component fitted in systems with a constant delivery fixed volume pump to control system pressure. It operates in two modes: cut in and cut out, regulating the flow of hydraulic fluid to maintain system pressure and provide an idling circuit for the pump when no services are selected. The script explains how the ACOV helps reduce pump wear by controlling the pump's operation based on system pressure.

💡Thermal Expansion

Thermal expansion is the process by which materials expand when heated. In the context of the hydraulic system, the script mentions that the fluid expands and contracts with temperature changes, which can affect the system's operation. The reservoir and accumulator help accommodate these volume changes, ensuring the system remains efficient and preventing damage due to pressure fluctuations.

💡Baffles and Fins

Baffles and fins are internal structures within the reservoir designed to prevent sloshing and foaming of the fluid during in-flight maneuvers. The script explains their purpose in maintaining fluid stability, which is crucial for the accurate operation of hydraulic components and to prevent air entrainment that could lead to cavitation.

💡Pressure Relief Valves

Pressure relief valves are safety devices that release excess pressure from the hydraulic system to prevent damage to components. The script implies their importance in maintaining system integrity by protecting against over-pressurization, which could occur due to various operational factors or component failures.

💡Non-Return Valve

A non-return valve, as mentioned in the script, is a type of check valve that allows fluid to flow in one direction only. It is used in the hydraulic system to hold pressure, such as in the accumulator, and to prevent backflow, which could disrupt the system's operation or lead to cavitation.

Highlights

The necessity of a reservoir in an aircraft's hydraulic system, even in a sealed system.

Fluid leakage across seals for lubrication and the expansion and contraction of fluid due to temperature changes.

The role of accumulators in storing variable amounts of fluid depending on the system configuration.

Prevention of cavitation in hydraulic pumps by maintaining a positive pressure at the pump inlet.

Pressurization of reservoirs with air from the engine compressor to increase fluid pressure at the pump inlet.

Bootstrap method for pressurizing reservoirs using a piston and actuator.

Functions of reservoirs including connections for suction and return pipes, and monitoring system fluid levels.

Importance of keeping hydraulic fluid free from foreign bodies to prevent damage to system components.

Use of filters in both suction and pressure lines to protect pumps and maintain fluid cleanliness.

The purpose of accumulators in storing hydraulic fluid under pressure and their two common types.

How accumulators help to dampen pressure fluctuations and provide emergency fluid supply in case of pump failure.

The function of an automatic cutout valve (ACOV) in controlling system pressure and providing an idling circuit for the pump.

The significance of the time between cut out and cut in of the ACOV as an indicator of system condition.

The role of filters in protecting system components from damage caused by foreign particles in the fluid.

Maintenance of filter elements and the use of ultrasonic cleaning as a recommended method.

The importance of correct pre-charge pressure in accumulators to prevent rapid fluctuations and potential system damage.

Transcripts

in this lesson we all now look in

greater detail at some of the other

components that make up an aircraft's

hydraulic system

the first component we are going to

discuss is the reservoir you may think

that in a sealed leak free system a

reservoir would not be necessary however

that is not true

firstly no system can ever be leak free

in fact we have already said that a very

small amount of fluid is allowed to leak

across the seals of actuators in order

to lubricate them

also the fluid expands and contracts as

it is heated and cooled

as we shall see later some types of

hydraulic actuator need more fluid when

they are extended than when contracted

the extra fluid is stored in the

reservoir when not required

most hydraulic systems also have

accumulators fitted and these will

contain variable amounts of fluid

depending on the system configuration

again we will discuss these in greater

detail shortly

when a hydraulic pump is operating as

well as supplying pressure at its outlet

it also of course sucks fluid in at its

Inlet

this suction causes a big drop in the

pressure of the fluid and can lead to

low temperature boiling and the

formation of bubbles

when these bubbles pass through to the

pressure side of the pump

they then implode with great force

this is known as cavitation and if

allowed to happen it can cause severe

damage to the pump

the sound of pumps operating while

cavitating can range from a low pitch to

steady knocking sound like on a door to

a high-pitched and random crackling

similar to a metallic impact

the reservoir helps to prevent

cavitation by providing a head of fluid

for the pump to do this the reservoir is

located higher in the system than the

pump to produce a positive pressure at

the pump Inlet

in many installations the reservoir is

also pressurized with air from the

engine compressor to further increase

the fluid pressure at the pump Inlet

thus reducing the possibility of

cavitation at high altitude

another way that reservoirs are

pressurized is by the bootstrap method

a piston sits on top of the fluid in the

reservoir it is pushed down by an

actuator powered from its own hydraulic

system pressurizing the fluid

when the system is not operating the

pressure is held by the non-return valve

you

the reservoir also contains connections

for suction pipes to the pumps and

return pipes from the system

a quantity transmitter unit to allow the

flight crew to monitor the system for

correct servicing and in-flight leaks

a filling Inlet normally sealed with the

filler cap note the strainer in the

filler Inlet preventing debris entering

the reservoir

and in some cases a temperature sensing

probe which may be used to operate a

fluid temperature gauge and or an over

temperature warning light

the baffles and fins are fitted to

prevent sloshing and foaming of the

fluid during in-flight maneuvering

in systems which are fitted with more

than one pump usually the main pump is

engine driven whilst the backup may be

electrically or pneumatically driven or

may even be a hand pump

the main pump will draw its fluids

through a stack pipe in the reservoir

we'll collect its fluid from the bottom

of the reservoir

this ensures that if fluid is lost from

that part of the system supplying the

main pump or supplied solely by the main

pump a reserve of fluid with a backup

pump will still be available

it is very important that hydraulic

fluid is kept free of foreign bodies any

debris would quickly damage the pump and

components it would also cause problems

in things such as pressure relief valves

as they can have very narrow passages

which are easily blocked by foreign

particles

Builders are fitted in both suction and

pressure lines that is on both sides of

the pump

the suction filter protects the pump and

the pressure filter ensures the

cleanliness of the hydraulic fluid

during use

there is also sometimes a filter fitted

in the fluid return line to the

reservoir to remove particles picked up

during component operation

filters remove foreign particles from

the fluid thus protecting the seals and

working surfaces of the components

some filters are fitted with a device

which senses the pressure differential

across the filter element and releases a

visual indicator in the form of a button

or illuminates a warning lamp when the

pressure differential increases as a

result of the filter becoming clogged

false indication of element clogging as

a result of high fluid viscosity at low

temperature is prevented by a bi-metal

spring which inhibits indicator button

movement and low temperatures

other filters are fitted with a relief

or bypass valve which allows unfiltered

fluid to pass to the system when the

element becomes cloaked in this type of

filter the element must be changed at

regular intervals before clocking occurs

some filters combine both systems with

the warning indicator set to operate

shortly before the bypass valve opens

individual components often have a small

filter fitted to their Inlet connection

and constant pressure pumps will have a

case drain filter to help monitor pump

condition we will cover the term case

drain when discussing pumps

filter elements may be manufactured from

paper felt or gauze from metal wire or

from a combination of these materials

all elements except those made from wire

are usually discarded when removed but

wire elements may usually be cleaned

cleaning by an ultrasonic process is

normally recommended but if a new or

cleaned element is not available when

the filter becomes due for check

maybe cleaned in a suitable solvent as a

temporary measure

an accumulator is a device used to store

hydraulic fluid under pressure

two different types of accumulator are

illustrated here but many other types

are used however the two shown are those

most commonly in use in aircraft systems

whatever its shape the chamber of the

accumulator is split into two parts by a

separator in the case of the cylinder a

floating piston is used whilst the

spherical accumulator has a flexible

diaphragm

for the purpose of this explanation we

will concentrate on the cylindrical type

as this is the most common of all

the volume on one side of the floating

piston is pressurized with air or

nitrogen and the other is connected to

the hydraulic system pressure line

accumulator is normally inflated through

a charging valve which may be attached

directly to the accumulator or installed

on a remote ground servicing panel and

connected to the accumulator by means of

a pipeline

the charging valve usually takes the

form of a non-return valve which may be

opened by means of a plunger in order to

relieve excessive pressure

to pre-charge or check the gas pressure

the system hydraulic pressure should be

released this will allow the gas

pressure to move the floating piston to

the bottom of the accumulator

when a pump is operating and hydraulic

pressure builds up in the system the gas

is compressed until fluid and gas

pressure equalized at normal system

pressure at this point the pump

commences to idle and system pressure is

maintained by the accumulator the

accumulator gas pressure gauge will now

read system pressure

turn valve is fitted upstream of an

accumulator in order to prevent fluid

from the accumulator being discharged

back through the pump to the reservoir

the accumulator carries out a number of

functions in the system it helps to

dampen out pressure fluctuations

thermal expansion and contraction of

fluid trapped in the pipes

can provide a small emergency supply of

fluid to the system in the event of pump

failure the initial gas charge of the

accumulator is greater than the pressure

required to operate any service and the

fluid volume is usually sufficiently

large to operate any service once except

that brake accumulators normally permit

a guaranteed number of brake

applications or the ability to stop the

aircraft during a rejected takeoff

if the system is using a constant

delivery fixed volume type pump with an

automatic cutout valve it will prolong

the period between cutout and cut-in of

the automatic cutout valve and so reduce

the wear on the pump operation of the

automatic cutout valve will be explained

shortly

the accumulator will also provide the

initial fluid when a selection is made

and the pump is cut out or at minimum

stroke

if a service is selected a supplier

fluid under pressure is available until

pressure drops efficiently to bring the

pump online

incorrect pre-charge pressure of the

main accumulator can cause the automatic

cutoff valve to cut in and out too

frequently

this may cause rapid fluctuations of

system pressure which can be felt and

heard as hammering in the system this

can rapidly cause damage to the system

if it is not dealt with

an automatic cutout valve a CoV is

fitted to a system employing a constant

delivery fixed volume pump to control

system pressure and to provide the pump

with an idling circuit when no services

have been selected

the a COV consists of a piston which is

sensing system pressure

this is opposed by a spring

the piston is controlling a poppet valve

the a COV operates in two modes these

are known as cut in and cut out

the valve is in the cut in position when

spring pressure is greater than pump

output pressure the piston is fully down

and the poppet valve is closed

cut out is when hydraulic pressure is

greater than spring pressure the piston

moves up and the poppet valve is open

this allows the pump output to return to

the reservoir

you

with the system not operating and the

pressure zero the ACO V will be in the

cut in position

when the pump starts operating pressure

will build up until when normal system

operating pressure is reached

the pressure on the bottom of the piston

will overcome the spring

the piston will move up opening the

poppet valve

and allowing the pump to return its

fluid output to the reservoir without

any load there will still be fluid

flowing through the pump to cool and

lubricate it this is the cut out

position

system pressure will be

held on the bottom of the piston by the

non-return valve

when a surface is operated

system pressure will fall

the spring force will now be greater

than the hydraulic pressure so the

piston will move down closing the poppet

valve

this will cut off the pump return to the

reservoir and allow it to operate the

service the a CoV will remain in the cut

in position until the service reaches

the end of its travel

and pressure builds up again

it will then return to the cutout

position

this type of system needs a reservoir of

fluid energy downstream of the automatic

cutout valve otherwise any slight

leakage through components or from the

system would result in frequent

operation of the valve and frequent

loading and unloading of the pump

an accumulator fulfills this as well as

a number of other functions

the time between cut out off load and

cut in on load of the a CoV is a good

indication of the condition of the

system

external leakage will cause a reduction

in the operating period with frequent

loading and unloading of the pump

internal leakage usually caused by a

piston seal failure will similarly cause

frequent loading and unloading of the

pump this could also cause an increase

in fluid temperature due to the extra

work being done by the pump

that is the end of the lesson on

components to summarize the main points

a reservoir provides both storage space

for the system fluid and sufficient air

space to allow for any variations of

fluid volume in the system

which may be caused by actuator RAM

displacement

or by thermal expansion

it compensates for small leaks

and it provides ahead of fluid for the

pump most reservoirs are pressurized to

provide a positive fluid pressure at the

pump Inlet in order to help prevent

cavitation

filters are used to protect pumps and

other system components from damage

caused by foreign particles in the fluid

cutout valve is fitted to a system

employing a constant delivery fixed

volume pump to control system pressure

and to provide the pump with an idling

circuit when no services have been

selected

finally an accumulator is fitted to

store hydraulic fluid under pressure in

order to dampen pressure fluctuations

to allow for thermal expansion

an emergency supply of fluid to the

system in the event of pump failure

period between cut out and cut in time

at the AC o V and so reduce the wear on

the pump

it also provides the initial fluid when

a selection is made and the pump is cut

out