REQUIRED PROPERTIES OF THE FLUID
Viscosity is internal resistance to flow. 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.
Chemical stability. It is the liquid’s ability to resist oxidation and deterioration for long periods.
Excessive temperature of the liquid, contact with air, water, salt, or other impurities or. Some
metals (zinc, lead, brass, and copper) 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.
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.
Additional requirements
•Good lubrication properties.
•Non toxic.
•Low freezing point.
•High boiling point.
•No foaming.
•Compatibility.
•Coloured.
SIMPLE HYDRAULIC SYSTEM
The hydraulic system shown in is a passive hydraulic system without a pressure pump. The
system is not pressurised unless the pump is operated.
Hydraulic fluid, stored in the reservoir, is drawn into the hand pump via a pipeline attached to
the bottom of the reservoir, through a non-return valve (NRV) and into the hand pump. The
pump pushes the fluid through another NRV, via the pressure pipeline, to a 3-position selector
valve. Depending on the position selected, it will either direct the fluid through a port, to one side
of the double-acting, linear actuator piston, or the other. Or it can be selected to the “Off”
position, which locks the fluid in the actuator and prevents any movement of the piston in either
direction. Fluid from the “non-pressure” side of the actuator piston, is diverted back to the
reservoir by another port in the selector valve via a return pipeline.
PASSIVE HYDRAULIC SYSTEM
The use of an EDP creates a problem in that the pump is still maintaining pressure in the system
when it is not needed during cruise flight, thereby wasting valuable engine power. This is an
active hydraulic system.
The pump absorbs very little power when it is not moving fluid against an opposition. This
problem is overcome by the installation of a pump, unloading valve. (Automatic Cut-out
valve). This valve relieves the pressure off the pump by diverting the fluid back to the reservoir.
The fluid circulates freely from the pump, to the reservoir and back to the pump again with no
opposition, thereby using very little engine power.
When the piston has reached the end of it’s stroke, pressure will build up in the system. This is
relieved by the system pressure relief valve, which dumps the excess pressure fluid back to
the reservoir.
The functions of reservoir:
stores the hydraulic fluid.
supplies fluid to the system through a pump and receives the return fluid from the system.
accommodates the extra fluid caused by thermal expansion and compensates for slight leaks,
which may occur throughout the system.
provides a reserve supply of fluid for emergency operation of systems which are essential for
flight control and landing
RESERVOIRS
Types of reservoirs
A Vented (unpressurised) would normally operate below 20,000 feet altitude.
The reservoir is located at a higher level than the EDP’s to ensure a positive “head of pressure”
supply of fluid throughout all normal flight manoeuvres.
Negative “g” forces or high roll angles, could cause a temporary loss of supply to the EDPs’
allowing them to “run dry”, resulting in pump inlet cavitation. To compensate for this, a lowpressure
pump is sometimes installed between the reservoir and the EDP’s to ensure a positive
head of pressure during such conditions.
A Pressurised reservoir is pressurised to prevent foaming of the fluid due to the low ambient air
pressure at high altitudes, and to prevent pump cavitation in it’s inlet.
There are several ways in which pressurisation can be achieved:
A nitrogen charged cylinder.
Cabin pressurisation air.
Engine Compressor/ Bleed air.
Hydraulic system pressure
RESERVOIR AIR PRESSURE SYSTEM
An air pressure system is provided to pressurize each hydraulic reservoir in order to ensure
adequate fluid supply to the pumps and to avoid cavitation effect.
RESERVOIR COMPONENTS
The following components are installed on a typical reservoir:
Reservoir pressure relief valve—prevents over pressurization of the reservoir. Valve opens at a
preset value.
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.
Reservoir quantity transmitter—transmits fluid quantity to the flight deck so that the flight crew can
monitor fluid quantity during flight.
POWER TRANSFER UNIT
PTU transfers hydraulic power from one of an
aircraft's hydraulic systems to another in the
event that second system has failed or been
turned off.
The PTU transfers hydraulic power without
intermixing of hydraulic fluid between the
systems
RAM AIR TURBINE
RAT may be used as an emergency source of hydraulic power in the case of major failure within
the normal system. The RAT consists of a turbine (similar in appearance to a small propeller) which
is normally stowed in a compartment in the fuselage. The RAT may be deployed automatically or by
manual selection. Pressure output is governed by varying the blade angle in response to aircraft
speed and pressure demand
RAM AIR
AIR-TURBINE DRIVEN PUMPS
Air-turbine driven pumps (ATDP) receives pressurised air from the aircraft’s main bleed air
system. The flow of air is controlled and modulated by a solenoid operated pressure regulator and
shut-off valve to maintain the turbine speed within set parameters. The turbine is connected by a
shaft to the pump
ACCUMULATORS
The accumulator serves four functions in a hydraulic system:
(a) It absorbs pressure surges, which occur when operation of a system component causes
a pressure drop, followed by a pressure rise as the pump control responds.
(b) It provides supplemental system pressure when large fluid demands are made, by
supplementing the fluid flow from the hydraulic pump.
(c) It maintains system pressure when the hydraulic pump is disconnected from the
system by the Automatic Cut-Out Valve.
(d) It serves as a pressure storage
unit to permit limited operation of
hydraulic services when
the pump is not operating.
This allows, for example, flap
operation for ground servicing
or brake operation during
towing, when the engines
are not running.
CHECK VALVE. ORIFICE CHECK VALVES (RESTRICTOR VALVES)
A check valve permits the flow of hydraulic fluid in one direction only.
An orifice check valve is designed to provide free flow of hydraulic fluid in one direction and
restricted flow in the opposite direction
CUT-OUT VALVES
(a) Provide a return line to the hydraulic reservoir.
(b) Close the return line and direct fluid into the system when pressure falls below a set figure.
(c) Give a smooth transition between pump and accumulator pressure control.
(d) Automatically cut out to off-load the pump and permit an idling circuit from pump to
reservoir. This will reduce pump wear and consumption of engine power and, at the
same time, prevent overheating of the hydraulic fluid.
(e) Cut in automatically when a service is selected and pressure is required to operate it.
(Cut-in will also occur if a leak causes a drop in hydraulic pressure).
(f) Act as a non-return valve during cut-out periods and seal off the system to maintain
pressure when the hydraulic pump is idling.
PRESSURE/THERMAL RELIEF VALVES 18
Pressure relief valve is used as a safety device and is designed to open when system
pressure reaches a preset value which is slightly higher than the intended system pressure.
Cracking pressure is the pressure at which a pressure relief valve begins to open.
Thermal relief valves are designed to open
when the pre-set pressure at the valve is
exceeded. In this case however the increase in
pressure is caused by an
increase in the temperature and subsequent
expansion of the fluid in a part of the system
where the fluid is trapped, typically between a
non return valve and an operating jack.
PRESSURE CONTROL VALVES 19
A full flow relief valve protects the system
against over-pressurisation due to a variable
displacement pump pressure control failure
which leaves the pump stuck on maximum flow.
This type of relief valve must be capable of
passing the maximum pump output while limiting
the pressure to approximately 10% above
normal working pressure.
Pressure reducing valves will reduce the main
system ressure to that required for a particular
service, for example the brake system.
The spring is designed to resist the required service
pressure. The main high pressure (HP) supply will, by
definition, deliver a greater pressure than the required
service pressure. When main high pressure is
supplied to the valve the piston is forced against the
spring which compresses, allowing the piston to
move. This movement closes off the HP supply port to
a position where the pressure of fluid entering the
valve is balanced by the spring pressure and the low
pressure (LP) output will therefore be required system
pressure. The movement of the piston will also
uncover the return port to route excess fluid back to
the reservoir
SHUT-OFF VALVES 20
Positioned between the hydraulic reservoir and the pump, shut-off valves are normally
operated electrically to cut off fluid supply to the pump.
They are used to facilitate ground servicing and to isolate the fluid supply in the event of
engine fire.
SHUTTLE VALVES 21
Shuttle valves are used to disconnect one source of hydraulic fluid whilst connecting another.
It would therefore be necessary to supply fluid from the bottom of the reservoir via the hand
pump. The necessary re-routing of the circuit would be achieved using shuttle valves. In the
event of a shuttle valve becoming stuck in its ‘normal’ position (as illustrated at Figure) it would
be incapable of connecting the emergency
supply to the system.
HYDRAULIC FUSES 22
Hydraulic fuses prevent fluid loss when a leak occurs downstream of the fuse. They are often
incorporated in braking, flap and thrust reverser systems and they operate when pressure drop
across the fuse exceeds a preset value
PRIORITY VALVES 23
On some aircraft systems a priority valve is used instead of a pressure maintaining valve. The
end result is the same but is achieved by different means. Let us assume that the system fluid
content is falling due to a leak. At a pre-determined level, a switch located in the reservoir will be
activated.
This will transmit a signal to the priority valve which then closes off the supply line to the
nonessential services.
SOLENOID OPERATED SELECTOR VALVE
In a complex hydraulic system, control valves or selector valves are needed to control fluid
flow. 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.
closed-center selector valve blocks the flow of fluid through the valve when it is in the
NEUTRAL or OFF position.
continue....
