11.11 Hydraulics Flashcards
What pressure do hydraulic systems normal work under?
3000psi
What two types of hydraulic pressure is there?
- Hydrostatic pressure
- hydrodynamic pressure
What type of pressure is produced by static or nearly static fluids?
Hydrostatic
What type of pressure is produced by moving liquids?
Hydrodynamic
What’s the advantage of using fluids rather than gases to transmit force?
Pressurise quicker and transmit force more efficiently
What are the colours of hydraulic fluid? [2]
- Mineral oil = red
- Synthetic oil = violet
What 2 types of hydraulic fluid are there?
- mineral oil
- synthetic oil
Can mineral oil and synthetic oil mixed?
No
What’s a typical name for synthetic hydraulic fluid? [2]
- Skydrol
- hyjet
What characteristics are required for hydraulic fluid? (7)
- low flammability
- good lubrication quality
- resistance to freezing
- non-evaporating
- non-foaming
- chemical stability (resist oxidation and deterioration)
- hygroscopic quality
If a fluid is hygroscopic, what does it have a tendency to do?
Absorb water
What is another name for packing?
Seals
What is the function of packing? [2]
- Prevent leaks from the components housing
- provide a seal between two different fluid pressures
What are seals usually made out of?
Rubber materials and back up rings from Teflon
What do manufacturers of packings have to consider? [2]
- Materials used depending on the job they do
- whether it’s for a mineral or synthetic oil
What must be considered when choosing seals? [2]
- Working temperature
- strength of the forces that act upon them
What are static seals used to seal against?
Steady and pulsating pressure
What are dynamic seals used to seal against?
Stroking or rotational movements
What can the pressure distribution section often called?
The manifold
What 1 main component does each hydraulic system have?
Reservoir completely independent from other reservoirs
What does the cylindrical shape of a reservoir resist?
Changes in differential pressure caused by different altitudes and temperatures
What is the purpose of the pressurisation of the reservoir?
To ensure a positive supply of fluid to the pumps
Where are the supply pipe to pressurisation pump and return pipe located on the reservoir?
At the bottom of the reservoir
What can you find in the reservoir to prevent hydraulic fluid from moving upwards if there was a sudden loss of altitude?
A sheet metal baffle
What components can you find on the reservoir? (5 components)
- Quantity gauge
- quantity transmitter
- low level switch
- drain valve
- reservoir pressurisation/air manifold
What does the pressurisation of the reservoir prevent? [3]
- Foaming
- evaporation
- cavitation of the fluid
What 2 types of methods are there for pressurising the reservoir?
- Hydraulic pressure
- air pressure
With air pressurisation in the reservoir, what is the main source of this air?
- One of the engines
- Pneumatic system will provide back up pressurisation
What pressure are reservoirs usually kept at?
50psi
What is attached to the reservoir pressurisation manifold? [4]
- check valves
- filters with a differential pressure indicator
- pressure reducing valve
- fluid separator (fitted with a drain valve and a ground connector)
When might you need to depressurise the hydraulic reservoir?
For maintenance of system components
What type of assembly is the depressurising valve?
Spring and piston assembly
What is a hydraulic accumulator?
A device that stores potential energy store
When are accumulators used?
To accumulate energy and to smooth out pulsations
How is the energy stored in an accumulator?
Available for instantaneous use, released at a great force that what can be supplied by the pump alone
What can accumulator also act as?
Surge or pulsation absorbers
What 4 types of accumulators principals are there?
- Weight loaded piston type
- diaphragm (bladder) type
- spring type
- hydropneumatic piston type
What are the functions of accumulators? (5 functions)
- stores energy
- absorbs pulsations
- cushions operating shock
- supplement pump delivery
- maintains pressure
What are the two types of hydraulic piston pumps?
- variable displacement
- fixed displacement
What is the mechanism called where movement is created? (VDP)
Wobble plate/yoke/swash plate
Does the wobble plate move with the pistons? (VDP)
No, it is a fixed component and doesn’t rotate with engine rotation
What happens to the fluid output when the wobble plate reduces the piston stroke?
Reduction in flow output
What is the boost pump called in the VDP?
Impeller
What does the impeller do in a VDP?
Precharges the fluid from the reservoir as it enters the pump, preventing cavitation
What does the pump compensator valve do?
Alters the angle of the wobble plate according the pressure of the system via the actuator piston
What can stop the VDP output?
A solenoid valve that can be activated in the flight deck
What happens when the captain activate the depressurisation valve?
Piston moves up allowing pressurised fluid go to the compensator valve. Moving 2 pistons, allowing fluid to push the actuator piston changing the angle of the wobble plate. And also shuts the flow via a block valve
What does a case drain do?
Drain line removing cooling fluid from the pump to the reservoir
What does the seal drain do?
Drains fluid which comes through the driveshaft sealing
How are the piston shoes lubricated and cooled?
Via very thin holes on the pistons
Where are FDPs usually installed?
On ground service panels
What are the main source of hydraulic power?
Engine driven pumps
What is type of pump is the engine driven pump?
Variable displacement pump
How is low pressure detected on the outside of all the system pumps?
Pressure switches
If an engine fire was to occur, what is necessary to do?
Cut off the supply of hydraulic fluid
When will the electric pump automatically start?
When the engine starts
How can an electric pump be cooled? (2)
- cooled by air or,
- hydraulic fluid that passes through
What will the temperature switch in the electric pump do?
Alerts the flight deck when the pump gets too hot, so they can switch the pump off
What is another name for electric pumps?
Demand pumps
When will air driven pumps activate?
When electric pumps can’t maintain the system pressure
What rotates the drive shaft of an air driven pump?
Air turbine driven by pneumatic system
What does the gearbox do on an air driven pump?
Reduces the speed of air turbine to a suitable level for the Variable displacement pump
What is the purpose of a shut-off and speed control valve in an air driven pump?
- Shut off valve = solenoid valve energised by the flight deck, controlling the supply of pneumatic air. When shut, the turbine stops spinning
- Speed control valve = controls the rotational speed of the air turbine
What are the 3 uses of auxiliary pressurisation?
- help main pressurisation if there is a high user demand
- replace main pressurisation when a fault occurs
- for maintenance purposes on the ground when the engines are not running
What does RAT stand for?
Ram air turbine
How is the RAT deployed?
Push button in the flight deck with spring tension to extend
How does the RAT operate?
Air stream turns the propeller driving a variable displacement pump
How is the RAT retracted?
Hydraulically
How long does it take for the RAT to deploy?
Less than 1 second
What is a PTU?
-Power transfer unit,
Transfer hydraulic power between 2 hydraulic systems with a motor/pump combination
What doesn’t a PTU allow?
Fluid transfer
What will you find in a PTU?
Fixed displacement pump/motor and variable displacement motor/pump
Can the PTU transfer hydraulic power in either direction?
Yes, the pump/motor functions switch depending on the direction of power
FD motor and VDP
VD motor and FDP
Why are filters important?
Prevent foreign particles from damaging important components
What are the main components of a filter? (4)
- filter head
- filter bowl
- filter element
- filter bypass
What is the purpose of the filter bypass?
To allow for unfiltered fluid to bypass the filter in the event of the filter becoming clogged and increasing pressure
How is high pressure hydraulic distribution achieved?
Via lines, hoses, valves and manifolds
What can the manifold include so that the main components are in one place? (3)
- switches
- filters
- valves
- transmitters
What components does a manifold have that a pressure module doesn’t?(3)
- Pressure transmitter,
- Fluid sampling valve
- Leak measurement solenoid valve
What does the HP manifold have that prevent reverse flow back to the pumps?
Check valve
What does the system pressure relief valve on the HP manifold do?
Prevent damage to the hydraulic system due to an over pressure
What does the leak measurement solenoid do?
Test the internal leakage of the hydraulic components
What is the purpose of a priority valve?
To close the hydraulic circuit to different users when there is a high demand in the system.
The valve closes the line that is less vital ensuring the more vital line gets enough pressure to operate
What does an accumulator do?
Acts as a damper to smooth out pressure fluctuations
What does the LP manifold combine?
All the return lines into a single line going back to the reservoir
What does the LP filter have that the HP filter doesn’t?
Filter bypass
Where can an indication of a pump malfunction be located?
LP manifold temperate sensor
When the case drain fluid enters the module, what does it enter following this?
Heat exchanger to cool it before returning to the reservoir
What are 2 types of lines used in hydraulics?
- hoses
- pipes
What are the 2 types of pipe fittings?
Disconnect able and non disconnect able
What do hoses usually consist of?
Inner tube = Teflon and outer protective = steel wire
What can you identify hydraulic lines?
With a sticker identification
Black circle, blue and yellow square
What is the purpose of fluid sampling?
Analyses the fluid for water contamination and reduction in fluid performance
What do aircraft have to have for a safe flight?
Enough redundancy systems similar in design
