Hydraulic Systems Flashcards
Hydraulic pumps and the 2 main types
Usually driven from the engine, RAT, PTU, hand driven
Or electrically driven for emergencies
2 main types = Constant delivery (fixed volume) Constant pressure (variable volume)
Open centre system
Only one service can be operated at a time - only used on light a/c
Flaps/ undercarriage systems
Thermal relief valves
Relieve excess pressure caused by the expansion of trapped fluid as ambient temp increases
Sense pressure not temp
Usually operate at 3350psi
Sequence valves
Makes everything work in the correct sequence
Works hydraulically or mechanically
Hydraulic fuse
Once operated - services downstream will be isolated
Systems upstream will still work
Used in leak situations - lose service but keep the hydraulic fluid
Indications of a fast external leak in the hydraulic system
Contents - rapid loss
Pressure - nil
Temp - N/A
Shuttle valve
Provides a method of splitting 2 different supplies (normal and emergency) to a single service
Indications of an internal leak in the hydraulic system
Within in the system
Contents - no change
Pressure - no change
Temp - increase
Purpose of fin and barrels in the reservoir
Prevents the fluid moving around and creating foam
Modulator - hydraulic systems
Basically a flow control valve
Specific to the brake system
Speeds up the initial operation of the anti skid system
Linear (spool) valve selector
Uses pistons, controlled by pilot input to selected the desired direction of flow
Pressure reducing valve
Reduces main system pressure to a valve req for the operation of some services such as wheel brakes
Non return valve and how to check flow direction
Force pushes a spring open - low pressure forces the seal shut
Direction of the flow is indicated by the arrow on the casing
Mechanical overload protection system (blowback)
A form of pressure relief valve fitted in flap circuits an some spoiler circuits to prevent over pressure from aerodynamic forces causing damage to the a/c
Flaps moving to an intermediate/mid posn - once slowed moves to selected posn
Constant delivery
Fixed volume
Req ACOV and accumulators to stop excess pressure building
Indications of a slow external leak in the hydraulic system
Contents - slowly decrease
Pressure - no change
Temp - no change
Variable volume (constant pressure) pump
Rotating with a series of pistons - driven by the engine - no ACOV/accumulator req
Swash plate is at an angle
As the pressure increases the control piston changes the angle of the swash plate and regulates the output volume as it senses the pressure downstream
Shut off valves
Similar to fuses in that it can isolate services
Isn’t self regulating - has an external controller (on the f/d)
Single acting actuator
Pressure to one side
Spring supplies the return force that returns the piston when pressure is released
Undercarriage door locks
Electrical selector
Uses solenoids to electrically direct the flow
Has to have a manual redundancy
Filters in the hydraulic system and 2 basic types
Removes fine particles from the fluid to prevent damage to seals and components
2 basic types
Paper element - replaceable
Metal element - cleanable
Before the pump = suction filter
After = pressure filter
Return line = return filter
Accumulator charge
Pre charged with nitrogen at half the system value
Why is the hydraulic reservoir pressurised
Prevents cavitation and allows for fluid expansion
Pressurised with bleed air
Double acting balanced
Pressure to either side - force is the same either side due to equal surface areas
Nose wheel steering - req equal forces on both sides
On start up the constant pressure pump q
The swash plate is at max deflection
Blocking valve off loads the pump
Pump is lubricated by hydraulic fluid
Hydraulic lock
When fluid is trapped between the piston and the jack - fluid can’t get out - can’t flow and hydraulic lock is formed
PTU
A back up power system
Uses a hydraulic motor connected to the remaining live system to drive the hydraulic pump in the failed system
Only transfers power not fluid
Flight deck indications of hydraulic systems
Temp
Pressure
Contents
ACOV
Automatic cut of valve
Directs the access fluid back to the reservoir in a hydraulic system using a constant volume pump
Actuators and the 3 different types
Convert pressure energy into linear motion
Piston with a seal around it
Also called hydraulic jacks
3 types =
Single acting
Double acting balanced
Double acting unbalanced
Double acting unbalanced actuator
Pressure either side - different surface areas = different forces in each direction
Typical psi for a large a/c hydraulic system
3000 psi
Quill drive
Pumps are driven by this
Has neck portion that will shatter/sear if the pump was to seize
Tell tail button
Button pops when the bypass relief valve has been operated
Bypass relief - indicates the filter has been blocked so can no longer pass through it
Indicates the use of dirty oil
Selectors
Purpose is to direct the hydraulic flow - dependant on the pilot selection
Full flow relief valve
Protects the system from over pressurisation
Excess pressure acts against a spring causing ball to unseal
Makes excess flow go back to the reservoir
Hydraulic fluid reservoir
Holds the fluid - will contain the correct fluid at the correct amount
Hold excess fluid to compensate for small leaks
Accumulators and purpose
Contains pressurised gas and fluid in a cylinder separated by a piston or rubber diaphragm
Stores fluid under pressure Provides a small amount of pressurised fluid for an emerg Dampens pressure fluctuations Allows for thermal expansion Allows for small internal leaks
Hydraulic motors
Operating screw jacks to drive flaps and slat mechanisms
Also known as an axel piston motor or bent axis motor
