11.11 Hydraulics Flashcards
What ATA chapter number is hydraulics?
ATA 29
What is the unit for pressure
Pascal or NM
What is hydraulic pressure measured in?
Bar
What is 1 bar in psi
14.5psi
What pressure do most civil aircraft operate at?
3000psi or 207 bar
What sort of pressure does the A380 operate at?
5080psi or 350 bar
Why do aircraft like the A380 increase the pressure in their system rather than stick to 3000psi
To generate the required force with out having to increase the size of hydraulic components
What is the formula for force
F=PxA
Force = pressure x area
What are the two types of hydraulic pressure
Hydrostatic pressure and hydrodynamic pressure
What type of pressure is used on an aircraft hydraulic system
Hydrostatic pressure
What is hydrodynamic pressure
Pressure produced by moving liquids ideally with no internal friction
Is it possible to transport force by using solid, liquids and gasses?
No, just liquid and gas
What is more easily compressible liquid or gas?
Gas is more easily compressible
What substance pressurises more quickly, liquid or gas?
Liquid
What two types or hydraulic fluid are there
Mineral and synthetic
What colour is mineral oil usually
Red
Can mineral oils be mixed together
There are many suppliers of mineral oils but they can be mixed together
What colour is synthetic oil usually?
Violet or purple
Can synthetic oils be mixed
Yes the same grade of synthetic oils can be mixed
Can synthetic and mineral oils be mixed?
No
What are the 2 of the most common name of synthetic oil
Skydrol and hyjet
What are the prefix’s used by Boeing and Airbus for synthetic oils?
Boeing - BMS
Airbus - NSA
What do the oil specifications describe?
The oils ability to perform a certain standard through a range of changing conditions
What types of performance characteristics must minders aircraft oils have?
- Low Flammability
- Good lubricating quality
- Resistance to freezing
- Non evaporating
- Non foaming
- Chemical stability
- Hygroscopic quality
What type of chemical stability must aircraft fluids have?
Resist oxidation and deterioration over long periods of time.
This will reduce the need of replacing the fluid as frequently
What does hygroscopic mean
The ability to absorb water
What is the biggest biggest disadvantage that synthetic oil has over mineral oil
Synthetic oil is more hygroscopic which is a disadvantage
Which oils is more toxic to humans mineral or synthetic
Synthetic
What does the term packing refer to
Within a component housing such as a filter, packing is used to make a seal between the environment and the fluid pressure inside it
What are packings also known as
Seals
What would be done if a single seal in a component is not strong enough to perform its task
A backup ring is used. This is also a seal
What seals are usually teflon
Backup rings
What are normal seals made from
Rubber
What is the most common way to damage a seal
Using the wrong hydraulic fluid
When choosing seals what things must be remembered
What type of temperature and force the seal will be subject to. Also what fluid will be used
How can you double check that the seal you are using is the correct one for the job
Check the associated part numbers
What types of pressures are static seals used on
Steady or pulsating pressure
What type of pressure are dynamic seals usually subject to
Stroking or rotational movement
How do Airbus name their hydraulic systems
Usually yellow,blue and green
How do Boeing name their hydraulic systems
The numbering system 1,2,3,4
Where are the main hydraulic components kept in Airbus aircraft
They are usually in the main gear bay
Do all hydraulic systems share a reservoir
No they have independent reservoirs to supply each system
What is the purpose of the hydraulic reservoirs
To keep enough fluid for all systems and redundancy fluid in case of a leak
What monitors the hydraulic fluid levels which is then sent to the cock pit
A quantity transmitter is located on the side of the reservoirs housing
We’re is the hydraulic reservoirs drain valve located
The lowest point of the reservoirs
Why is the hydraulic reservoir pressurised
To ensure a positive supply of hydraulic fluid always reaches the pump
It also prevents foaming, evaporation and cavitation
What are the two methods of pressuring the hydraulic reservoir
One by air and one by hydraulic pressure
What is the most common way to pressurise the hydraulic reservoir? What happens if this fails
By the engines if they stop providing pressurisation it is done by the pneumatic system
What pressure will the hydraulic reservoir be pressurised to
Around 50 psi
What is an accumulator
A device which potential energy is stored in the form of compressed gas or spring
Why are accumulators used
- Absorb pulsations
- cushion operating shock
- supplements pump delivery
- maintains pressure
What are the 4 main types of accumulators
- Weight loaded piston type
- Diaphragm or bladder type
- Spring type
- Hydropneumatic piston type
What are the two categories or accumulators
Pneumatic and Mechanical
What type of accumulators are the Bladder and Piston
Pneumatic
What are the two mechanical accumulators
Weight and spring
If you are using a pneumatic system will you need a reducing valve ?
No, they are already regulated
What are check valves used for?
To prevent air from the hydraulic system going back in to the engine
Can you disconnect a hydraulic pump in flight
Yes but if you want to reconnect it must be done on the ground
What must be done prior to checking accumulator base pressure
The system must be unpressurised
What are responsible for the system pressurisation
Hydraulic pumps
What do hydraulic pumps do
They create high pressures and fluid flows in the system
How are the hydraulic pumps usually powered
Either by the engine or electric motor
What are the two types of hydraulic piston pump?
variable displacement and fixed displacement
What is an IDG
Integrated Drive Generator
What is an EDP
Engine Driven Pump
What is a VDP
Variable Displacement Pump
What is an FDP
Fixed Displacement Pump
What does a fixed displacement pump consist of
A pump unit and a handle
What would a fixed displacement pump usually be used for?
Manual filling of the reservoir or manual opening of the cargo doors
How is a fixed displacement pump regulated
It is regulated by the mechanic operating it, there is no internal regulation
How many pumps are used for power and what are they
3 main pumps
Hydraulic
Pneumatic
Electric
What type of pump is the EDP
Permanently connected variable displacement pump
When is it necessary to cut off the supply of hydraulic fluid to an EDP
If the engine which it is connected to has a fire
Hat type of pump Is the electric pump?
Also a variable displacement pump
Electric pumps generate a lot of heat, what ways are used to prevent overheating?
They are cooled by air or hydraulic fluid. If they become too hot they must be shut off completely
What are electric pumps also called
Demand pumps
Where is the on/off switch for the electric pump
The cockpit
What is the ADP
Air Driven Pump
How are the ADP’s switched on or off
They cut in automatically if the EDP’s cannot maintain 3000psi or they can be turned on manually in the cockpit
Where does the ADP get its air/pressure
The pneumatic system
What is used to slow down the air from the turbine of a Air Driven Pump to a suitable speed for the VDP?
A gearbox
What are the 3 uses for auxiliary pressurisation?
- To help maintain pressurisation if there is high user demand
- To replace the main pressurisation when a fault occurs
- For maintenance purposes on the ground when engines are not running
How is auxiliary pressurisation produced?
Ram Air Turbine (RAT), a power transfer unit and an electric pump. Also from sources on the ground.
What type of pump is the RAT
Variable displacement pump
How can the RAT be retracted after it has been deployed
It is retracted hydraulically and can only be done on the ground by a mechanic
How long will the RAT take to deploy once the button in the cockpit has been pressed?
Less than 1 second
Is there a way of preventing the RAT being accidentally deployed on the ground?
Yes a device is installed, usually made of wood
What does PTU stand for?
Power Transfer Unit
What is the PTU used for
It is another way for providing auxiliary pressurisation
Transfers hydraulic power between two systems.
NO FLUIDS are transferred
How is the power regulation of the PTU done
It is automatic
When using ground power to gain hydraulic pressure where would the cart be connected to?
The ground service panel
There are two connections on the service panel for the hydraulic pressurisation what are they for
One is for pressure connection and the other is a return connection
These allow fluid circulation in the system
When using the ground power unit for pressurisation which of the 2 connections on the cart has a bigger diameter?
The return line to reduce flow resistance
What are filters used for?
Filters are screening devices used to remove foreign particles from air and fluid systems
Why are filters so important?
The foreign material that they remove could cause serious damage to vital system components
What are the main components of a filter?
- The filter head
- The filter bowl
- The filter element
- The filter bypass
They also have a differential pressure indicator (pop out indicator)
The filter element is the most important part of the filter, what are they made from?
- Metal
- Plastic
- Paper material
What happens if a filter gets clogged?
The pressure increases in the unit and the bypass is activated
What are the two types of pressure indicator used to tell if a filter is clogged?
Pop out and electrical
What components are used to move hydraulic fluid from the pumps to the users?
- Lines
- Hoses
- Valves
- And Manifolds (Airbus) / Modules (Boeing)
Why do manifolds/modules combine the major distribution components such as valves, filter, switches and transmitters in one place?
To make maintenance easier
What two sections are manifolds divided in to?
High pressure (HP) and Low pressure (LP)
What manifold distributes pressurised hydraulic fluid to the users?
The HP manifold
What are Boeing modules subdivided into?
Pressure, return and case drain modules
What similar components do the high pressure manifold and pressure module share?
- A system pressure relief valve
- A pressure switch
- A filter
Airbus manifolds have some additional components over the Boeing modules what are they?
- Pressure transmitter
- Fluid sampling valve
- Leak measurement solenoid valve
What component on the HP manifold ensures the flows in the right direction to the users?
The check valve
What component prevents the system from being damaged due to overpressure?
The pressure relief valve
What does the HP manifolds leak measurement solenoid valve do?
Check for internal leakage. Can be opened or closed from cockpit
What type of manifold is the priority valve fitted to?
Not a part of the HP manifold or module but fitted to different manifolds depending on aircraft type
What is the purpose of the Priority valve?
To give priority of a more important hydraulic user in at times of great hydraulic demand
What type of systems would the Priority Valve give priority to?
Flight control surfaces
What sensors would be found on a HP manifold
- Two pressure switches which detect low system pressure caused by pump malfunctions or system leaks
- A pressure transmitter which monitors the pressure at all times.
All data is sent to the cockpit
