11.11 HYDRAULIC POWER Flashcards
What is Hydrostatic pressure
Pressure produced by static build up in aircraft hydraulic system
What is hydrodynamic pressure
Pressure produced by moving liquids ideally with no friction
Explain open-centre
- series
- only operate 1 component at a time
- slower as pump
- selector valve for each system
Explain open-centre
- series
- only operate 1 component at a time
- slower as pumps Needs to supply high pressure (delay)
- selector valve for each system
Explain closed centre
- parallel connection
- multiple sub-systems can be used at one time
- Pressure build up if selector valve is neutral
- quicker response due to pressure build up
Explain viscosity
- Internal resistance to flow
- Viscosity increases as temperature decreases
Explain flash point
Temperature at which liquid gives off vapour in sufficient quantity to ignite momentarily
Explain Fire point
- temperature at which a substance gives off vapour in a sufficient quantity to ignite and continue to burn
3 types of hydraulic fluids
- minerals
- polyalphaolefins
- phosphate esters (skydrol)
What are the 2 classes of Phosphate Ester-based fluid (skydrol)?
- class 1 fluids are low density
- class 2 fluids are standard density
2 types of main servicing activities for hydraulic systems
- Reservoir filling
- Fluid sampling
What are the 2 ways reservoirs can be filled
- manually
- pressurised
Where is the hydraulic sampling valve located
- HP manifold of each hydraulic system
What are we looking for when carrying out a Hyrdraulic fluid sampling
- checks for water contamination
- reduction in fluid performance
How often is Hydraulic sampling scheduled
- every 1 year
- every 3000 flight hours
If hydraulic contamination is suspected what should be done
- fluid should be drained and replaced
- samples should be taken too
What is the function of the Hydraulic Reservoir
- storage
- replenishes fluid lost through leakage for operational needs
When are non-pressurised reservoirs used
- aircraft that do not fly at high altitudes
When are pressurised reservoirs used
- high altitude
- usually reservoirs located in non-pressurised areas like wheel wells
What is a pressure relief valve
- prevents excess pressure
How do you de-pressurise between the pump and the user?
- operate flight control to force fluid back to reservoir
What are the 2 servicing activities for hydraulic systems
- Reservoir filling
- Fluid sampling
Where is the Reservoir pressurisation module installed and what is its function
- Installed close to reservoir
- supplies aircraft bleed air to reservoirs
What are the 2 types of accumulators used on aircraft
- Spherical
- Cylindrical
What is the accumulator charged with
Compressed nitrogen
What are the 2 ways hydraulic pumps are classified
- positive displacement
- non-positive displacement
Explain constant delivery pumps
- deliver fixed quantity of fluid per revolution despite pressure demands
Explain gear type power pump
- e.g. of positive displacement pump and delivers constant volume
- 2 meshed gears, during gear driven by aircraft engine etc
Explain RAT
- variable displacement pump
- retract only on ground
- provides electrical and hydraulic power
Explain PTUs
- transferring power
Explain electrically drive hydraulic pumps
- electrical motor that’s mechanically connected to hydraulic pump
Where does the tiny particles of metal from hydraulic fluid usually come from
Normal wear of selector valves, pumps, and other components
What are hydraulic system filters for
- filter out contamination from hydraulic fluid
Explain wire mesh filters
- made of steel
- usually used for surface cleaning
Explain stainless steel filters
- like wire mesh but only stainless steel is used
Explain micro glass filters
- provides the best filtration quality
- prevent passage of solids >10 microns
Explain cellulose paper filters
- made from resin-impregnated cellulose filter
What are the common pressure relief valves
- ball type
- sleeve type
- poppet type
What are hydraulic seals
- to prevent fluid from passing a certain point
- to keep air and dirt out the system
Seals are divided into 3 main classes
- Packings
- Gaskets
- Wipers
Hydraulic seals used internally on sliding assembly are called
Packing seals
Hydraulic seals between non-moving fittings and bosses are called
Gaskets
Red o-ring
- for fuel
Blue o-ring
- For hydraulic fluid
Advantage of back up rings
- do not deteriorate with age
What are backup rings
- ## helps support the seal and acts as a seal
When would a back up ring be used
- when a single seal itself is not enough
- e.g. pressure acting on the right of the seal pushing it out of position which could damage it
When are static seals used
Against steady or pulsating pressure
When are dynamic seals used
- Seal where stroking or rotational movements occur
Gaskets are used as static or dynamic seals?
Static
Which valve is hydraulically operated
- shut off valve
- mechanically operated
- priority valve
Priority valve
Explain non-return valves
- allow fluid to flow in one direction and prevents/restricts flow in another
What are hydraulic fuses
- shuts off fluid flow if it detects a sudden increase in flow
Hydraulic actuators page 143
