Module 11.11 Hydraulic power Flashcards
What is the only loss of efficiency in hydraulic systems?
Fluid friction
What are the two types of hydraulic forces?
- Hydrostatic
- Hydrodynamic
How should pressurisation sources be installed?
In parallel
What are the pumps designed to supply?
The max demand pressure
How is the pressure reduced from the highest demand to the required pressure?
Pressure reduction valve
What is the purpose of the pressure relief valve?
To stop over pressurising of the system and relieves due to thermal expansion
What is an open centre system?
Is a system with fluid flow but no pressure in the system when the actuators are idle
What does the selector valve do?
Directs pressure to actuator and decides the pressure and return
What component is controlled to vary the outputs of the system?
Directional/selector valve
What will happen to the pressure when the selector valve moves?
It will drop
What is the reason for pressure surges?
Movement of the selector valve
Aircraft have multiple hydraulic systems are these interconnected?
No they are separate
When are power boost systems used?
To overcome high control forces, but flight crew still actuate via cable and push rod
What are the benefits to hydraulic power pack systems?
- No need for a central system
- Reduced weight as no need for long stretches of lines
What are the disadvantages of an open centre system?
- One surface operated at a time
- Slow actuation
How are the selector valves connected in an open centre system?
In series
What is the main advantage of a closed centre system?
Multiple user systems can be actuated at the same time
How are the selector valves connected in a closed centre system?
In parallel
What system is the most common on aircraft?
Closed centre system
What is the difference between flash and fire point?
Flash point, the vapour ignites momentarily
Fire point, the vapour ignites and continues to burn
What are the 3 categories of hydraulic fluids?
- Mineral based
- Polyoalphaolefin
- Phosphate ester
Is Skydrol fireproof?
No
What are the two types of Phosphate -Ester fluids and their subclasses?
Type 4
- Class 1 Low density
- Class 2 Standard density
Type 5
Can different types of hydraulic fluid be mixed?
No
Where should contamination checks be taken from?
Reservoir and other locations
What are the two main servicing activities on hydraulic systems?
- Reservoir filling
- Fluid sampling
How is pressure refilling done?
Use of Ground Service Equipment
Where should the filling be done from and why?
Either in the suction or the return portion of the closed centre system, as between the pump and user there it is system pressure
Manual filling is done by what?
Hand pump
Refilling is never by done directly into what on large aircraft?
The reservoir
What should be done to the system before refilling?
Depressurised
How may a leak be found in a hydraulic system?
- Visual inspection
- Leak measurement valve
Where is the leak measurement valve located?
Between the user and the selector valve
During testing, pressure is directed through what?
The leak measurement valve
How are the specific parts isolated to test for leaks?
Manual selector valves
If there are 2 pumps connected too a reservoir how many suction and return lines are there?
Suction: 2
Return: 1
Where would non pressurised reservoirs be used?
Small aircraft with low flying altitudes
How is the fluid level checked in a non pressurised reservoir?
Visual gauge
Why is there a need for pressurised reservoirs?
To supply a positive fluid supply at high altitude
What is the purpose of the check valves in a hydraulic system?
To allow fluid to travel one way but not the other
How may a hydraulic reservoir be pressurised?
- Pneumatically
- Hydraulically
How is a hydraulically pressurised reservoir pressurised?
Using system pressure
In a hydraulically pressurised reservoir how can the level be indicated?
The small piston extension
Where would an accumulator be?
In between the pump and the user
What is the purpose of an accumulator?
- Smooth pulsations
- Store energy
- Recover pressure surges
How are the chambers separated in a:
- Spherical accumulator?
-Cylindrical accumulator?
Spherical: A diaphragm
Cylindrical: A piston
How is the piston sealed?
Double sealed to stop leaks
What are the 3 principle methods of operation?
- Diaphragm
- Spring type
- Hydro-pneumatic
What can be said about the nitrogen pressure with respect to the system pressure?
The nitrogen pressure is less than the system pressure
What must be done before checking the nitrogen base pressure?
Completely depressurise otherwise you will get a false reading
How are the pumps classified?
- Positive displacement
- Non-positive displacement
What types of delivery/volume does a positive displacement pump give out?
- Constant
- Variable
Gear type power pump and Gerotor are examples of what?
Positive displacement fixed output
What is the purpose of the shear sectioning the drive coupling?
To stop damage to the pump or drive shaft if a blockage or unusual conditions occur
What dictates piston extension in a piston pump?
The Swashplate
In a bent axis pump what causes pistons to suck then pressurise the fluid?
Angular housing configuration
Vane type is an example of what?
- Constant
- Positive
How is the VDP different from a constant displacement pump?
The outlet pressure and flow rate can be altered
How is the output varied in a VDP?
By changing the angle of the Swashplate
What is an EDP?
Engine driven pump:
- Variable displacement
- Permanent engine connection
What is the drive shaft turned by in an electric/Alternating Current Motor Pump (ACMP)?
An electric motor
Why would an air driven pump be used?
To supplement the EDP if it can not maintain system pressure
- Can be auto or manually operated
What are the types of hand pumps?
- Rotating
- Single acting
- Double acting
What is a pump system on airbus aircraft?
Power Transfer Unit
What is the purpose of the Ram Air Turbine (RAT)?
Too provide electrical and hydraulic power if primary sources of power are lost
What precaution should be taken before maintenance is carried out on systems that may deploy the RAT?
Pull CB’s or place inhibitors
What is the main output of the RAT?
Hydraulic pressure
When can the RAT be retracted?
Only on the ground
What are the methods of pressurising the reservoir with air?
- Engine/APU bleed air
- GSE
What are the different types of material used in filters?
- Wire mesh
- CRES
- Microglass
- Cellulose paper
What are the main components of a filter assembly?
- Filter head assembly
- Filter bowl
- Filter element
- Filter bypass valve
Where may a filter be located?
LP - after the user
HP - after the pump
When would the filter bypass valve be opened?
When the filter is clogged and the pressure is building up.
Dirty fluid is preferred to no fluid
If there is a differential pressure in a filter, what happens?
Filter bypass opens and the red indicator pops out
this also occurs for thermal lockout
What are the valve types used in systems?
- Ball type
- Sleeve type
- Poppet type
Why may a pressure relief valve be used?
As a:
- System relief valve
- Thermal relief valve
When would a relief valve be opened?
When the pressure in the line exceeds the max/system pressure
What is the purpose of the pressure regulator?
Used to keep output pressure within a certain threshold and permit pump to turn without resistance
Where would pressure relief and regulators be installed?
After the pump in the pressure subsystem
Why would a pressure reducer valve be used?
To supply systems with a lower pressure than systems pressure
What is the purpose of shutoff valve?
Two shutoff the system. Two positions on or off
What is the purpose of each part in a 4 way valve?
- Pressure
- Return
- 3rd and 4th connected to actuator
Is there fluid flow in a closed centre system when valves are in neutral or off?
No fluid flow
In fly-by-wire how are selector valves positioned?
Solenoid operated
What is the purpose of a sequence valve?
Control the sequence of operation between two branches in a circuit
What is given priority by a priority valve?
Critical systems over non critical when pressure is low
How can a sequence valve be controlled?
- Mechanically
- Pneumatic
What is a use of a sequence valve?
To ensure that the landing gear doors open first before the gear moves
Where are quick disconnect valves used?
In suction and pressure lines either side of the pump
How is the piston moved in the pressure controlled sequence valve?
When pressure builds up in the primary unit it lifts up the piston allowing the pressure to head to the secondary heavier unit
What are non-return valves?
Valves that only permit flow in one direction
What does a shuttle valve consist of?
2 inlets, 1 outlet
What is a hydraulic fuse?
Safety device that shuts off supply when a fuse detects a sudden flow increase such as a burst in the pipe
Where are hydraulic fuses found?
- Brakes
- Flaps and slats
- Nose landing gear extension and retraction
- Thrust reverser pressure and return
In a single action actuator how is the actuator returned?
Via a spring
How is a double acting actuator returned?
Hydraulically
All seals can be classified into 3 groups what are they?
- Gaskets
- Packings
- Wipers
When would a gasket be used?
To seal two components that don’t move relative to each other
When would a packing seal be used?
Seal internally on sliding components
What should be done when installing/removing o-rings?
- Avoid use of sharp tools
- Use specialised tools
- Inspect and clean parts that get new seals
- Immerse o-ring in hydraulic fluid be fore installation
What is the purpose of a back up ring?
To support the o-ring when it is subjected to differential loads
Where should a pressure switch be installed?
After the pump
When would the pressure circuit close the circuit to raise the alarm?
When the pressure is above or below a certain point
What does a pressure transducer do?
Samples and records the pressure i.e. they display the actual pressure in the flight deck like 2900 psi
What provides a pressure warning?
The pressure switch
What does the power by wire system utilise?
Hydraulic packs local to the area
