46 Questions
What is the primary purpose of a 'cut-out' in a hydraulic system?
To relieve pump load when services are complete and the accumulator is charged
How can hydraulic pressure be restored in a system?
By compressing the air charge in an accumulator
What is the color of the band on a hydraulic seal?
Green
What is the function of a maintainer valve in a hydraulic system?
To allow continued pressure to essential systems during reduced supply
In a hydraulic system, what component prevents creep in jack-operated services with multiple positions?
Cut-out
What is the purpose of a 'cut-out' in limiting fluid loss during a pipe fracture?
It is a secondary purpose
Where is the temperature overheat switch located in a hydraulic pump?
In the case drain
What happens if a component has an internal hydraulic leak?
Increase in fluid temperature
What happens to hydraulic fluid when it is overheated?
Increase acidity
How is the hydraulic pump depressurising solenoid actuated?
Energised Closed for fail-safe
How do you prevent hydraulic fluid frothing?
By pressurising
Accumulators as fitted to aircraft hydraulic systems...
store fluid under pressure
A Skydrol hydraulic fluid seal would be made of...
butyl rubber, ethylene propylene or Teflon
Synthetic hydraulic fluid subjected to overheating is high in...
acidity
A normal hydraulic system will...
show pressure and source of hydraulics
A constant volume hydraulic system uses what to relieve pressure in the system when no services are being used?
ACOV
What is the purpose of using two spanners to stop a pipe from twisting?
To secure pipe connections
How is air removed from hydraulic fluid?
Pass over a tray
What is the function of a pressure control relay in a hydraulic pump?
De-energised to start the pump
How is the output of a constant delivery pump varied?
By regulators
What does a check flow valve in a hydraulic system prevent?
Reverse flow
When does an hydraulic de-pressurising valve get energised?
When selected off
In a closed hydraulic system, what happens at the selector valves when no functions are required?
Pressure is maintained at the selector valves at all times
What components constitute a hydraulic power pack?
Reservoir, pump, selector valve, filter
What is one possible issue if an accumulator loses air pressure?
Rapid depressurisation of system
For what purpose would you fit a thermal relief valve in an hydraulic system?
Relieve excess pressure
What is the correct fluid to use in aircraft hydraulic systems?
Only the specified fluid
What are the essential components of a hydraulic system of the constant delivery type?
Accumulator and automatic cut-out valve
What is the primary function of an automatic cut-out valve in a hydraulic system?
Prevent hydraulic lock formation
Why is a maintaining valve fitted to a hydraulic system?
Ensure a supply of fluid to essential services in case of system failure
What is the purpose of a thermal relief valve in a hydraulic system?
Senses fluid pressure
How are hydraulic systems typically flushed?
The same type of fluid as used in the system
What kind of seals are required for Phosphate Ester hydraulic fluid?
Butyl Rubber
What is the primary purpose of a RAT hydraulic pump?
Flying controls if hydraulic power lost.
Where does the hydraulic hand-pump fluid supply usually come from?
The bottom of the reservoir.
Why are hand pumps in hydraulic systems typically double-acting?
Double acting.
What is the purpose of a shuttle valve in a hydraulic system?
Changing over from main to auxiliary system in case of failure
Why are restrictor valves used in a hydraulic system?
To control the rate of system operation
What is the purpose of a non-return valve in a hydraulic system?
Direct fluid to the hydraulic actuator
Why are the end fittings on a non-return valve different?
To prevent incorrect installation
In a hydraulic accumulator, what happens when the hydraulic system pressure reaches 3000 PSI?
It remains at 3000 PSI
What causes automatic cut-out valve hammering in a hydraulic system?
Low accumulator pressure
Why are hand pumps in a hydraulic system usually single-cylinder double-acting?
Two non-return valves are fitted
During a leak test of an hydraulic system, what should the system pressure be?
Maximum pressure
What is required for constant delivery hydraulic pumps in a system?
An automatic cut-out valve and accumulator
Where are pressure relay valves fitted in a hydraulic system?
Before a pressure gauge
Study Notes
Hydraulic Pump and System Components
- The temperature overheat switch in a hydraulic pump is located in the case drain.
- A component with an internal hydraulic leak will result in an increase in fluid temperature.
Hydraulic Fluid Properties
- Overheating hydraulic fluid will increase its acidity.
- Synthetic hydraulic fluid subjected to overheating is high in acidity.
Hydraulic System Operation
- Hydraulic systems normally operate at 3000 PSI.
- A normal hydraulic system will show pressure and source of hydraulics.
- When no services are being used, a constant volume hydraulic system uses an Accumulator Charge Valve (ACOV) to relieve pressure in the system.
Hydraulic Pump Types
- A fixed volume axial pump contains a drive shaft, swash plate, and cylinder block.
- A variable angled pump starts at maximum stroke.
Hydraulic System Components
- A duplex seal consists of a rubber 'T' section ring sandwiched between two Teflon rings.
- A shuttle valve is used to change over from main to auxiliary system in the case of failure.
- A restrictor valve is used to control the rate of system operation.
- A mechanical sequence valve is used to ensure the operation of landing gear and doors.
Hydraulic System Safety
- An accumulator is charged with air pressure to absorb pressure surges.
- A thermal relief valve is fitted to relieve excess pressure due to excessive temperature.
- A hydraulic de-pressurising valve is energised when selected off.
Hydraulic System Maintenance
- When flushing hydraulic systems, the same hydraulic fluid should be used.
- When replenishing a hydraulic system, the same/correct hydraulic fluid should be used.
- When replacing a flexible pipe, use two spanners to stop the pipe from twisting.
Hydraulic System Troubleshooting
- If an accumulator loses air pressure, there will be rapid fluctuations of instruments.
- Automatic cut-out valve hammering is caused by low accumulator pressure.
- A maintaining valve is fitted to ensure a supply of fluid to essential services in the event of system failure.
Hydraulic Systems MC
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