11.11 Hydraulic Power (ATA 29) Flashcards
What is the purpose of a hydraulic reservoir?
It compensates for variations in the need for hydraulic fluid caused by the operation of components
When the gear is up, would the fluid in the reservoir show more or less hydraulic fluid?
It would show less hydraulic fluid in the reservoir
What are hydraulic systems on aircraft used for?
- brakes
- Flight controls
- cargo doors
- landing gear
What is 1 bar in psi?
1 bar = 14.5psi
What is 3000psi in bar?
207bar
What is 5080psi in bar?
350bar
What are the two types of hydraulic pressure?
- hydrostatic pressure
- hydrodynamic Pressure
What colour is mineral oil?
Red
What colour is synthetic oil?
Violet/purple
What are the two names used for synthetic oils?
Skydrol and Hyjet
What is it called when a fluid has the tendency to absorb water?
Hygroscopic
What is the disadvantage of using synthetic oil?
They are hygroscopic and this changes the freezing and lubrication characteristics
What are the oil characteristics required?
- low flammability
- good lubrication
- resistance to freezing
- non evaporating
- non foaming
- chemical stability
- hygroscopic quality
What are packings?
Also called seals - prevent leaks from component housings and provide a seal between different fluid pressures
(O-ring)
In areas where there is a high differential pressure, what additional packing can also be added if the seal alone cannot withstand it?
A backup ring made from Teflon
What material are packing seals made from?
Rubber
When are dynamic seals used?
Where stroking or rotational movement occur
When selecting seals to use, what should be considered?
- the working temperatures
- the forces acting upon them
How are airbus hydraulic systems identified?
Colours - green, yellow and blue
How are Boeing hydraulic systems identified?
Numbers - 1,2,3,4
In hydraulic systems, what is meant by each system being independent?
Fluid exchange does NOT take place
What is the pressure distribution section of a hydraulic system called?
A manifold
What is the purpose of the cylindrical shape of the hydraulic reservoir?
The shape resists changes in differential pressure caused by changes in altitude or temperature
What is mounted inside the reservoir to prevent the sudden movement of hydraulic fluid if the aircraft suddenly loses altitude?
A sheet metal baffled mounted horizontally
What is mounted on the inside of the hydraulic reservoir to indicate the levels of fluid to the crew in the cockpit?
A quantity transmitter - mounted on the side
Where is the drain valve located on a hydraulic reservoir?
At the bottom
What is the purpose of the quantity gauge on the side of the reservoir?
It provides a visual indication of the fluid level
Why is the hydraulic reservoir pressurised?
To ensure a positive supply of hydraulic fluid always reaches the pumps.
It also prevents foaming, cavitation and evaporation
What are the two methods of pressurising the hydraulic reservoir?
Hydraulic = using a piston to create pressure
Air pressure = via one of the engines (or pneumatic system if engines failed)
What is a reservoir pressurisation manifold also known as on some aircraft?
A reservoir pressurisation module
What is the purpose of a hydraulic restrictor?
It reduces the amount of airflow through the pipe. This means it prevents total loss if there is an air leak from the engine to the manifold.
Why is the air from the engine required to be regulated before it enters the reservoir?
It is at a much higher pressure than needed
What component regulates the air from the engine for the hydraulic reservoir?
The pressure reducing valve
Why do aircraft that only use pneumatic systems to pressurise the hydraulic reservoir not need to regulate the air?
The air is already regulated
What is the purpose of the fluid separator in a hydraulic system?
It removes any water contained in the air.
Where is the water collected from the fluid separator stored?
Into a container and is drained via the drain valve
If pressurising the hydraulic reservoirs on the ground, what is used?
Nitrogen bottles
What is the purpose of a check valve in a hydraulic system?
To maintain the air pressure in the reservoir
How can a hydraulic reservoir be depressurised for maintenance?
Via the depressurising valve located on each of the reservoirs
What is a hydraulic accumulator?
A device which potential energy is stored to exert a force on an incompressible fluid
What are the 4 types of hydraulic accumulators?
- weight loaded
- spring type
- piston type
- bladder type
Which types of hydraulic accumulators aren’t widely used today?
- weighted type
- mechanical spring type
What are the main functions of hydraulic accumulators?
- stores energy
- absorbs pulsations (from pumps)
- cushions operating shock
- supplements pump delivery (smaller pump can be used)
- maintains pressure
What is important to remember about reading the pressure gauge on a hydraulic accumulator?
The gauges aren’t usually calibrated and should be used for quick reference only.
Manufacturer pressure/temperature charts should be referred to before taking readings
What should be done before charging a hydraulic accumulator with nitrogen?
Purge the hose
NEVER USE OXYGEN OR AIR
What are hydraulic reservoirs pressurised to?
50psi
What are the two types of hydraulic pump?
- fixed displacement
- variable displacement
What is the process called where fluid is forced out of the cylinders by the pistons in a hydraulic pump called?
Fluid displacement
How does a hydraulic pump work?
A wobble plate, yoke or swash plate (depending on manufacturer) is displaced causing movement of the pistons to displace hydraulic fluid
What is the boost pump of a VDP also known as?
An impeller
What is the purpose of a boost pump in a VDP?
It prevents cavitation and increases the pressure of the fluid (known as pre-charging)
What is cavitation of hydraulic fluid?
The formation of bubbles in a liquid
Can cause damage to pumps etc
Where is the pump compensator valve calibrated to activate at 3000psi?
It is set in the workshop
What is the purpose of the filter inside the case drain of a hydraulic pump?
It provides the mechanic with indication of how much wear is occurring inside the pump
What is a fixed displacement pump (FDP)?
A hydraulic pump that has a fixed angle of displacement of the pistons. Found usually on cargo doors etc.
The mechanic varies the input to the pump by manual operation
What is an engine driven pump (EDP)?
It is a type of VDP that is permanently connected to the engine
If an engine fire occurs, how does the pilot turn off the EDP?
An electric motor operates a shut off valve located between the EDP and the reservoir, activated by the fire handles in the cockpit
What are electric pumps also known as?
Demand pumps
What is the purpose of electric hydraulic pumps?
They assist with maintaining the pressurisation when the EDP’s cannot do this
What is the problem with using electric pumps?
They get very hot and cannot be used for too long
What is an ADP?
An air driven pump
- a type of VDP
- another form of demand pump when the EDP cannot maintain the main pressure
How does an air driven pump work?
It uses high pressure air from the pneumatic system to drive a turbine (which in turn drives the pump)
How is the speed of the turbine of an air driven pump altered?
By the speed control valve (opening and closing) which changes the airflow
What is the purpose of Auxiliary pressurisation of hydraulic systems?
- to maintain pressurisation at high user demand
- replace main pressurisation when the system has a fault (RAT)
- for maintenance when engines aren’t running
How is the RAT deployed?
By a push button in the cockpit
How long does the RAT take to deploy when the pilots have pressed the push button?
Less than 1 second
What type of pump is the Ram Air Turbine (RAT)?
A VDP
What is a power transfer unit (PTU)?
It transfers hydraulic power between two systems via pump-motor combinations.
No transfer of fluid occurs
Pistons arranged like a VDP - can reverse direction (motor/pump)
If there is a serious fault with one of the hydraulic systems and the PTU automatically powers on, what can be done to prevent further damage to the failed system?
A cockpit switch de-energises the PTU
What is a ground-cart used for?
During maintenance - to pressurise hydraulic systems
What are the four components of a filter?
- filter bowl
- filter head
- filter element
- filter bypass
What is another name for a differential pressure indicator on a filter?
A pop-out indicator
What is the most important part of a filter, and what is it made up of?
The filter element
- metal
- plastic
- paper material
What is a high pressure manifold?
It moves the pressurised fluid from the hydraulic pumps to the hydraulic users
What is he purpose of a priority valve on a hydraulic manifold?
Its purpose is to close the valve when there is high demand in the system. It closes off to the less-vital users.
- it does not destroy pressure like the pressure relief valve, instead it adjusts the distribution to high/low priority
The high pressure filter (Airbus) cleans the fluid entering the manifold. What do Boeing aircraft call this filter?
The pressure filter
What are the two types of sensors found on a high pressure manifold?
- two pressure switches which detect low system pressure caused by pump malfunctions or system leaks
- pressure transmitter monitors system pressure at all times
