13.14 Hydraulic Flashcards
What are the major advantages of hydraulic actuation?
Provision of smooth and steady movement
Confined to pipelines and components, and does not require widespread significant structural strengthening
Installation is simpler than mechanical power transmission systems
Variations in speed and power output can be made without the need for complex and heavy gearboxes
Power for hydraulic systems can be provided from many separate sources, for both normal and emergency operation
What is a piston rod often referred to and what is the principle often referred to where an actuator has different force on each side of the rod for the same pressure?
Jack ram
Jack ram displacement
By approximately how much will hydraulic oil compress when subjected to 5000 psi of pressure?
1%
What does Pascal’s Law state?
Pressure in an enclosed container is transmitted equally and undiminished to all parts of the container, and acts at 90° to all confining surfaces
What are the general classifications of hydraulic fluids?
Natural
Mineral
Synthetic
What are the four main properties by which a hydraulic fluid is judged?
Viscosity
Chemical stability
Flash point
Fire point
What is the flash point of a fluid?
Temperature at which a liquid gives off vapour in sufficient quantity to ignite momentarily or flash when a flame is applied
What is the fire point of a fluid?
Temperature at which a substance gives off vapour in sufficient quantity to ignite and continue to burn when exposed to a spark or flame
What should you ensure when adding fluid to a system?
Use the type specified in the AMM or on the instruction plate affixed to the reservoir or unit being serviced
What are the properties of an ideal fluid?
Incompressible
Reasonable density with little or no variation due to temperature changes
Viscosity should have a low rate of change with changes in temperature
Large working temperature range
Good lubrication properties
Not a significant health hazard
Flash point above 100°C, but preferably non–flammable
Should not foam
Chemically stable under all operating conditions
Should not harm, nor be harmed by materials used in the system pipelines and components
Good storage life
What type of oil must be used with natural rubber seals?
Natural oil
What type of seal must be used with mineral oils?
Synthetic rubber
What type of seals will a synthetic oil attack?
Natural and synthetic rubber
What type of seals must be used with synthetic oils?
Butyl
Ethylene
Propylene
Teflon
What is a synthetic oil?
Phosphate–ester based oil
What is cavitation defined as?
Gas coming out of solution to form free bubbles
What are the basic components of a hydraulic circuit?
Reservoir
Pump
Selector valve
Actuator
What is the pipeline called which goes from a reservoir and a pump?
Suction line
What is the pipeline called which goes from a pump to a selector?
Pressure line
What is the pipeline called which goes from a service back to a reservoir?
Return line
What does a pump provide?
Flow, not pressure
What are the requirements of reservoirs?
Maintain a head of pressure, under all conditions
Prevent the possibility of the oil becoming aerated during manoeuvres
Prevent cavitation in the supply pipeline to the pump
Prevent a lowering of hydraulic fluid boiling point at high altitude
What are the four methods of pressurising reservoirs?
Aspirator
Permanent gas charge
Bleed air from the engine compressor
Hydraulic system pressure
What is the source of air normally found in an aspirator?
Cabin air
How is pressure maintained in a compressor bleed air reservoir, if engine is not being run to provide the high pressure bleed air?
Schrader valve charge point and gauge are usually supplied to enable pressurisation of the reservoir from an external source
When is hydraulic pressure created?
When an attempt is made to compress a fluid
What is the inefficiency of a single action hand pump?
Draws fluid into the pump on one stroke and pumps that fluid out on the next stroke
What type of pumps deliver a fixed quantity of fluid into the system at a particular speed of rotation, regardless of system requirements?
Power Driven Fixed Volume or Constant Displacement Pumps
Describe the shear section (Quill) of the Power Driven Fixed Volume or Constant Displacement Pump?
Located midway between the two sets of splines
Smaller in diameter than the splines
Shears, preventing damage to the pump or driving unit if it becomes unusually hard to turn or becomes jammed
What is the purpose of ‘case pressure’ in a spur gear pump?
If the shaft or seal becomes scored, fluid will be forced out rather than air drawn into the pump
Why is a small amount of fluid allowed to leak past the gears and around shafts in a spur gear pump?
Allow lubrication and cooling
In a variable volume pump, how is supply varied to suit system requirements?
Angle between the yoke and the cylinder block is varied to increase or decrease pump stroke to suit system requirements
What is the bias of a variable volume pump and how is this achieved?
Biased to deliver maximum output to the system
Spring pressure on the control piston turns the yoke to its maximum angle and the pistons are at full stroke
Why is there a small flow allowed through a variable volume pump?
Lubricate working parts
Overcome internal leakage
Dissipate heat
What would be the result of a damaged spring in the variable volume pump?
Sloppy delivery
Swashplate would be moving around erratically
In a variable load pump, what component would allow the facility to be ‘off loaded’ to reduce drag on the engine, for example during engine starting?
Solenoid–operated depressurising valve
In general, what do pumps provide?
Rotary motion into hydraulic flow
What is the principle by which blockage indicators work?
Differential pressure
What element of a twin element filter will allow unfiltered fluid to flow through the system and what is this called specifically?
Inner element
Belvalve
What is the principle of the two stage filter?
Allows the use of an extremely fine element for low flow rates, but bypasses this filter for high flow rates without causing excessively high pressure
Normal cruising, requires relatively low flow rate, so flows through both stages
Excess flow rate will allow excess flow to bypass the first stage, through the second coarser stage only
What are the four functions of an accumulator?
Accumulator ensures smooth operation where an automatic cut–out is used in conjunction with a pump (hammering)
Storage of hydraulic pressure which can be used to operate systems in an emergency
Provides initial impetus to a system until the pump begins to provide sufficient pressure/flow
Damp out pulsations from the pump preventing fatigue damage to pipelines, pipe clamps and the aircraft structure
What are the three types of linear actuator?
Single acting
Double acting
Balanced
What is the typical application of a single acting actuator and how does it work?
Landing gear up–lock
Lock is engaged by spring pressure and released by hydraulic pressure
How does a double acting actuator work, and what is the typical application?
Presence of the piston rod means the area of the top of the piston is greater than that under
More force can be applied during extension, operation which offers greater resistance is carried out in the direction in which the piston rod extends
Raising landing gear
How does a balanced actuator work, and what is the typical application?
Equal force is applied to both sides
Nose–wheel steering and flying control boost systems
What is the principle of operation of a hydraulic motor?
Takes hydraulic pressure, and converts to rotary motion
What common components are used to achieve pressure control?
Automatic Cut–Out Valves (ACOV)
Pressure Relief Valves (PRV)
Thermal Relief Valves (TRV)
Pressure Reducing Valves (PRV)
Where are Thermal Relief Valves usually fitted?
Into parts of circuits where no other form of pressure relief exists
What is the purpose of a restrictor device in a TRV?
Prevents major loss of pressure should the valve jam in the open position
What are the two ways that a pressure reducing valve reduces pressure?
High pressure fluid enters a larger chamber, reducing pressure
If pressure continues to increase, piston is lifted against spring pressure, closes off the pressure inlet but opens the return line
What are the three types of mechanical selector in a hydraulic system?
Rotary
Poppet
Spool
What causes slide valve movement with the solenoid de–energised in a single solenoid selector?
Differential area on the piston
What does a double solenoid selector allow?
A neutral setting
How is the direction of flow normally indicated on a non–return valve?
Usually indicated by means of an arrow moulded or stamped on the valve body
When is a hydraulic fuse used?
In pipelines to seal them off if a serious leak occurs
Where might a Pressure Relay Valve be used and what is its purpose?
Pipelines to direct reading gauges or pressure transmitters
Prevents fluid loss if a leak occurs
What are System Isolation Valves used for in a hydraulic system?
Shutting off of parts for:
Maintenance purposes
In the event that a fire should occur in certain areas
Where are drain cocks usually fitted?
In the lowest part of the system
What are the two classes of seals?
Static
Dynamic
What is the correct seal/fluid combination of castor based fluid DTD900/4081?
Natural rubber
What is the correct seal/fluid combination of mineral based fluid MIL–H–5606?
Synthetic rubber
What is the correct seal/fluid combination of phosphate ester based fluid (SKYDROL)?
Butyl, ethylene, propylene or teflon
How is the possibility of an ‘O’ ring seal becoming extruded (squeezed) through any small gap prevented?
Using back up rings
What are back up rings normally made of?
Tufnol
Teflon
Micarta
Where is a duplex seal especially useful and where is it often used?
Oil/gas interfaces
Accumulators
What type of leak is a duplex seal especially useful in preventing?
Internal leakage
How can you ensure ‘U’ or ‘V’ section seals can provide static and dynamic sealing in both directions?
2 seals must be fitted back to back
Where will a wiper ring be used and what is it useful in providing?
Fitted into the body of any component that has a moving ram or rod
Any dirt or other contaminants picked up on the ram will be removed by the wiper ring, preventing damage to internal seals
What is the primary purpose of the fuel cooled oil cooler and the secondary advantage?
Cool the oil
Heats the fuel, preventing icing and aiding efficient combustion
What are standby or auxiliary hydraulic systems said to be?
Separate power systems which have their own dedicated reservoir/pump etc
How is it ensured that the electrically driven hydraulic pump can be operated where one engine has been lost?
Cross connect the pumps to either engine output
Engine driven pump comes from one engine, but the electrically driven pump comes from the other engine
What are Power Transfer Units (PTUs) and what is the secondary benefit of it?
2 hydraulic pumps mounted back–to–back
Allows the engineer to power all systems off of just one functioning engine
How does a RAT maintain constant RPM?
Centrifugal force will be felt on weights which change the blade pitch
Why is a volumetric fuse fitted to the RAT?
Allows the propeller to attain operating speed before being loaded by the pump
Allows full system pressure to be achieved approximately 4–5 seconds after deployment into the airstream
What is the purpose of the standpipe in a hydraulic reservoir?
Should the main system’s fluid supply become depleted, adequate fluid is then left for operation of the emergency system
In a pressure indicator of a hydraulic system, what is a electrically signalled system?
Bourdon tube connected to a variable resistor, and a voltage proportional to pressure is sent to a moving coil type gauge
What is viscosity?
Internal resistance to flow
Why do hydraulic reservoirs need to be pressurised?
Temporary pressure loss might occur if during banking the reservoir is below the height of the pump, therefore positive pressure is required
How does a bootstrap reservoir work?
Large piston on top of the oil, connected by a ram to a small piston upon which system pressure is acting
System pressure on small ram ensures pressure will be felt on the stored fluid
Servo pressure is felt on which component of the variable volume pump?
Control piston
What two things occur when the primary element of a twin element filter becomes blocked?
Primary bypass valve will open, directing fluid to the inner element
Pressure differential will cause clogging indicator to show
What action needs to be carried out should the blockage indicator pop?
Investigation into the cause
What type of hydraulic circuit requires an automatic cut out valve?
Where the pump is a fixed volume type and does not have the ability to self–idle
How is the ACOV prevented from hammering?
An accumulator ensures smooth operation
Where might a sequence valve be fitted and why?
Undercarriage raising/lowering circuit
It is necessary to ensure that the undercarriage strut has retracted before the fairing door closes
What valve enables an alternative source of pressure to be used in an emergency?
Shuttle valve
What is the purpose of the pressure maintaining valve?
Ensure flow/pressure to the primary/essential operating systems whenever any is available
How is a hydraulic pipeline identified?
Black disc on a white background
Adhesive or shrink–fit tapes, with the word hydraulic in black on a white or blue and yellow background
System name or code and an arrow to show direction of flow
What three parameters will a hydraulic system monitor?
Quantity
Temperature
Pressure
What is the prime advantage of modular valve assemblies?
Simplifies fault diagnosis and requires only one component to be replaced to restore system function
Where might you find a suction filter?
In a pipeline between the pump and the reservoir in a self–idling system
In a non self–idling circuit, what are the three connections to the engine driven pump?
Suction
Bypass
Pressure
What will leakage across dynamic seals cause?
Purely internal
Cause slow or sluggish operation of actuators
May cause overheating due to increased flow rates
When sampling a fluid, what is it usually necessary to ensure?
They are at their working temperature
How is sampling a fluid at its working temperature ensured?
Within 20 minutes of system shut down
If a hydraulic system is cold, how do you get the fluid to its working temperature, for sample taking purposes?
Ground run
When might a sample of hydraulic fluid be required?
Called for by maintenance schedule
Suspected contamination
What is the Millipore patch test?
A comparison test against a known standard
Why must the configuration of an aircraft be checked prior to filling a reservoir with fluid?
Fluid might remain in actuators or within the system, which when returned to the reservoir could leave insufficient space
Fluid will be released to atmosphere through the reservoir relief valve
How is internal test of a hydraulic system tested?
Run the system to full pressure and then switch off the pressure supply
System pressure drop rate is then timed and compared to values specified in the AMM
What is the name given to an internal leak check?
Leak rate test
What might indicate aeration of a fluid or excessive internal leakage?
Sluggish, slow or erratic operation
What are the two general types of contamination?
Particulate
Liquid or soluble contaminants
What are the more common sources of contamination?
Foreign matter introduced during servicing
Solid particles caused by natural wear of moving components
Incorrect fluids used during system filling
Overheating of system fluid leading to chemical changes
Incorrect handling of ground maintenance and servicing equipment
What are the steps for flushing a system?
Drain
Replace filters and filter elements
Refill the system to a specified level
Operate appropriate systems for a specified time
Drain all fluid
Replace filters and filter elements again
Refill with clean fluid to the specified level
Run system for specified time and sample
What two things must be checked prior to topping up a reservoir?
Check that ground equipment contains the correct specification fluid
Ensure that the aircraft is in the correct configuration for filling
What would determine the system is free from air following a bleed?
An air free flow of oil is obtained
For safety, what must be done prior to disconnection of any hydraulic pipeline?
All hydraulic pressure has been released, including that held in accumulators
