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Module 13 EASA Part 1 > 13.14 Hydraulic > Flashcards

13.14 Hydraulic Flashcards

1
Q

What are the major advantages of hydraulic actuation?

A

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

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2
Q

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?

A

Jack ram

Jack ram displacement

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3
Q

By approximately how much will hydraulic oil compress when subjected to 5000 psi of pressure?

A

1%

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4
Q

What does Pascal’s Law state?

A

Pressure in an enclosed container is transmitted equally and undiminished to all parts of the container, and acts at 90° to all confining surfaces

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5
Q

What are the general classifications of hydraulic fluids?

A

Natural

Mineral

Synthetic

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6
Q

What are the four main properties by which a hydraulic fluid is judged?

A

Viscosity

Chemical stability

Flash point

Fire point

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7
Q

What is the flash point of a fluid?

A

Temperature at which a liquid gives off vapour in sufficient quantity to ignite momentarily or flash when a flame is applied

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8
Q

What is the fire point of a fluid?

A

Temperature at which a substance gives off vapour in sufficient quantity to ignite and continue to burn when exposed to a spark or flame

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9
Q

What should you ensure when adding fluid to a system?

A

Use the type specified in the AMM or on the instruction plate affixed to the reservoir or unit being serviced

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10
Q

What are the properties of an ideal fluid?

A

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

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11
Q

What type of oil must be used with natural rubber seals?

A

Natural oil

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12
Q

What type of seal must be used with mineral oils?

A

Synthetic rubber

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13
Q

What type of seals will a synthetic oil attack?

A

Natural and synthetic rubber

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14
Q

What type of seals must be used with synthetic oils?

A

Butyl
Ethylene
Propylene
Teflon

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15
Q

What is a synthetic oil?

A

Phosphate–ester based oil

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16
Q

What is cavitation defined as?

A

Gas coming out of solution to form free bubbles

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17
Q

What are the basic components of a hydraulic circuit?

A

Reservoir

Pump

Selector valve

Actuator

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18
Q

What is the pipeline called which goes from a reservoir and a pump?

A

Suction line

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19
Q

What is the pipeline called which goes from a pump to a selector?

A

Pressure line

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20
Q

What is the pipeline called which goes from a service back to a reservoir?

A

Return line

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21
Q

What does a pump provide?

A

Flow, not pressure

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22
Q

What are the requirements of reservoirs?

A

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

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23
Q

What are the four methods of pressurising reservoirs?

A

Aspirator
Permanent gas charge
Bleed air from the engine compressor
Hydraulic system pressure

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24
Q

What is the source of air normally found in an aspirator?

A

Cabin air

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25
Q

How is pressure maintained in a compressor bleed air reservoir, if engine is not being run to provide the high pressure bleed air?

A

Schrader valve charge point and gauge are usually supplied to enable pressurisation of the reservoir from an external source

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26
Q

When is hydraulic pressure created?

A

When an attempt is made to compress a fluid

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27
Q

What is the inefficiency of a single action hand pump?

A

Draws fluid into the pump on one stroke and pumps that fluid out on the next stroke

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28
Q

What type of pumps deliver a fixed quantity of fluid into the system at a particular speed of rotation, regardless of system requirements?

A

Power Driven Fixed Volume or Constant Displacement Pumps

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29
Q

Describe the shear section (Quill) of the Power Driven Fixed Volume or Constant Displacement Pump?

A

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

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30
Q

What is the purpose of ‘case pressure’ in a spur gear pump?

A

If the shaft or seal becomes scored, fluid will be forced out rather than air drawn into the pump

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31
Q

Why is a small amount of fluid allowed to leak past the gears and around shafts in a spur gear pump?

A

Allow lubrication and cooling

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32
Q

In a variable volume pump, how is supply varied to suit system requirements?

A

Angle between the yoke and the cylinder block is varied to increase or decrease pump stroke to suit system requirements

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33
Q

What is the bias of a variable volume pump and how is this achieved?

A

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

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34
Q

Why is there a small flow allowed through a variable volume pump?

A

Lubricate working parts

Overcome internal leakage

Dissipate heat

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35
Q

What would be the result of a damaged spring in the variable volume pump?

A

Sloppy delivery

Swashplate would be moving around erratically

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36
Q

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?

A

Solenoid–operated depressurising valve

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37
Q

In general, what do pumps provide?

A

Rotary motion into hydraulic flow

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38
Q

What is the principle by which blockage indicators work?

A

Differential pressure

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39
Q

What element of a twin element filter will allow unfiltered fluid to flow through the system and what is this called specifically?

A

Inner element

Belvalve

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40
Q

What is the principle of the two stage filter?

A

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

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41
Q

What are the four functions of an accumulator?

A

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

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42
Q

What are the three types of linear actuator?

A

Single acting
Double acting
Balanced

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43
Q

What is the typical application of a single acting actuator and how does it work?

A

Landing gear up–lock

Lock is engaged by spring pressure and released by hydraulic pressure

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44
Q

How does a double acting actuator work, and what is the typical application?

A

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

45
Q

How does a balanced actuator work, and what is the typical application?

A

Equal force is applied to both sides

Nose–wheel steering and flying control boost systems

46
Q

What is the principle of operation of a hydraulic motor?

A

Takes hydraulic pressure, and converts to rotary motion

47
Q

What common components are used to achieve pressure control?

A

Automatic Cut–Out Valves (ACOV)
Pressure Relief Valves (PRV)
Thermal Relief Valves (TRV)
Pressure Reducing Valves (PRV)

48
Q

Where are Thermal Relief Valves usually fitted?

A

Into parts of circuits where no other form of pressure relief exists

49
Q

What is the purpose of a restrictor device in a TRV?

A

Prevents major loss of pressure should the valve jam in the open position

50
Q

What are the two ways that a pressure reducing valve reduces pressure?

A

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

51
Q

What are the three types of mechanical selector in a hydraulic system?

A

Rotary

Poppet

Spool

52
Q

What causes slide valve movement with the solenoid de–energised in a single solenoid selector?

A

Differential area on the piston

53
Q

What does a double solenoid selector allow?

A

A neutral setting

54
Q

How is the direction of flow normally indicated on a non–return valve?

A

Usually indicated by means of an arrow moulded or stamped on the valve body

55
Q

When is a hydraulic fuse used?

A

In pipelines to seal them off if a serious leak occurs

56
Q

Where might a Pressure Relay Valve be used and what is its purpose?

A

Pipelines to direct reading gauges or pressure transmitters

Prevents fluid loss if a leak occurs

57
Q

What are System Isolation Valves used for in a hydraulic system?

A

Shutting off of parts for:

Maintenance purposes

In the event that a fire should occur in certain areas

58
Q

Where are drain cocks usually fitted?

A

In the lowest part of the system

59
Q

What are the two classes of seals?

A

Static

Dynamic

60
Q

What is the correct seal/fluid combination of castor based fluid DTD900/4081?

A

Natural rubber

61
Q

What is the correct seal/fluid combination of mineral based fluid MIL–H–5606?

A

Synthetic rubber

62
Q

What is the correct seal/fluid combination of phosphate ester based fluid (SKYDROL)?

A

Butyl, ethylene, propylene or teflon

63
Q

How is the possibility of an ‘O’ ring seal becoming extruded (squeezed) through any small gap prevented?

A

Using back up rings

64
Q

What are back up rings normally made of?

A

Tufnol

Teflon

Micarta

65
Q

Where is a duplex seal especially useful and where is it often used?

A

Oil/gas interfaces

Accumulators

66
Q

What type of leak is a duplex seal especially useful in preventing?

A

Internal leakage

67
Q

How can you ensure ‘U’ or ‘V’ section seals can provide static and dynamic sealing in both directions?

A

2 seals must be fitted back to back

68
Q

Where will a wiper ring be used and what is it useful in providing?

A

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

69
Q

What is the primary purpose of the fuel cooled oil cooler and the secondary advantage?

A

Cool the oil

Heats the fuel, preventing icing and aiding efficient combustion

70
Q

What are standby or auxiliary hydraulic systems said to be?

A

Separate power systems which have their own dedicated reservoir/pump etc

71
Q

How is it ensured that the electrically driven hydraulic pump can be operated where one engine has been lost?

A

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

72
Q

What are Power Transfer Units (PTUs) and what is the secondary benefit of it?

A

2 hydraulic pumps mounted back–to–back

Allows the engineer to power all systems off of just one functioning engine

73
Q

How does a RAT maintain constant RPM?

A

Centrifugal force will be felt on weights which change the blade pitch

74
Q

Why is a volumetric fuse fitted to the RAT?

A

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

75
Q

What is the purpose of the standpipe in a hydraulic reservoir?

A

Should the main system’s fluid supply become depleted, adequate fluid is then left for operation of the emergency system

76
Q

In a pressure indicator of a hydraulic system, what is a electrically signalled system?

A

Bourdon tube connected to a variable resistor, and a voltage proportional to pressure is sent to a moving coil type gauge

77
Q

What is viscosity?

A

Internal resistance to flow

78
Q

Why do hydraulic reservoirs need to be pressurised?

A

Temporary pressure loss might occur if during banking the reservoir is below the height of the pump, therefore positive pressure is required

79
Q

How does a bootstrap reservoir work?

A

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

80
Q

Servo pressure is felt on which component of the variable volume pump?

A

Control piston

81
Q

What two things occur when the primary element of a twin element filter becomes blocked?

A

Primary bypass valve will open, directing fluid to the inner element

Pressure differential will cause clogging indicator to show

82
Q

What action needs to be carried out should the blockage indicator pop?

A

Investigation into the cause

83
Q

What type of hydraulic circuit requires an automatic cut out valve?

A

Where the pump is a fixed volume type and does not have the ability to self–idle

84
Q

How is the ACOV prevented from hammering?

A

An accumulator ensures smooth operation

85
Q

Where might a sequence valve be fitted and why?

A

Undercarriage raising/lowering circuit

It is necessary to ensure that the undercarriage strut has retracted before the fairing door closes

86
Q

What valve enables an alternative source of pressure to be used in an emergency?

A

Shuttle valve

87
Q

What is the purpose of the pressure maintaining valve?

A

Ensure flow/pressure to the primary/essential operating systems whenever any is available

88
Q

How is a hydraulic pipeline identified?

A

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

89
Q

What three parameters will a hydraulic system monitor?

A

Quantity
Temperature
Pressure

90
Q

What is the prime advantage of modular valve assemblies?

A

Simplifies fault diagnosis and requires only one component to be replaced to restore system function

91
Q

Where might you find a suction filter?

A

In a pipeline between the pump and the reservoir in a self–idling system

92
Q

In a non self–idling circuit, what are the three connections to the engine driven pump?

A

Suction
Bypass
Pressure

93
Q

What will leakage across dynamic seals cause?

A

Purely internal

Cause slow or sluggish operation of actuators

May cause overheating due to increased flow rates

94
Q

When sampling a fluid, what is it usually necessary to ensure?

A

They are at their working temperature

95
Q

How is sampling a fluid at its working temperature ensured?

A

Within 20 minutes of system shut down

96
Q

If a hydraulic system is cold, how do you get the fluid to its working temperature, for sample taking purposes?

A

Ground run

97
Q

When might a sample of hydraulic fluid be required?

A

Called for by maintenance schedule

Suspected contamination

98
Q

What is the Millipore patch test?

A

A comparison test against a known standard

99
Q

Why must the configuration of an aircraft be checked prior to filling a reservoir with fluid?

A

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

100
Q

How is internal test of a hydraulic system tested?

A

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

101
Q

What is the name given to an internal leak check?

A

Leak rate test

102
Q

What might indicate aeration of a fluid or excessive internal leakage?

A

Sluggish, slow or erratic operation

103
Q

What are the two general types of contamination?

A

Particulate

Liquid or soluble contaminants

104
Q

What are the more common sources of contamination?

A

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

105
Q

What are the steps for flushing a system?

A

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

106
Q

What two things must be checked prior to topping up a reservoir?

A

Check that ground equipment contains the correct specification fluid

Ensure that the aircraft is in the correct configuration for filling

107
Q

What would determine the system is free from air following a bleed?

A

An air free flow of oil is obtained

108
Q

For safety, what must be done prior to disconnection of any hydraulic pipeline?

A

All hydraulic pressure has been released, including that held in accumulators

Module 13 EASA Part 1 (20 decks)

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