HYDROLICS general

Question 1

hydrolics is:

Answer 1

• study of behaviour of liquids under certain conditions.

all fluids have static pressure due:
- to their own weight
- temperature

Question 2

hydrolic system

Answer 2

aircraft systems provide a means of power transmission through hydrolic system (via pipelines and actuators)

System does:
- transmit force from one part to another part
- applies considerable amount of force on services

Question 3

where do hydrolic systems provide power to

Answer 3

to systems that need:
* High Power
* Accurate Control
* Rapid Response rate

examples:
* landing gear
* flaps spoilers
* flight controls
* wheel brakes
* nose gear stirring
* primary flight control
* windshield wipers
* cargo doors

Question 4

Hydrostatic Pressure

Answer 4

In an open container,

pressure exerted BY A FLUID is dependent on the height of the fluid,

**Irrespective of the sizes of the varying containers. **

this means that the force causing the hydrostatic pressure is the ‘weight’ of the fluid

Question 5

Pascal’s law

Answer 5

an force applied to a liquid in** closed container**, the force will be felt equally on all directions

pressure applied from anywhere, in a confined space, will spread in equal directions in the fluid

Question 6

Hydraulic power relies on 3 things

Answer 6

Area=area force is applied on
force (input and output)
pressure (energy built up in the fluid)

Question 7

what is the force employed in hydraulic system?

Answer 7

Pressure
= force per unit area (force/area)

Question 8

So what is force?

Answer 8

Force is total load available

pressure x area

unit= N

Question 9

So how is pressure built up in hydraullic system? using Pascal’s law..

Answer 9

in a closed container, if a **force is applied to the liquid **(without a mean for liquid to escape), there is kinetic energy

the pressure will build up equally on all directions.

If force is the same, but area increases = pressure decreases

If force is the same, but area decreases, pressure increases

If area is the same, but force increases = pressure increases

If area is the same, but force decreases = pressure decreases

Question 10

Braham’s Press

Answer 10

The smaller the area under the load
The greater the pressure

the larger the area under the load
the greater the force

pressure is excerted by the fluid, not the piston. Pressure is the same throughout the fluid.

Question 11

what is ‘work done’ by a machine

Answer 11

it’s the force applied x distance moved

force x distance = Joules

it’s a type of energy

Question 12

for a given fluid pressure, the forced produced can be varied how?

Answer 12

by adjusting the piston area

Question 13

What is passive hydraulic system

Answer 13

Found on light aircraft braking circuit

‘human’ powered

system has no pump, only a pistons

Pressure is produced only when force is applied to a piston A
Applied when it’s desired to move the load (piston b)===doing the work

Generating pressure is ‘required’

as opposed to maintaining pressure all the time and only using it when something needs to be moved

Question 14

What is active hydraulic system

Answer 14

Most aircrafts have this

2) 1) When a system has a pump, to deliver a flow of fluid into a system

3)pumps are engine driven or electric motor driven

2) some restrictions are required to create pressure by **Pistons that travel froward and backwards in cylinders (hydralic jacks and actuators).

Active system supplies pressure all the time OR they supply pressure on demand

Used in:
Undercarriages
flaps
nose steering
power flying control units
spoilers

Question 15

critical hydraulic systems carry how many pumps

Answer 15

carry at least one BACK UP pump

Question 16

the efficiency of hydraulic systems is goverend by?

Answer 16

resistance of the motion encountered by the fluid

Question 17

hydraulic fluids are not

Answer 17

**NOT compressable
**
except in high pressures (but even then it’s a small change in volume)

Question 18

compression of liquid vs air

Answer 18

liquid is compressed by 1% of it’s original volume, 99% remain

air is compressed by 99% of its volume, 1 % remain

Question 19

When fluids resist compression there is still some force expended..where?

Answer 19

between:

1)Piston and cylinders
Can’t be complely eliminated but lessend with good design/workmanship

2)Piston rods and seals/bearing
Can’t be complely eliminated but lessend with good design/workmanship

3)fluid and pipes
Depend on
velocity of fluid in pipes, length and finish of the pipe
number of bends
viscosity of fluid/oil

Question 20

How to counteract this loass of effiency

Answer 20

use right oil/fluid

use glands, seals and bearing rings

Question 21

frictionless pistons are possible but…

Answer 21

incur high weight and high cost

Question 22

Characteristics of Hydraulic fluid

Answer 22

• Low viscosity (to pump easily, reduce power consumption and resistance to flow)
• Lubricative (to reduce wear of rubber and metal)
• Incompressable as much as possible (to transfer power)
• High boiling point
• Low freezing point
• good storage properties
• High flash & fire points (doesn’t ignite easy)
• Thermal stability
• Low imulsifying (doesn’t blend with other fluids)
• compatible with other material
• non-crossive
• resistant to sludge or foaming
• Low volatility
• resistant to evaporation

Cons
can be corrosive in high temperature (so should be wiped of any surface
*ruin paint work
* Skin/’eye irration
* cannot mix with other fluids - as in they are NOT INTERCHANGEABLE - must use the type advised by design.
* If mixed, you must flush the system
* fluid can attack electrical wiring coating or PVC or rubber material
* when overheated can be acidic and form sludge

Question 23

Seals

Answer 23

Help** prevent leakage of fluid** by being squeezed between 2 surfaces

there are V and U types for one direction fluid pressure

O for fluid pressure applied in both directions

Question 24

Seals

Answer 24

Purpose:
* Help** prevent leakage of fluid**

How:
* by being squeezed between 2 surfaces

Types:
* there are V and U types for one direction fluid pressure

• O for fluid pressure applied in both directions

To retain longevity
* seals need to be slightly lubricated so some fluid iseepage is ok.
* damaged easily by grit or the wrong hydraulic fluid

Question 25

Choice of aircraft hydraulic fluids is influenced by…

Answer 25

materials of seals, rings, seats, etc.

Question 26

Types of hydraulic fluids in ACs

Answer 26

Mineral & synthetic

DTD 585 - mineral:
petroleum/mineral based
red in colour
works with synthetic rubber seals (Neoprene)

Skydrol - synthetic:
Phosphate ester based oil
works with syntheitc rubber seals (Butyl & teflon)
Type 500A & B - FRESH = purple colour
Type 700A = green

Question 27

what happens if you uuse the wrong hydraulic oil in the system

Answer 27

**Only one type of oil can be used **in the system

**incorrect one is added
**- seals will damage
- fluid leakage occurs internally and externally in actuators

How to know if the wrong fluid is added?
- fluids have their own colour
- can be confrim by AC manual
- use fluid from sealed containers

Question 28

Low and high pressure hydraulic systems

Answer 28

Low: up to 2000psi
High: Above 2000psi and avg 3000-4000psi

A380 operates at 5000psi

Question 29

what’s the advantage of high pressure system

Answer 29

the actuator can be made small, meaning less fluid is required and pipes can be smaller

weights less and saves space

Disadvantage: can cause shear

Question 30

Open system hydraullics

Answer 30

**good for **light/mid aircrafts **that don’t require constant pressure esp if flaps and gear are powered for short time

works for Active systems generate pressure only when it’s demanded by the service

simple system

but only one service can be operated at a time

**low pressure **of 2000psi

how it works:
IF a service is not selected:
fluid passes from reservoir then thru the pump
fluid passes through the valve (past the NRV) and into the selector(s).
If service is not operating or selected, then fluid continues to move through the valves and return to reservoir
here we are at lowest pressure, but fluid still moves to lubricate the pump

if a service is selected
fluid passes from reservoir then thru the pump
fluid passes through the valve (past the NRV) and into the designated selector (belonging to the service).
If service is selected, then fluid goes through one side of the Actuator (piston moves).
From the other side of the actuator, the fluid flows into the other selector, returns back thru the valves, and back to resourvoir

here we are at high pressure

Selector returns to ‘neutral’ to offload the pump and allow other services to be selected.

The relieve valve will relief pressure if the selector is not returned to it’s neutral posisition.

Question 31

Closed system

Answer 31

Use on large aircrafts

multiple services can be operated at same time

the use of this system is only limited by the flow capacity of the pump.

pressure in this system stays limited to the part of teh system that leads to the selector valves. So there is a lot of pressure and load on the pump.

Need to relief the pump from this pressure by:

1.Off loading valve:
allows pump to unload while service is not being used

2. ACOV
   In a fixed volume pump, an automatic cut-out valve is fitted (ACOV) to divert pump output to the reservoir when pressure has reached normal operating pressure

Option 2:
In other system a Variable volume pump is used.
This reduces fluid delivery flow as pressure increases

Once operation is complete, there is no need to return the selector valve to neutral because the pump system is designed to relieve pressure increase.

But in some system,s the selector valve might be returned to neutral = hydrolic lock (fluid is locked on both sides of the actuator) and non return valve, hence no undemanded moved will be done by the actuator and piston cannot move

In some simple aircrafts:
A simply hydrollic power pack is used
this is an electric pump operated by pressure operating switch - with a reservoir and accumulater
it’s when

Question 32

Reservoir

Answer 32

**space to store fluid. **

fins and baffles help reduce swirling of fluid when aircraft is moving

supplies fluid to the pump

Stores extra to compensates for small leaks

**has extra airspace to account for Actuator fluid coming back and thermal expansion

Reservoirs are pressurized:
1. to avoid fluid from boiling at altitude
2. provides positive pressure at inlet to the pump to to avoid bubbles in fluid (cavitation) in altitulde
Reservoirs are pressurized to ensure pump inlet is provided with continious and adequate supply of fluid free of foam

the fluid coming in, flows thru a ‘de-arator to remove the bubbles

has relief valve to relieve over pressure

May have a **temperature & content sensor **

In case of major fluid loss, there is a 2nd lower exit pipe operated by a hand pump to acompany the main pump, so in case fluid is not supplied by main pump, a reserve of fluid is available for the heand pump

Fluid level depends on:
* Thermal expansion = higher temp, fluid expands
* When system is energized (pump starts pumping) = fluid decreases
* When actuators are operated = fluid fluctuates

Question 33

Filters

Answer 33

Fitted for both Sucktion and Pressure lines (both sides of the pump)

—–One for protecting the pump

——-One for keeping fluid clean and remove particles (protect seals)

some other individual components may be fitted with their own filters.

has pressure differential sensor that lights up/visual indicator that indicates it is clogged

sometimes there is a relieve valve to allow unflitered fluid to pass if system is clogged

note:
indicator can light up if fluid has high viscocity in low temp

Question 34

Vlaves

Answer 34

NRV -
* prevents reverse flow of fluid
* a ball push by fluid pressure against the spring.
* when pushed, it allows fluid into the valve
* when pressure is reduced, and fluid has moved on, the ball returns back to its place to close the valve

selector valve: directs pressure to the appropriate side of the accutator.
Mechanical or electrical driven

Pressure relieve valve
Dumps excess pressure into the resrvoir

Question 35

Selector valves

Answer 35

Closed Centre Selector

Open Centre Selector

Linear Valve Selector

Question 36

Pumps

Answer 36

Driven from:
accessory gearbox

Usually called:
Engine driven pumps

Pumps can be:
* Constant delivery Fixed volume
* Constant pressure Variable volume

Piston type: for large aircraft
rotary/spur type for small aircraft

Pressurizes fluid flow in the circuit

*has overheat detection

draws oil from reservoir and delivers a supply of fluid to the system

Can be:
* *hand operated
* engine driven
* electric motor driven (used for light aircraft power pack or for emergency)
* ATM - air turbin motor (pneumatically operated)
* RAT - ram air turbine
* PTU - power transfer unit (hydraullical pumps)

ATM, RAT, PTU - provide and alternate supply for an Engine driven pump as part of hydraulic system redundancy provision in the event of engine pump failure

RAT: provides emergency hydraulics and electric power to flight controls and breakls only

Question 37

Pumps - Hand

Answer 37

hand pump used in small aircrafts

in large aircraft used in:
1. for ground servicing to take place without engine running
2. for lines and joints to be tested
3. cargo doors to be operated witout power

Hand pump is usually
- a double acting pump- delivers oil on 2 strokes
- Non return valve
- relief valve (to relieve pressure = 10% of normal system pressure

In the centre of the piston assembly, there is a Trasnfer Valve, which is spring loaded to the closed position.

HOW IT WORKS
* Fluid transfers from reservoir via the Inlet, where the RV1 sits.
* Fluid then travels to the Piston assembly.
* When the operating handle is moved up,
* It push the Piston assembly down
* this increases pressure below the piston and holds the RV1 closed.
* Fluid is trapped between the RV and Under the piston, which builds up pressure
* It is felt by the transfer ball at the bottom of the transfer valve
* which pushes the ball up towards the transfer valve up, creating a gap for the fluid to move up the piston
* then fluid flows out the RV2 outlet

• Once the piston moves to the bottom of its travel, the handle moves in the opposite direction,
• moving the piston in the assembly up
• As the piston moves up, the pressure above the piston incerases
• the ball is pushed into the transfer valve, closes the transfer valve
• Pressue above the piston is great, pushing fluid to RV2 outlet
• RV1 opens up becuase of suction felt below the piston, allowing fluid to flow from reservoir

Question 38

Engine driven pumps

Answer 38

1) Constatn delivery (fixed volume) pump for low hydraullic pressure aircrafts
* Uses spur rotary gears. They rotate at constant speed
* Therefore they supplies fluid at a constant rate
* Volume of fluid passing depends on the speed of rotation
* Needs an automatic cut-out to control it’s output or relief valve to return the fluid to the reservoir
* when the system is not operating, it requires an idling circuit. This pump is usually a single or double stage gear pump giving a large flow at a small pressure, typically up to 2000 psi.
* delivers same amoutn of fluid without regard to the fluid required by the system, unused fluid returns to reservoir via relief valve
* wastes energy

Made with 2 gears.
1 gear driven by the power source (ie engine), Gear 2 is powered by gear 1
the 2 gears rotate
fluid travels from inlet, through the gear teeth, then out from the outlet
Ths system gives great flow rate but Low pressure rate
This system supplies fluid at constant rate irrespective of demand
If this system is mechanically driven by the engine in a ‘closed hydraullic system’, it needs an ACOV to return fluid to the reservoir when the system is not operating

Constant Pressure (variable volume) pump for high pressure hydraullic system modern aircrafts
* large modern aircraft (for hydraulic systems operation at 3000 psi +)
* supplies fluid at a variable volume - on demand
* controls its own pressure
* can alter the outlet flow as more services are operated. It will increase flow to maintain working pressure
* ACOV and accumulator not needed

Question 39

Most transport aircrafts are fitted with wich type of pumps

Answer 39

Constant pressure pumps
Demand pressure pumps (better option)

Question 40

Automatic Cut Out Valves (ACOV)

Answer 40

Used in Constant Deliver (fixed volume) engine driven pumps.

in there, it controls system pressure

it also provides the pump with idling circruit when no service is selected

however for this reason, it needs an accumulator to maintain pressure, and take in some slight leakage due to the frequent operation of the ‘cut - out’

maintains cooling and lubrication of output pumped fluid

–HOW IT WORKS–
when the cut out is in possition, fluid goest thru the pump, then thru the NRV.
Pressure is built, till it pushes the piston upwards till the poppet valve opens and the output pumped fluid passes to the reservoir

Question 41

how do we know the condition of the system?

Answer 41

The time between cut-out (off load) and cut-in (on load) (periodicity) of the ACO valve

Question 42

how do you know there is external leakage

Answer 42

• cause a **reduction in the operating period **with frequent loading and unloading of the pump;
• also with a loss of system fluid.

Question 43

how do you know there is internal leakage

Answer 43

• caused by a piston seal failure
• causes frequent loading and unloading of the pumps
• increase in fluid temperature.
• Decrease system pressure

Note: No fluid loss

Question 44

Accumulator (fitted with NRV)

Answer 44

Contains:
Has piston moving freely inside a sealed cylinder

onse side has gas (precharged nitrogen or air), and other has fluid

How it works

*works as an energy storage

When hyraulic system pressure increases
the piston moves up and compresses gas
the piston stops when gas pressure = fluid pressure and remain in this state during normal operations

If hydraulic pump fails, the accumulator has ‘reserve/limited supply of pressure fludi’ to operate essential services

when it’s powered: it shows hyrdraulic system pressure

when it’s not powered: it shows nitrogen pre-charge pressure

Inciorrect recharge:
will cause hammering in the system
affect the placement of the piston inside accumulator

If in excess: then there is insufficient fluid stored
if low: insufficient fluid pressures for emergencies

needed for:

• breaking system
• allowing thermal expansion
• allow internal small leaks
• store pressurized hydraulic fluid/gas & dampen pressure pulses fluctuation
• provide emergency supply of fluid in case of pump failure
• prolongs the period between cut out-cut-in of ACOV to reduce wear of the pump
• provide initial fluid when service is selected and pump is in Cut Out
• fitted with NRV to not allow fluid to return back to the reservoir

Question 45

2 types of accumulators

Answer 45

Separator/floating piston

flexible diaphragm

Question 46

The gas inside accumulator

Answer 46

Gas inside is Air or Nitrogen charged with predetermined pressure

**Gas pressure is usually greater than the pressure to operate any service
**
Pump is idle: when gas is compressed and hydraulic fluid and gas pressure equalize

Pump operational: when service is selected - fluid under pressure is available until pressure drops sufficiently to bring the pump on line.

fluid volume is sufficient to perate a serivce once

Question 47

Brake accumulators

Answer 47

permit a guaranteed number of brake applications (sufficient fluid to operate service multiple times) or stop aircraft during a rejected take off

Question 48

Incorrect pre-charge pressure of the main accumulator can cause

Answer 48

• the ACOV to cut in and out too frequently.
• This may cause rapid fluctuations of system pressure
• Can be felt and heard as ‘hammering’ in the system.

Question 49

Actuators

Answer 49

Physically attached to the service component that needs to be moved

convert fluid flow into linear motion
* Has piston
* Rod/Ram
* Seals

Three types:
Single:
powered in one direction by hydraulic power. The return movement is under string force. Used for locking device (ie landing gear downlock)

Balanced
* Piston moves in both direction
* The rod is on the sides of the piston/Ram head
* Equal force applied on both sides of the piston
* Either sides may be connected to a mechanism
* Use for N**ose wheel steering **& control boost systems

Double Unbalanced:
Piston moves in both directions
Only one piston attached to the RAM
used in most aircraft systems.
Offers more force/resistance to be applied to extensions (ie raising landing gear)
Area of the top of the piston in greater than underneath the piston
* used for rectractable landing gear

Question 50

Differential Areas.

Answer 50

It should be noted that the area of the upper side of the piston is greater than the area of the lower side by the amount equal to the area of the piston rod; therefore the force acting on it will be greater on the larger area.

Question 51

hydraulic lock

Answer 51

When fluid is trapped between the piston of the jack and a non-return/locked selector valve valve

Because the fluid is incompressible and is unable to flow through the system, the piston cannot move even if a load is applied to it and is therefore locked in its position.

Question 52

Hydraulic Motors

Answer 52

These are a form of rotary actuator= generate rotary motion

• needed to operate screw jacks to drive the FLAP slat mechanism

called axial piston motor

, and are sometimes connected through gearing to operate a screw jack, or to drive generators or pumps. In some aircraft they are used for driving a hydraulic pump unit, thus enabling power to be transferred from one hydraulic system to another without transferring fluid. The construction of a hydraulic motor is generally similar to the construction of a variable volume multi-piston pump. The speed of a hydraulic motor is dependent on the flow rate of oil into it.

Question 53

Hydrollic pack

Answer 53

Used for **very light aircraft **that maybe only need to retract/extend landing gear

powered by DC electric motor

Flow & valves complete the system

landing gear is in rectractable position by Hyraullic lock

thermal relief valve used to relieve pressure

pumps come on line when needed to restore system pressure

landing gear is lowered & locked mechanically.
At this point, electrical supply to the pump is switched off.

Question 54

shut off valve

Answer 54

**Shuts off services or hydraulic supplies to specific areas.
**
must be initiated by ‘external controller’
It’s not self regualting

ex: isolate fluid in the event of engine fire

Question 55

hydraulic fuese

Answer 55

shuts off flow in case of leak between upstream and downstream components. it avoids total fluid loss.

It is self regulating

if it sense excessive flow downstream, it will shut off and provide no operation there

Question 56

shuttle valve

Answer 56

allows 1 services to be operated by 2 supplies. if one fails, the other can replace.

ex: main & emergency system

main system pressure has higher pressure than emergency system pressure, therefore the valve moves and seals off the emergency supply. if main fails, the valve seals the main, and opes up the emergycy system.

used for switching emergency system of landing gear and flap lowering.

Question 57

priority valve

Answer 57

it ensures fluid is sufficiently supplied to main controls and brakes.

if system pressure drops, it closes and isolates non essentials circuits

Question 58

restricter/choke valve

Answer 58

reduces fluid flow to slow the operation of a service. (slowing down extension of a gear)

a one-way restrictor allows full flow to one direction and slows down the flow in another

Question 59

relief valve

Answer 59

releases excess pressure

when the ‘balls unseats’ due to high pressure, it allows the fluid to return to the reservoir and pressure is relieved. once relieved, the ball re-seats, consequenly closing the return to the reservoir