CAP 5 hydraulic System

Question 1

Power transfer unit(PTU)

Answer 1

Power transfer unit ,transfers hydraulic power to other systems without mixing hydraulic fluids

Question 2

Type of actuators

Answer 2

> single acting,

> double acting balanced
Double acting unbalance

Question 3

Single acting actuator

Answer 3

Hydaulic pressure in one direction against a spring force

Question 4

Double acting balance actuator

Answer 4

Selectable hyd pressure in both directions but equal force

Question 5

Double acting unbalance

Answer 5

Equal hyd pressure in both directions but unequal force

Question 6

Non return valve

Answer 6

Allow fluid to flow only in one direction

Question 7

Selector valves?

Answer 7

Closed centre selector valves: constant pressure all the time
Open centre selector: pressure only when demand

Question 8

Open centre hydraulics circuit

Answer 8

Only one actuator is operated at a time

Question 9

Closed centre hydraulic sys

Answer 9

Services are supplied in parallel so they can be operated simultaneously.
And an off loading valve allows to unload the pump when is not used
Off load condition reduces engines power required

Question 10

Light a/c hydraulic sys?

Answer 10

May have only one active hyd for landing.
Uses a self contained hyd POWER PACK.
operated by a reversible DC eletric motor,which reverses the polarity so changing the flow direction

Question 11

Hydraulic reservoir

Answer 11

Hydraulic reservoir is pressurised to stop boiling.

“Stack pipe” are walls that divedes the main hydraulic sys with the emergency sys within the reservoir

A de-aeretor tray is fittide to remmove the air bubbles.

Relief valve protects the reservoir from over pressurisation.

Accumulator charge ,pressure and temperature affects the volume of fluid in the reservoir

Question 12

Accumulator

Answer 12

Pre charged with nitrogen
Act as a store of hydraulic fluid under pressure
Provides limited supply of pressure if the pump fails.
They are fitted downstream of the non return valve.
When pumps are running and the sys is operating ,gas pressure equals hyd pressure.

Dampens out pressure fluctuations.
Allow for thermal expansion.
Cater for small internal leaks

Question 13

Hydraulic pumps

Answer 13

Provive fuel flow and pressure

Primary pumps are driven by engine driven pumps

Alternatively Electrically driven
Hand operated
Driven by ram air turbine
Driven by hydraulic motor (power transfer unit PTU)

Question 14

Two hydraulic pumps

Answer 14

~constant delivery (fixed volume) pump
Are spur and rotary pumps often found in light a/c

~Constant pressure (variable volume) pump
Piston pumps more commonly used in larger a/c

Unless is small a/c electrically pump normally used as emergency back up.Hand pumps tends to be used for maintenance purporses

Question 15

Fixed volume

Answer 15

Fixed volume -constant fluid supply
Flow rate dependant on speed of rotation
Gears normally rotate at constant speed hence pumps emits a constant volume of fluid
Normally requires an automatic cut out valve(ACOV)

Question 16

ACOV(automatic cut out valve)

Answer 16

Are fitted in conjunction with a fixed volume pump (so after the pump) to regulate the pressure.
When pressure increases above a pre set level a poppet valve opens to create an idling circuit.
System leaks will cause the acov to operate excessively

Question 17

Constant pressure (variable volume)

Answer 17

Variable volume pump does NOT required and ACOV.
Used in large a/c with sys at 3000psi or more

Output volume NOT dependant upon pump RPM
Volume can be altered
Self regulates and only supplies fluid needed to maintain constant working pressure.
Engine driven
On start up pump swash plate is at max deflection.
“Case drain” allows to cool down at minimu necessery when the hydraulic is not used for long time and the pump is rotating

Question 18

Hydraulic motor

Answer 18

Uses hyd pressure to generate rotatory motion.
Hyd pressure forces down the pistons in the cylinder block.
Because of the angle in the swash plate this translates linear piston movement into rotatory movement.
~hyd motors are useful for operating screw jacks to drive the flap and slat mechanism
~the speed of the motor is dependant on fluid flow rate
~a selctor valve can charge direction of rotation

Also known as an axial psiton motor

Question 19

Hyd fiilters

Answer 19

Filtration and hyd circuit protection
Keeps the fluid free of contamination
Normally fitted downstream of the hydraulic pump

If clogged a differential pressure sensor operates a warning light in the cockpit.or a red filter pop out button will pop out
A bypass valve automatically allows unfiltered fluid directly through the head

Question 20

Pressure relief valve

Answer 20

Protects from over pressure.
Pressure acts against the spring causing the ball to open.

FFRV(full flow relief valve, can relieve the equivalent of total pump output.
Thermal relief valve (TRV) dumping excess heat thermal expansion

Question 21

Restrictor valve(chocke valve)

Answer 21

Reduces flow to slow the speed of operation of services in one direction the other direction is unaffected.
Usually used in landing gear extension

Question 22

Priority valve (pressure maintaining valve)

Answer 22

Is also called pressure maintaining valve
Ensure sufficient fluid at all times to the primary controls and wheel brakes.

It closes and isolates non essential circuit if sys pressure drops below set limit

Question 23

Shuttle valve

Answer 23

Allows one service to be operated by two independent supplies

In the exams is describes as&raquo_space;Allows the most appropriate pressure

Question 24

Hydraulic fuse

Answer 24

Upstream of components to shut off flow and prevent total fluid loss in the event of a major leak.

Question 25

Shut off valve

Answer 25

Similar to hyd fuses but is operated manually by pilot.
Might be used to stop hyd fluid flow to a service in the event of a fire or to isolate a hyd service during service /maintenance

Question 26

Larger a/c hyd sys

Answer 26

Use a closed centre system in which full sys pressure is always available.
~multiple different circuits
~separate EDP’s supply each system
~ram air turbine provides emergency power after pump failure
~ptu are fitted to cater for pressure loss in one or other sys

Answer 27

Control panel will have the hydraulic pump switches and overheat warning lights

Question 28

Emergency and auxiliary power sources

Answer 28

~hand pump
~RAT> used to drive an auxiliary hyd pump or electrical generator.blade pitch can be adjusted to control pump rpm
~PTU>uses a hyd motor to drive a hyd pump on another sys
~Air turbine motors > high pressure bleed air from an engine powers an air turbine which in turn powers an hyd pump

Question 29

Monitored parameters of fluid sys?

Answer 29

Temp
Pressure
Quantity

Question 30

The hyd reservoir

Answer 30

Is pressurised with pneumatic system