Hydraulic Systems

Question 1

Why are there two separate hydraulic systems

Answer 1

Redundancy. Can still provide hydraulic power to the most essential components

Question 2

How are pumps driven

Answer 2

Engine driven pump are driven by a gearbox.

Gas turbines are driven by accessory gearbox.

Question 3

What are actuators and what are the 3 types

Answer 3

Convert hydraulic pressure into movement.

Single-acting, Double-acting balanced, Double-acting unbalanced

Question 4

What are single-acting actuators used for?

Answer 4

Move only in one direction. Spring applies return force. Normally used as a locking device eg landing gear.

Question 5

What are Double-acting Balanced actuators?

Answer 5

Hydraulic pressure moves pistons in both directions. Pressure gives equal force. Used for nose steering wheel and flight control systems.

Question 6

What are double acting unbalanced actuators?

Answer 6

Equal hydraulic pressure generates different forces on each side of piston area due to different surface area on piston face. Actuator moves left. Allows for different forces to be produced for example on extending/retracting. landing gear

Question 7

What are non-return valves.

Answer 7

Allows fluid to flow only in one direction. Fluid pressure unseats ball under spring pressure to allow flow. When system pressure falls below spring pressure ball reseats preventing reverse flow.

Question 8

What are selector valves

Answer 8

Direct hydraulic fluid to various parts of system. Can be operated mechanically or electrically. Can be used to hydraulically lock position of actuator.

Question 9

What are the types of hydraulic circuit

Answer 9

Passive hydraulic system-No pump, power provided by Pilot

Active hydraulic system- Uses one or more pumps

Question 10

What is an open-centre hydraulic system

Answer 10

2000psi. Used on small-midsized aircraft. Operates landing gear or flaps but not at same time. When one is used other is locked.

Question 11

What is an on-demand system

Answer 11

Pressure only generated when a service is required. System pressure remains low until service selection made. Off load condition off-loads pump reducing engine power required to drive it and also reducing wear on pump

Question 12

What is a closed centre Hydraulic system

Answer 12

Maintains a constant supply of pressure using a variable pump. Services can be operated simultaneously.. Off-loading valve allows pump to unload when not in use.

Question 13

What are light aircraft systems

Answer 13

Often only one active hydraulic circuit used to extend/retract Landing gear. Uses self-contained hydraulic power pack operated by reversible DC electric motor.

Question 14

What are the main points of a light aircrafts hydraulic system

Answer 14

Landing gear held in retracted position by hydraulic lock.
Pump comes online when required to restore system pressure.
Thermal relief Valve protects against over pressure.
When landing gear lowered mechanically locked and electric supply to pump switched off.

Question 15

What is the hydraulic reservoir in large aircraft system

Answer 15

Stores H.fluid. Allows thermal expansion, holds excess fluid to compensate for leaks, accepts fluid returning from low pressure side.
Lower exit inside to ensure emergency supply if major fluid loss

Question 16

What do fins, baffles and de-aerator tray do

Answer 16

Reduce fluid swirl due to aircraft manoeuvring.

Removes bubbles from returning fluid.

Question 17

Why are hydraulic reservoirs pressurised

Answer 17

Prevents fluid from boiling at altitude. Pressurised by bleed air from engine. Relief valve protects from over pressurisation.

Question 18

What are accumulators

Answer 18

Comprises piston moving freely inside sealed cylinder. On one side pre-charged inert gas (nitrogen) other hydraulic fluid.

Question 19

What is the purpose of an accumulator?

Answer 19

Store fluid under pressure.
Provide limited supply of pressurised fluid in an emergency.
Dampen pressure fluctuations in system.
Allow for thermal expansion and small internal leaks.

Question 20

What are the two types of engine driven pumps

Answer 20

Engine driven constant delivery (fixed volume)
Constant pressure (variable volume)

Question 21

How do constant delivery pumps work

Answer 21

Constant fluid supply. Flow rate dependent on speed of rotation. Spurs rotate at constant speed the pump emits a constant volume of fluid.
This pump requires a automatic cut out valve.

Question 22

What is an ACOV

Answer 22

When system pressure increases above pre-set level poppet valve opens creating an idling circuit which dumps excess pressure. Valve open= ACOV “kicked out”
System leaks will cause ACOV to operate for greater periods.

Question 23

What is a constant pressure (variable volume) pumps

Answer 23

Commonly used on large modern aircraft. Operate 3000psi+ Demand pump which delivers hydraulic pressure on demand.

Question 24

How does constant pressure pumps work

Answer 24

Driven by gearbox connected to engine. Series of pistons in sleeves with each piston end in a sliding shoe which presses against swash plate. If swash plate at slight angle stroke of piston is minimal resulting in an idling state in which pump produces no output.

Question 25

How does a hydraulic motor work

Answer 25

Uses hydraulic pressure to generate rotary motion. Useful for operating screw jacks that operate flaps and slat mechanisms.