Chapter 1 - Aircraft Hydraulic System

Question 1

What do hydraulic systems provide?

Answer 1

A means of transmitting, and often multiplying, a force by the use of fluids.

Involve the generation, modulation and control of pressure and flow of the fluid to provide a convenient means of transmitting power for the operation of a wide range of aircraft services.

Question 2

What are the advantages of hydraulic power?

Answer 2

Capable of transmitting very high forces

Rapid and precise response to input signals:

• Good power to weight ratio
• Simple and reliable
• Not affected by electromagnetic interference

Question 3

What does Pascal’s law/principle state?

Answer 3

For an incompressible fluid at rest, the difference in pressure is proportional to the difference in height and this remains he same whether or not the overall pressure of the fluid is changes by applying an external force.

I.e. by increasing the pressure at any point in a confined fluid, there is an EQUAL pressure increase at every other point in the container.

Question 4

What is the formula for pressure produces by a fluid contained in a cylinder with force applied by a piston and what does this equation imply?

Answer 4

P = F/A

Therefore, F = P x A

Force applied to a small piston will produce a much greater force at a larger piston in the same system.

Big piston would move less (ratio of the sizes of the piston areas, e.g. 1:4, but would exert a greater force during this movement).

This is the principle of a hydraulic lever, which is the principle of any hydraulic system.

Question 5

What is hydraulic fluid?

Answer 5

INCOMPRESSIBLE oil

Question 6

As well as being incompressible, what characteristics are required of aircraft hydraulic oil?

Answer 6

Low flammability

Boiling and freezing points outside operating parameters

Question 7

What hydraulic oil is used by the Grob 120TP Prefect?

Answer 7

Oil Mineral (OM) 15 (Aeroshell 41)

Question 8

What factors can influence/limit the effectiveness of a hydraulic system?

Answer 8

Temperature and aeration - Fluid vapour and absorbed air given off and carried in the liquid as it approaches its boiling point, introduces an unacceptable degree of compressibility

Contamination - Must filter for particles that could cause mechanical damage

Flammability

Hazardous liquids - Hydraulic fluids are active solvents and many are also corrosive, need careful handling during maintenance

Temperature and Aeration, Contamination, Flammability, Hazardous Liquids

Question 9

What are the components found in a hydraulic system?

Answer 9

Reservoir

Filters

Pump

Accumulator

Valves - Pressure control valves, Direction control valves, NRVs, Selector valve

Actuator - Converts the hydraulic power into usable mechanical power

Question 10

What are the majority of engine or motor driven pumps in a hydraulic system?

Answer 10

Rotary Swash Plate

Question 11

What is the purpose of an accumulator?

Answer 11

Absorb shocks and sudden changes in system pressure

Smooths out pressure ripples and acts to maintain pressure (e.g. parking brake) + absorb any spikes in hydraulic pressure

Question 12

What is a typical accumulator filled with?

Answer 12

Nitrogen - Due to its compressibility (high or low? Check notes)

Question 13

What type of valves are found in a hydraulic system?

Answer 13

Control valves:
Pressure and Thermal relief valves

Non-return valves

Question 14

When are selector valves used in a hydraulic system + what is the most common type?

Answer 14

In areas where it is necessary to vary direction of hydraulic flow to a component, e.g. raising and lowering control surfaces

Rotary selector valve

Question 15

Where are hydraulic systems typically used on aircraft?

Answer 15

Powered Flying Control Unit (control surfaces)

Landing gear extension/retraction

Hydraulic braking

Question 16

How long must back-up hydraulic systems work if used to power the movement of control surfaces?

Answer 16

Indefinitely

Question 17

How long must back-up hydraulic systems work if used to power undercarriages or brakes?

Answer 17

Enough for one landing

Question 18

What system safety features are used in hydraulic systems?

Answer 18

System redundancy (e.g. control surfaces will either have an option to revert to manual control of a second, independent hydraulic system)

Emergency power - Backup hydraulic pumps are driven by separate electrical systems, e.g. RAT, EPU or secondary power system.

Blow Down system

Question 19

What is a Blow Down system?

Answer 19

Energy is stored in accumulators, or ‘blow down’ Nitrogen cylinders (nitrogen used to reduce the risk of a hydraulic fire).

One use - Releases high pressure air which pressurises the system sufficiently for the undercarriage to be lowered