15.12 Air Systems

Question 1

What is the purpose of engine air distribution?

Answer 1

To provide cooling to internal combustion engines due to intense heat generated during fuel and air combustion.

Cooling is essential to maintain safe operating temperatures and prevent engine damage.

Question 2

How is engine air distributed in gas turbine engines?

Answer 2

Through zone cooling, where large volumes of air are passed through the engine to maintain acceptable temperature ranges.

The ideal air/fuel ratio is 15:1; however, more air is necessary to cool the engine.

Question 3

What temperature range is maintained by cooling air in gas turbine engines?

Answer 3

800 to 1150 °C

This range is critical for safe engine operation.

Question 4

Where is the hottest section of a gas turbine engine located?

Answer 4

Around the turbines.

Despite cooling, high temperatures are still present due to heat conductivity.

Question 5

What is the function of secondary air in a gas turbine engine?

Answer 5

To cool the combustion chamber liners.

Air induces a thin, fast-moving film over the liner surfaces.

Question 6

What type of burners helps promote high combustion efficiency in gas turbine engines?

Answer 6

Annular-type burners with a centre tube.

These burners facilitate rapid dilution of hot combustion gases.

Question 7

True or False: Secondary airflow for cooling is provided by bleeding air from the later stages of the compressor.

Answer 7

False

Air is bled from the early stages of the compressor.

Question 8

What critical lubricating point in a gas turbine engine requires special cooling?

Answer 8

Exhaust turbine bearing.

High temperatures necessitate additional cooling methods.

Question 9

What is the role of cooling air inlets in a gas turbine engine?

Answer 9

To allow air to cool the turbine case, bearings, and turbine nozzle.

These inlets help manage engine temperatures effectively.

Question 10

Fill in the blank: The engine compartment is divided into a cold section at the front and a _______ section at the back.

Answer 10

hot

The hot section is where the turbine is located.

Question 11

How are reciprocating engines cooled?

Answer 11

By passing air over fins or using liquid coolant through jackets surrounding the cylinders.

This method is effective due to the intermittent nature of combustion.

Question 12

What are the primary zones in turbine engines that isolate each other?

Answer 12

Fan case compartment, intermediate compressor case compartment, core engine compartment.

These zones help manage temperatures and prevent harmful vapour build-up.

Question 13

Describe the cooling mechanism in Zone 1 of a turbine engine.

Answer 13

Vented using ram air through an inlet tube and exhausted through a vented panel.

A pressure relief door opens if pressure exceeds preset limits.

Question 14

How is Zone 2 of a turbine engine cooled?

Answer 14

By fan air from the upper part of the fan duct, exhausted back into the fan air stream.

This zone contains fuel and oil lines to remove vapours.

Question 15

What is the airflow process in Zone 3 of a turbine engine?

Answer 15

Air enters from the exhaust of the pre-cooler and is exhausted through the aft edge of the thrust reverser inner wall.

This zone also contains fuel and oil lines.

Question 16

What is required for proper cooling of each section of a turbine engine?

Answer 16

An intricate internal air system that routes ram and/or bleed air to various components.

Effective cooling is essential for the compressor, combustion, and turbine sections.

Question 17

What is the purpose of the anti-ice system in turbine-engined aircraft?

Answer 17

To prevent ice formation in the engine inlet duct and inlet guide vanes.

Question 18

What can happen if ice builds up in the engine inlet duct?

Answer 18

It can disrupt airflow into the compressor, reduce engine efficiency, and cause damage to compressor blades.

Question 19

At what temperature can ice form in the engine inlet during high-speed operation on the ground?

Answer 19

Up to 5 °C.

Question 20

How is the anti-icing system typically controlled during flight?

Answer 20

Automatically by a switch on the flight deck.

Question 21

What triggers the operation of the anti-icing system?

Answer 21

A temperature sensor sensing a freezing condition.

Question 22

What happens when the anti-icing system is activated?

Answer 22

A bleed valve opens, allowing hot air from the compressor to flow to the inlet leading edge, nose dome, and inlet guide vanes.

Question 23

Where does the heated air exit when the anti-icing system is on?

Answer 23

Through a grill on the nose cowl.

Question 24

Can flight crew manually control the engine anti-ice function?

Answer 24

Yes, they can manually select the engine anti-ice function if required.

Question 25

What indicates that the anti-icing valve is open?

Answer 25

An indication can be viewed on the engine parameters page.

Question 26

Fill in the blank: The anti-icing system is designed to prevent ice from disrupting airflow into the _______.

Answer 26

[compressor]

Question 27

What is the purpose of the air system in internal cooling?

Answer 27

To cool various components of the engine.

Question 28

Name the subsystems of internal engine cooling.

Answer 28

* Compressor cooling * Combustion chamber cooling * Turbine disc cooling * Turbine Nozzle Guide Vanes (NGV) and blade cooling * Accessory section

Question 29

What is the temperature range at the rear stages of the compressor?

Answer 29

Approximately 350 °C to 400 °C.

Question 30

How is cooling achieved in the compressor?

Answer 30

By providing cooling air from an intermediate compressor stage.

Question 31

What is the structure of the combustion section?

Answer 31

An outer casing with a perforated inner liner.

Question 32

What are the two airflow paths in the combustion chamber?

Answer 32

* Primary airflow * Secondary airflow

Question 33

What percentage of incoming air is directed as primary airflow in the combustor?

Answer 33

Approximately 20% - 30%.

Question 34

What is the velocity of the secondary airflow entering the combustor?

Answer 34

Up to several hundred metres per second.

Question 35

What two ways does cooling occur in the combustion chamber?

Answer 35

* Cooling air removes heat from the combustion liner * Cooling air creates a film acting as a thermal barrier

Question 36

What is a major limiting factor of a gas turbine engine?

Answer 36

The temperature of the turbine section.

Question 37

How does turbine cooling affect engine performance?

Answer 37

It allows turbine vanes and blades to operate at higher temperatures.

Question 38

What is the common method of cooling components in the turbine section?

Answer 38

Using engine bleed air.

Question 39

How are turbine discs cooled?

Answer 39

By directing cooling air over each side of the disc.

Question 40

What is the typical temperature range for turbine blades and discs?

Answer 40

300 °C to 400 °C above the limits of their metal alloys.

Question 41

How are Nozzle Guide Vanes (NGV) cooled?

Answer 41

By compressor discharge air flowing into a cavity from above and below.

Question 42

What is a challenge in cooling turbine blades compared to guide vanes?

Answer 42

Air is supplied through drilled holes on the disc front surface.

Question 43

What is the cooling method for engine accessories and electrical generators?

Answer 43

Air is ducted from intakes in the fan cowlings or from a specific stage of the compressor.

Question 44

What is used for oil cooling in generators?

Answer 44

An air-cooled oil cooler.

Question 45

How are accessory units cooled during flight?

Answer 45

By atmospheric air.

Question 46

What is a solution for cooling accessory units during ground running?

Answer 46

Allowing compressor air to pass through nozzles in the cooling air outlet duct.

Question 47

How is air used for internal sealing?

Answer 47

Pressurised air acts on the air seals and oil seals to prevent oil from escaping from the bearing chamber. ## Footnote The process involves using bleed air from the LP compressor.

Question 48

What is the purpose of the LP compressor bleed air?

Answer 48

It is used for the sealing of the engine bearing chambers. ## Footnote This helps maintain the integrity of the bearing compartments.

Question 49

What additional effect does pressurised air have in sealing?

Answer 49

It prevents oil from escaping out of the bearing chamber. ## Footnote This is achieved by acting on both air seals and oil seals.

Question 50

Where does the air that enters the bearing compartments vent?

Answer 50

It vents overboard through the hollow N1 rotor shaft. ## Footnote This is part of the typical sealing airflow process.

Question 51

Fill in the blank: The air seals and oil seals in the bearing chamber are acted upon by _______.

Answer 51

pressurised air ## Footnote This sealing mechanism is crucial for maintaining functionality.

Question 52

What is the secondary function of the compressor in an aircraft engine?

Answer 52

To supply air to the various systems on the aircraft.

Question 53

Where can bleed air be tapped off in the compressor?

Answer 53

At any of the pressure stages of the compressor.

Question 54

What determines the exact location of bleed ports in the compressor?

Answer 54

The pressure or temperature required for a particular system.

Question 55

From which stage is air often bled due to maximum pressure and temperature?

Answer 55

The final or highest pressure stage.

Question 56

What must be done to high-pressure air used for cabin pressurisation?

Answer 56

It may need to be cooled.

Question 57

What system is used to cool high-pressure bleed air before entering the cabin?

Answer 57

An air conditioning unit.

Question 58

List some external air services provided by engine bleed air.

Answer 58

* Cabin pressurisation, heating, and cooling * De-icing and anti-icing equipment * Pneumatic starting of engines * Auxiliary Drive Units (ADU)

Question 59

What is the purpose of the external cooling system in an aircraft engine?

Answer 59

To cool and ventilate the external areas between the cowlings and the engine cases.

Question 60

How are the areas below the engine cowlings divided?

Answer 60

Into two compartments: the fan compartment and the core compartment.

Question 61

What does the external cooling system ensure regarding engine components?

Answer 61

That they are sufficiently cooled.

Question 62

How does the external cooling system prevent flammable vapours from collecting?

Answer 62

By separating fan compartments and core compartments with bulkheads and fire seals.

Question 63

What type of air intakes are used to cool the fan compartment?

Answer 63

National Advisory Committee for Aeronautics (NACA) air intakes.

Question 64

What is the role of ram air in the cooling process?

Answer 64

It cools and ventilates the fan compartment and discharges overboard.

Question 65

How is the core compartment typically cooled and ventilated?

Answer 65

By fan air picked up from the fan discharge duct.

Question 66

When does positive ventilation of the core compartment occur?

Answer 66

When the engine operates.

Question 67

What condition can occur after an engine shutdown due to different expansions?

Answer 67

Bowed rotor.

Question 68

What can a bowed rotor condition lead to during engine start-up?

Answer 68

High vibration indications or hung start.

Question 69

What do some engine manufacturers recommend after engine shutdown?

Answer 69

A minimum waiting time before the subsequent engine start.

Question 70

How does the engine cool after shutdown?

Answer 70

By convection.

Question 71

What happens to the warm air in the engine compartments after shutdown?

Answer 71

It leaves via air outlets and draws in cold ambient air.

Question 72

What critical situation can arise due to temperature differences in the engine after shutdown?

Answer 72

The lower part of the engine becomes colder than the upper part.