15.11 Fuel Systems

Question 1

What is the primary function of the engine fuel system?

Answer 1

To increase or decrease power to obtain the required thrust for any operating condition.

Question 2

What happens if the quantity of fuel becomes excessive?

Answer 2

It can exceed the limiting temperature of the turbine blades or cause a compressor stall.

Question 3

What is a rich blowout?

Answer 3

A condition where the mixture is cooled below combustion temperature by excess fuel, causing insufficient oxygen for combustion.

Question 4

What are the three categories of gas turbine engine fuel control systems?

Answer 4

• Hydromechanical
• Hydromechanical/electronic
• Full Authority Digital Engine Control (FADEC)

Question 5

What does FADEC stand for?

Answer 5

Full Authority Digital Engine Control.

Question 6

How does the fuel metering device in a FADEC system operate?

Answer 6

It meters fuel using only outputs from the Electronic Engine Control (EEC).

Question 7

What is the role of fuel nozzles in the fuel system?

Answer 7

To atomise the fuel for efficient ignition and combustion.

Question 8

What is the purpose of the hydromechanical fuel control assembly?

Answer 8

To provide the fuel metering function of the Electronic Fuel Control Unit (EFCU).

Question 9

True or False: Hydromechanical fuel controls are becoming more common in modern engines.

Answer 9

False.

Question 10

What is the function of the fuel shut-off section in the hydromechanical fuel control assembly?

Answer 10

To allow manual control of fuel flow to the engine.

Question 11

What does the EEC do in a FADEC system?

Answer 11

It calculates the amount of fuel flow by adjusting the fuel metering valve.

Question 12

What is the function of the vane fuel pump assembly?

Answer 12

To provide high-pressure fuel to the engine fuel control system.

Question 13

Fill in the blank: The fuel temperature sensor thermocouple sends information to the __________.

Answer 13

EEC

Question 14

What does the manual mode solenoid valve do in a hydromechanical/electronic fuel control system?

Answer 14

It enables the mechanical speed governor to function as an all-speed governor.

Question 15

What is the purpose of the flow divider and drain valve assembly?

Answer 15

To proportion fuel to the engine’s primary and secondary fuel nozzles and drain at engine shutdown.

Question 16

What is the function of the start fuel enrichment solenoid valve?

Answer 16

To provide additional fuel flow for engine cold starting or altitude restarts.

Question 17

What is the role of the torque motor in the fuel control system?

Answer 17

To increase or decrease fuel flow in the automatic mode.

Question 18

What type of memory does the EEC contain?

Answer 18

Both volatile and non-volatile memory.

Question 19

What does the EFCU control during normal engine operation?

Answer 19

Thrust setting, speed governing, acceleration, and deceleration.

Question 20

What is the advantage of FADEC systems over traditional controls?

Answer 20

FADEC systems provide more accurate scheduling of fuel by sensing many engine parameters.

Question 21

What does the crosstalk logic in the EEC do?

Answer 21

Compares data from channels A and B to determine which channel is best for controlling outputs.

Question 22

What is the significance of the fuel control run cut-off switch?

Answer 22

It controls the high-pressure fuel shut-off valve to start or stop fuel flow.

Question 23

What is the primary component of the FADEC engine fuel control system?

Answer 23

The Electronic Engine Control (EEC).

Question 24

What is the function of the minimum pressure and shut-off valve in the fuel metering unit?

Answer 24

To start or stop fuel flow based on signals from the EEC.

Question 25

Fill in the blank: The EEC receives electronic signals from the flight deck to set engine power or _______.

Answer 25

thrust

Question 26

What is the function of the off valve in the fuel metering unit?

Answer 26

To start or stop fuel flow ## Footnote The off valve is crucial for controlling the fuel supply to the engine.

Question 27

What types of sensors does the EEC use to receive position feedback for engine components?

Answer 27

Rotary differential transformer, linear variable differential transformer, thermocouples ## Footnote These sensors provide essential engine parameter information.

Question 28

What does the fuel control run cut-off switch control?

Answer 28

The high-pressure fuel shut-off valve ## Footnote This valve allows or cuts off fuel flow to the engine.

Question 29

Where is the fuel temperature sensor thermocouple located?

Answer 29

On the fuel outlet line at the rear of the fuel/oil cooler ## Footnote This sensor sends temperature information to the EEC.

Question 30

What are the two main connections shared by both channels of the EEC?

Answer 30

Programming plug, test connectors ## Footnote These connections are essential for EEC functionality.

Question 31

What is the purpose of the programming plug in the EEC?

Answer 31

To select the proper software for the thrust rating of the engine ## Footnote The programming plug ensures the EEC is configured correctly for specific engine performance.

Question 32

How is the programming plug attached to the engine?

Answer 32

With a lanyard ## Footnote This attachment method ensures the plug stays with the engine when the EEC is removed.

Question 33

What are the two subsystems of the engine fuel system?

Answer 33

* Fuel distribution subsystem * Fuel control subsystem

Question 34

What are the three main tasks of the engine fuel distribution system?

Answer 34

* Safely supply the fuel to the combustion chamber * Pressurise the fuel for vaporisation * Heat the fuel to prevent ice build-up

Question 35

What is the main purpose of the secondary fuel system?

Answer 35

To supply fuel to the engine combustion chamber and control the quantity of fuel for correct engine operation.

Question 36

What component feeds fuel into the main fuel supply line?

Answer 36

Low-pressure fuel shut-off valve

Question 37

What does the low-pressure fuel pump do?

Answer 37

Increases the fuel pressure that comes from the fuel tank boost pumps.

Question 38

What is the dual function of the oil cooler in the fuel system?

Answer 38

* Cools the engine oil * Heats the fuel to prevent ice build-up

Question 39

What is the role of the high-pressure fuel pump?

Answer 39

Increases fuel pressure to the level required for correct vaporisation in the combustion chamber.

Question 40

What does the fuel control unit do?

Answer 40

Meters the fuel needed for combustion and controls the supply and shut-off of fuel to the fuel nozzles.

Question 41

Fill in the blank: The _______ measures the actual fuel flow and transmits signals to the flight deck.

Answer 41

fuel flow transmitter

Question 42

Where are all components of the fuel distribution system located?

Answer 42

In the area of the accessory gearbox on the engine.

Question 43

What is the function of the pressure relief valve in the high-pressure fuel pump?

Answer 43

Protects components in the high-pressure fuel system against overpressure.

Question 44

True or False: The low-pressure fuel pump prevents cavitation at the inlet of the high-pressure fuel pump.

Answer 44

True

Question 45

What types of fuel filters are commonly used?

Answer 45

* Micron filter * Wafer screen filter * Plain screen mesh filter

Question 46

What is the purpose of the micron filter?

Answer 46

Removes foreign matter measuring from 10-25 microns.

Question 47

What is the main function of heat exchangers in gas turbine engine fuel systems?

Answer 47

To prevent the formation of ice in the fuel filters.

Question 48

What types of heat exchangers are used in fuel systems?

Answer 48

* Air-to-liquid exchanger * Liquid-to-liquid heat exchanger

Question 49

What is the primary function of fuel spray nozzles?

Answer 49

Inject fuel into the combustion area in a highly atomised spray pattern.

Question 50

What is a fuel manifold?

Answer 50

Distributes fuel to the individual fuel nozzles on the combustion chamber.

Question 51

What are the two types of fuel nozzles commonly used?

Answer 51

* Simplex * Duplex

Question 52

What is the disadvantage of the simplex nozzle?

Answer 52

Can only provide good atomisation in a small fuel flow range.

Question 53

What type of nozzle combines the functions of primary and secondary nozzles?

Answer 53

Duplex nozzle

Question 54

What is the main function of the fuel nozzle in terms of atomisation?

Answer 54

Change solid fuel flow into a mist of microscopic fuel droplets.

Question 55

What type of nozzles do modern engines typically use?

Answer 55

Duplex nozzle ## Footnote Duplex nozzles combine the functions of primary and secondary flow nozzles into one component.

Question 56

What are the two components of a dual-flow nozzle?

Answer 56

Two discharge orifices and two internal fuel supply lines ## Footnote The primary orifice is smaller and located at the center, while the larger secondary orifice is arranged around it.

Question 57

What controls the secondary fuel flow in a duplex nozzle?

Answer 57

A spring-loaded pressurising valve ## Footnote This valve is also known as the flow divider valve and is typically an internal component of the fuel nozzle.

Question 58

What happens to the flow divider valve as engine speed increases?

Answer 58

It opens ## Footnote The valve closes when the engine speed decreases.

Question 59

What role does airflow play in fuel nozzles?

Answer 59

It cools the nozzle and removes residual fuel ## Footnote Airflow enters the combustion chamber through slots and holes in the nozzle tip.

Question 60

What is the purpose of the check valve in the fuel nozzle?

Answer 60

To prevent fuel from entering the combustion chamber by gravity ## Footnote The check valve closes when fuel pressure in the fuel manifold decreases.

Question 61

How does the air-spray nozzle achieve atomisation?

Answer 61

By using a high velocity of air ## Footnote This method allows for good atomisation even at low fuel flows.

Question 62

What feature of the air-spray nozzle aids in fuel atomisation?

Answer 62

A large inner airflow that rotates after passing through the inner swirler ## Footnote This airflow meets the fuel at the nozzle tip, turning it into small droplets.

Question 63

What does the outer airflow do in an air-spray nozzle?

Answer 63

It passes through an outer swirler ## Footnote This airflow further enhances the atomisation of fuel.

Question 64

Is the air-spray nozzle a single-flow or dual-flow nozzle?

Answer 64

Single-flow nozzle ## Footnote It can supply sufficient fuel for all engine operating conditions.

Question 65

What prevents carbon formation at the tip of the air-spray nozzle?

Answer 65

Bypassing airflow ## Footnote Similar to the fuel spray nozzle, this airflow cools the nozzle.