FUEL SYSTEMS

Question 1

Where is the fuel control unit of an engine usually mounted?

Answer 1

To the engine fuel pump on the accessory gearbox.

On turbofan engines, usually hydromechanical component

Question 2

In a hydromechanical fuel control unit what are all control functions done by?

Answer 2

Fluid pressures and mechanical components.

Question 3

What are the 2 different types of fuel control unit?

Answer 3

Speed governed fuel control unit (older engines)

Constant thrust fuel control unit (modern engines)

Question 4

What do speed governed fuel control units mainly use for steady state fuel metering?

Answer 4

N2 demand signal and N2 speed feedback signal.

Constant thrust fuel control, additional thrust feedback signals such as N1 speed, ambient pressure and temp signals

Question 5

What do temperature signals like CIT or fan inlet temperature convert the air temperatures to in a fuel control unit?

Answer 5

Fuel pressure signals.

Question 6

What do hydromechanical fuel control units use for operation?

Answer 6

Hydraulically, Pneumatically and mechanically

Question 7

What 2 parts can the fuel control unit be split into?

Answer 7

Fuel metering section

Computing section

Question 8

What can the computing section of the fuel control system be further divided into?

Answer 8

Governing section

Limiting section

Question 9

What does the fuel metering system do in a fuel control unit?

Answer 9

Ensures necessary fuel gets to the fuel nozzles and all fuel not needed for combustion returns to the fuel pump.

Question 10

What does the governing section of the fuel control unit do?

Answer 10

Makes sure that the selected power is controlled.

Question 11

What is the limiting section for in a fuel control unit?

Answer 11

ensures the engine always operates within safe limits.

Question 12

What does FADEC stand for?

Answer 12

Full Authority Digital Engine Control.

Question 13

What is the digital computer in FADEC system normally called?

Answer 13

ECU or EEC.

Question 14

What is a FADEC made up of?

Answer 14

EEC or ECU

FMU (fuel metering unit)

Question 15

Where are the electronic control units and FMU located?

Answer 15

Fan stator case of the engine.

Question 16

What can a small permanent magnet alternator on the accessory gearbox provide for FADEC?

Answer 16

A power supply.

Question 17

What are engine interface units (EIU) for?

Answer 17

Transmit the data between the FADEC system and the aircraft.

Control power supply from aircraft to ECU.

Question 18

What can the engine fuel system be separated into?

Answer 18

• fuel distribution subsystem

- fuel control system

Question 19

What 3 main tasks does the fuel distribution system have?

Answer 19

• safely supply fuel to combustor
• pressurise fuel
• heat the fuel

Question 20

What does the low pressure fuel pump do?

Answer 20

Increases the pressure that comes from the tank boost pumps

Question 21

What two functions does the oil cooler have?

Answer 21

Cold fuel cools the engine lubrication system

Puts fuel above freezing so it doesn’t block the fuel filter

Question 22

Where is the filter located in comparison to the oil cooler?

Answer 22

Downstream.

Ensures that ice cannot block the filter as fuel is warmed above freezing point

Question 23

What is a high pressure fuel pump?

Answer 23

Increases fuel pressure so it’s able to vaporise in the combustion chamber

Question 24

What is the purpose of the fuel control unit?

Answer 24

Meters the fuel which is needed for combustion. Also responsible for supply and shut off.

Question 25

The fuel control unit needs high pressure fuel to be used as servo pressure for internal mechanisms. To avoid ice build up, what can be added?

Answer 25

An additional servo fuel heater.

Question 26

What does a fuel flow transmitter measure?

Answer 26

Measure the actual flow of fuel and transmits to cockpit

Question 27

What is the fuel manifold?

Answer 27

Distributes fuel to individual fuel nozzles on the combustion chamber.

Question 28

What is the advantage of having an oil cooler downstream of the high pressure pump?

Answer 28

Requires less external fuel lines because the low and high pressure pumps are in one housing but the Danger of fuel leaks is greater in a low pressure system

Question 29

What PSI does the low pressure fuel pump increase the fuel pressure to before supplying fuel to the high pressure stage?

Answer 29

175psi

Question 30

Why is it necessary for the low pressure fuel pump to pressurise before the high pressure stage?

Answer 30

So the HP does not have to draw via suction and prevents cavitation at the inlet.

Question 31

At max engine speed, what does the high pressure pump increase the fuel pressure to?

Answer 31

900psi

Question 32

On the high pressure pump, when will the overpressure relief open?

Answer 32

When the system reaches 1250psi | Some fuel is released back to the inlet port of the high pressure pump

Question 33

How much fuel will the pump supply in comparison to the amount that is needed?

Answer 33

Always supplies more than needed. | Fuel that isn’t needed returns via the bypass return line

Question 34

Fuel that is passed down the bypass line can be used to cool what?

Answer 34

The IDG oil cooler

Question 35

In low engine power conditions, bypass fuel is returned to the tank. How is it ensured that fuel is not too hot when re-entering?

Answer 35

Cold fuel from the LP stage is added to the hot bypass fuel