AGK Engines - Gas Turbine Fuel System

Question 1

Role of the fuel system is to…

Answer 1

Provide correct quantity of fuel at correct pressure under all conditions

~Alt
~Airspeed
~Air temp
~Engine temp
~Engine speed
~Thrust settings

Everything is automatic, apart from thrust

Question 2

What is fuel delivery dependant on?

Answer 2

Core Rotation Speed (N2)

Your looking at N1, but the fuel system is looking at N2 so it can produce the power you want the N1 to be at

Question 3

How is fuel delivered onto the engine?

Answer 3

Boost pumps in the main tanks
~30-50psi fuel pressure in the LP (Booster) system
~50psi in central tank so it drains first
~115V AC single phase electric pumps

Question 4

Where does the fuel go before it reaches the HP system, ready for metering

Answer 4

Goes through a fuel heater and filter

Question 5

Three different types of HP systems include…

Answer 5

Hydromechanical Unit - uses Fuel Control Unit (FCU) to regulate fuel to the fuel nozzles

Hydromechanical Mechanical Engine Controller (MEC) - an electronic version of FCU

FADEC - uses Electronic Engine Controller (EEC) and Fuel Metering Unit (FMU)

Hydromechanical systems are considered obsolete in modern engines

Question 6

Hydromechancial (FCU)

Answer 6

Utilises a swashplate pump as a HP source and as speed changes, swash angle changes which causes varied fuel flow

Air/Fuel ratio kept correct to keep engine working in line

As pilots demand more thrust, a governor within FCU commanded more fuel to be injected

Subsequently EGT increases which is sensed by FCU in a closed loop control system

Engine speed would increase to reflect changes

Question 7

What does the FCU sense?

Answer 7

Thrust lever angle
Air temp and pressure
Rapid acceleration changes
Engine speed
EGT
Compressor delivery pressure (P3)

Question 8

Power Management Control System

Answer 8

Control a multi spool engine where FCU system can only efficiently deal with a single spool engine

Allows fine control of N2 in order to meet thrust demands and N1 rotation

Some systems refer to Mechanical Engine Control Unit (MECU) the systems are broadly the same

Sensing is achieved using signals converted in voltages

Question 9

FADEC stands for

Answer 9

The Full Authority Digital Engine Control

Question 10

What does FADEC compromise of?

Answer 10

Electronic Engine Controller (EEC)

1 FADEC system per engine

2 Channels (2 Separate Computers) (changes channel on each start)
~Each with their own power supply
~Both channels check input/output
signals for validity and errors

Question 12

Is FADEC highly redundant?

Answer 12

Allows a high degree of reducnacy as the likelihood of total failure is remote

Question 12

When signals are processed in FADEC where are they delivered to?

Answer 12

Fuel Metering Unit (FMU) situated with the HP fuel system

Question 13

FADEC performs the following…

Answer 13

Delivery of fuel to the Engine Fuel Nozzles

Maintain fuel pressure under all engine RPM’s

Shut-off fuel when commanded

Engine Overspeed Protection

Controlling Compressor Airflow (Bleed valves)

Reverse Thrust

Active Clearance Control (ACC)

Ensuring Manifold Fuel draining during shut-down

Question 14

FADEC based fuel system diagram

Answer 14

Question 15

FADEC reduced functionality

Answer 15

Single channel - no loss of function (uses other channel)

Dual channel - Loss of engine control, but last demanded engine setting is still available (EPR or N1)
~Loss of associated engine instruments

Power supply loss - FADEC has a dedicated alternator, failure will lead to duel channel failure

Signal loss - loss of an input signal may nor have a huge impact but signal losses will cause associated displayed parameters to be lost

Question 16

Jet fuel

Answer 16

Jet Kerosene or AVTUR (Aviation Turbine Fuel)

Specific gravity of 0.62 or 0.88

Most common : JetA , JetA-1(uses additives)

JetA - used in US
JetA-1 - used in the rest of the world

~Both have a flashpoint not less than +38°C
~JetA has a waxing point of -40°C
~JetA-1 has a waxing point of -47°C

Question 17

What does Jet fuel contain?

Answer 17

Fuel System Icing Inhibitor (FSII)

Anti Static additives

Oxidation and corrosion inhibitors

Question 18

Jet B uses and characteristics

Answer 18

Used in cold climates

JetB or AVTAG (Aviation Turbine Gasoline) is known as a wide cut fuel: this means its a blend
~70% Gasoline
~30% Kerosene
~Low flashpoint and high volatility

JetB banned under EU-OPS for commercial operations due to safety and environmental concerns

Question 19

Fuel Waxing

Answer 19

Exremtly low temps, kerosene begins to change state (wax)

Fully waxed, it resembles Vaseline and will clog components

Therefore crucial to maintain fuel temps in extended cruises

Question 20

How to prevent waxing

Answer 20

Recirculating fuel, cooling the IDG and placing hydraulic lines within fuel tanks are methods of heating fuel and maintaining suitable temps

Fuel temp can be monitored don the flight deck and raise warnings/cautions when necessary

Additionally, water present in the fuel will form slush way above the waxing point
~Fuel heater is now placed before the
filters to prevent fuel starvation