15.11 Flashcards
The fuel quantity changes with what?
Ambient pressure and temperature
What can happen if the quantity of the fuel is excessive?
Rich blow out (turbine temp is exceeded) and stalling.
When does a rich blow out happen? (2 things)
The amount of oxygen in the air supply isn’t good enough.
Or if the mixture is cooled below the temperature for combustion by excessive fuel.
When must energy/fuel be transferred to the turbine more than necessary?
When rapid acceleration occurs
When does an over rich condition occur?
If there is too much fuel
What can an over rich condition cause? (2 things)
A rich blow out
Compressor stall
What is the hydromechanical/electronic fuel control?
A hybrid of the two fuel systems
What does the dual mode in hydromechanical/electronic allow?
The inputs and outputs to be electronic
What is the best affect if the hydromechanical/electronic system fails electronically?
It still has mechanical linkages
What allows the inputs and outputs in a hydromechanical/electronic fuel system to control fuel rate?
Servo valves
Even though hydromechanical/electronic uses servo to control the inputs, what does FADEC use fir the same function?
Electronic sensors
What do the inputs from the sensor of the computing sections of FADEC rely on?
ECC which metres the fuel flow
Despite the different types of fuel control systems, what is the overall goal for the fuel systems?
To have enough fuel for the power of the engine
What are the two sections of el control?
Metering and computing
What fuel system has no electronic interface?
Mechanical
What is the hydromechanical fuel control system controlled by?
The gas generator gear train (Gearbox as its the only thing in the engine that will drive the mechanical pumps)
Disadvantage of hydromechanical fuel control system?
It Is less accurate
What directs the fuel flow in a hydromechanical/electronic fuel system?
The EEC
Where is the ECC located in the hydromechanical/electronic fuel system?
Remote of the engine
3 functions of the engine fuel systems?
To pressurise,
metre fuel flow
Send atomised fuel to the combustion section of the engine (for combustion and cooling)
What provides high pressure fuel to the fuel control system?
The vane fuel pump
What type of pump is the vane fuel pump?
Fixed displacement
What does the filter bypass valve allow for in the fuel pump?
Fuel to bypass the filter if it’s blocked (Pressure drops)
What two things happen when a blockage occurs?
Differential pressure indicator says theres a block
A pin extends from the fuel filter bowl
In the electrical aspect of the hydromechanical/electronic fuel system. what does the EEC consist of?
PLA Potentiometre
Where is the PLA potentionmetre mounted?
On the aircraft on the throttle quadrant
Where are control signals sent to for interpretation?
EFCU (throttle levers will be pushed to a certain position, then this position will be collected to the potentionmetre which is included in the EEC and from here will be sent to the EFCU which receives the control signals to then metre fuel flow because it uses the position of the thorttlke to assume the required fuel)
When the EFCU decides a power change is needed (more fuel) what modulates the differential pressure at the head sensor?
Torque motor
Where is the EFCU located?
Remotely on the airframe
What does the FADEC use to provide information to the EEC?
Electronic sensors
What gives commands to the fuel metering valve in FADEC?
EEC
What does the fuel metering valve measure?
Fuel flow
What fuel systems do turbofan high bypass engines use?
FADEC
What is the EEC?
A computer that controls the operation of the engine
How many channels does an EEC contain?
2 (A and B)
How is the EEC cooled?
By convection
Where is the EEC stored?
Engine nacelle
What supplies the EEC in FADEC with the proportion of the thrust lever position?
Thrust lever angle resolver
What does the crosstalk in EEC’S decide?
Which channel s best to use for control of output
Does the EEC have volatile or non volatile memory?
Both
What does the primary channel control?
ALL output drivers
Where is the FMU mounted?
On the front face of the gearbox attatched to fuel pump
What starts/stops fuel?
Shut off valve
In a FADEC system what provides feedback?
LVDT, RVDT and thermocouples (mounted on the fuel outlet line of the rear oil cooler and sends information to the EEC)
What does the programming plug do?
Selects proper software in the EEC for thrust rating
When removing an EEC where does the programming plug stay?
On the engine fan case via a lanyard
What 2 subsystems can fuel be split into?
Distribution and control
Fuel distrbution has what three tasks?
Pressurise the fuel
Supply fuel to combustion chamber
Heat the fuel (no ice in nozzles)
Where does the fuel system usually start?
Behind LP fuel shut off valve
What does the LP fuel shut off valve do?
It feeds the fuel into the fuel main supply line
What component comes after the LP fuel shut off valve?
The LP pump
What comes after the LP pump?
Oil cooler
What is the dual function of the oil cooler?
What come after?
Cools the oil and heats the fuel
HP fuel pump
What occurs in the HP fuel pump?
What comes after?
Fuel is pressurised to level required for vapourisation in combustion chamber
FCU
What 2 things does the fuel control unit do?
What comes after?
Metres the fuel needed for combustion
Responsible for supply/shut off of fue to fuel nozzles
FFT
What does the fuel flow transmitter do?
Measures fuel flow and indicates to flight deck
What happens after the fuel flow transmitter?
The fuel is directed to the engine fuel manifold
What does the engine fuel manifold do?
Distributes to individual nozzles
What PSI does the low pressure fuel pump pressurise to?
175 psi
What does LP pump distribute to?
Oil cooler, fuel filter and HP pump
When is high pressure fuel pump used?
When engine at max speed
What PSI is the HP pump pressurising to?
900
What does the overpressure relief valve do in the HP compressor?
Prevent damage to fuel systems
Fuel from HP pump flows where?
to the metering section of FCU
How much does the pump, pump fuel for combustion?
More than enough (incase of rapid acceleration)
Un needed fuel goes where?
Back to the fuel pump via the fuel bypass return line
What lets only metered fuel enter the nozzles?
The metering section
Two types of pressurised pumps?
Constant displacement
Non Constant displacement
What decides which pump is used?
Where they are in the engine fuel system
Where is a non constant displacement (Centrifugal) used?
At the inlet of a pump to provide positive pressure
What is the centrifugal pump (non constant displacement) referred to as?
Boost stage pump
How are fuel pumps lubricated?
By passing fuel so must never be run dry
What psi do the LP pumps increase the system pressure to?
175
Where does the LP pump fuel flow to?
The oil cooler and the fuel filter after being pumped through HP pump
What psi do the HP pump increase system pressure to?
900psi
What does the HP pump have that the LP pump doesnt?
An overpressure relief valve to prevent damage to the systems
Where is a LP filter positioned?
In between supply tank and engine fuel system to protect engine driven pumps
Where is the HP filter installed?
Between the fuel pump and fuel control unit to protect from any contaminants arising from LP pump.
What filter has the greatest filtering action?
Microns
Porous cellulose material used in construction of cartridges is can remove what size micron?
10-25
What is required for filters with minute openings? (Microns)
Bypass valve (clogging)
What mesh filters are most widely used?
200 and 35 mesh microns
What are micron filters made up of?
Mesh steel wire
What prevents the formation of ice in fuel systems?
Heat exchangers
What is the source of heat for heat exchangers?
Bleed air or engine oil
How do fuel nozzles ensure that the combustion is done evenly?
It sprays in a highly atomised spray pattern
What does the fuel manifold do?
Distribute fuel to the nozzle
What does keeping the flame within the liners and in the centre prevent?
Hot spots
Where is a single fuel manifold found?
Modern gas turbine engine
What does the single fuel manifold surround?
The combustion case
Two method of protecting connections from leaks?
Shrouded connections
Double sealed connection
What is on the fuel lines to catch any fluid leaks? And where are the leaks sent?
Shrouds and they are sent to the drain mast.
What do shrouded connections do and don’t do?
They don’t prevent leaks but they catch and carry them away via the engine drain system
What prevents leaks from the coupling connector nut getting onto the combustion case?
O-ring
On older aircraft how many lines and manifolds were used?
2
Two types of fuel nozzles?
Simplex (Single valve)
Duplex (Double manifold and pressurising valve/divider to seperate secondary and primary)
Where is a fuel nozzle mounted externally?
Via a mounting pad on the CASE or the INLET AIR ELBOW
Where are fuel nozzles mounted internally?
On the Liner dome (remove chamber cover for maintenance)
Where is fuel suppled in the combustion chamber?
Forward part
What is a disadvantage of simplex fuel nozzles?
Covers a small range as discharge orifice is small. Good for ignition only, not reaching take off power
What can you use to cover a good fuel range?
Two sets of simplex nozzles
What are primary fuel nozzles used for (In two sets of simplex nozzles)?
Small discharge orifice so start up, to idle
What are secondary fuel nozzles used for (In two sets of simplex nozzles)?
Acceleration to maximum power
Modern aircraft use duplex nozzles. What are they?
Dual flow nozzles in one
What prevents carbon formation?
A check valve in the fuel nozzle
When does the check valve close?
When the HP fuel shut off valve is closed (pressure decrease)
What does the check valve ensure?
That no fuel flows to the combustion chamber from manifold by gravity.
What is the dual function of the oil cooler?
Oil heats the fuel and the fuel cools the oil
