TASK: 2.1
The purposes of twin engine aircraft fuel system are to confirm the desire supply of fuel at all
altitude. It helps the aircraft to maintain the fuel system in a systematic way. All the time of flight
engine fuel requirement is not same. So a complete and well active fuel system is needed for
safe flight operation.
Major components of twin engine jet aircraft fuel system:
Fuel shutoff valve: Fuel Shutoff Valve is electrically controlled by the Engine Start Lever and
the Engine Fire Warning Switch
Fuel pump: Aircraft with fuel-pump systems have two fuel pumps. The main pump system is
engine driven with an electrically driven auxiliary pump provided for use in engine starting and in
the event the engine pump fails. It also known as booster pump.
Fuel heat exchanger: A fuel heat exchange system is proposed for an aircraft gas turbine
engine for heating fuel leaving the aircraft fuel tanks before it enters the engine fuel system.
Fuel filter: It filter the fuel to protect foreign object enter in to the engine.
Fuel flow transmitter: It transmits the fuel flow rate.
TASK: 2.2
Fuel flow system:
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At first fuel leaves the fuel tank and passes through the Engine Fuel Shutoff Valve,
which is located at the engine mounting wing station.
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Then fuel passes from the first stage of the engine driven fuel pump through a fuel/oil
heat exchanger to a filter. Provisions are made to bypass the heat exchanger or the filter
in the event of failure or blockage
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The second stage fuel pump provides high pressure fuel to the Main Engine Control
(MEC). As the fuel leaves the second stage fuel pump, a portion of the fuel is diverted to
operate the MEC. This fuel is filtered again and then routed through the fuel heater for
the second time.
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The MEC, in conjunction with the Power Management Control (PMC), uses thrust lever
angle, fan inlet pressure and temperature, N1 RPM and N2 RPM to meter the correct
amount of fuel to the combustor. Fuel flows from the MEC through the MEC fuel shutoff
valve. The MEC fuel shutoff valve is mechanically controlled by the Engine Start Lever.
A fuel flow transmitter measures the rate of fuel flow from the MEC.
TASK 4.1:
Continuous loop of fire detection system:
A continuous-loop detector or sensing system permits more complete coverage of a fire hazard
area than any type of spot-type temperature detectors. Continuous-loop systems are versions of
the thermal switch system. Two widely used types of continuous-loop systems are:
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The Kidde system and
The Fenwal systems.
The Kidde system:
In the Kidde continuous-loop system two wires are imbedded in a special ceramic core within
an Inconel tube.
One of the two wires in the Kidde sensing system is welded to the case at each end and acts
as an internal ground.
The second wire is a hot lead (above ground potential) that provides a current signal when the
ceramic core material changes its resistance with a change in temperature.
Fig:- Kidde sensing element.
Fenwal system:
Another continuous-loop system, the Fenwal system, uses a single wire surrounded by a
continuous string of ceramic beads in an Inconel tube.
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Consists of small, lightweight, flexible income tube with a pure nickel conductor wirecenter conductor.
The space between the nickel conductor and tubing wall is filled with porous aluminiumoxide, ceramic insulating material.
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Any voids or clearances are saturated with a eutectic salt mixture which has a low
melting point.
The tube is hermetically sealed at both ends with insulating material and threading
fittings.
When heated sufficiently, current can flow between the center wire and the tube wall
because the eutectic salt melts, and the resistance drops rapidly.
The increased current flow provides a signal which is used in the control unit to sound
the alarm system.
Once the fire is extinguished or the over-heat condition is corrected the eutectic salt
increase its resistance and the system will return to a stand-by.
Fig:- Fenwal sensing element
TASK 4.2:
Aircraft cargo overheats detection system:
Overheat warning systems are used on aircraft to indicate high area temperatures that may lead
to fire. The number of overheat warning system varies from aircraft to aircraft. The cargo
overheat warning system of BOEING 767-300 is given below.
BOEING 767-300 cargo compartment overheats detection system:
1. CARGO FIRE ARM switches & lights
Push FWD ARMED- Light illuminates
 Arms all cargo fire extinguisher bottles
 Turns off both recirculation fans
Push AFT ARMED- Light illuminates
 Arms all cargo fire extinguisher bottles
 Turns off both the recirculation fans
 Inhibits high flow operation of both packs
Off- normal position
2. CARGO FIRE warning lights (FWD or AFT)
Illuminated (red) – smoke is dectected in associated cargo compartmen
3. CARGO FIRE bottle discharge (BTL DISCH) switch
Push – discharges the fire extinguisher bottles into the ARMED cargo compartment
4. CARGO FIRE bottle discharged (DISCH) light
Illuminated (amber) – either extinguisher bottle has discharged or has low pressure
Smoke detector test circuit.
TASK 5.1
Explanation the operation of fuel feeding to engine:
Fuel is supplied under pressure by fuel pumps located in the fuel tanks. The fuel flows through a
spar valve, located in the main tank. It then passes through the first stage (low-pressure) engine
fuel pump where additional pressure is added. The fuel is then managed as follows:
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Engine automatically heat the fuel by bleed air as it flows through the fuel/oil heat
exchanger.
A low-pressure fuel filter then removes contaminants it removes comparatively large
objects.
Final pressure is generated by a second stage (high-pressure) fuel pump.
The fuel is then controlled by the Fuel Metering Unit to meet existing thrust
requirements. The more thrust is required the more fuel is needed.
The fuel flows through the Engine Fuel Valve and Fuel Flow Meter.
The fuel flows through a second, high-pressure fuel filter.
Then the high pressure fuel enters in to the engine combustion chamber.
Fuel Control Unit
The engine fuel control system incorporates a hydro-mechanical fuel control unit, which
operates in conjunction with the EEC and ELC units. The fuel control system schedules fuel flow
to meet engine thrust requirements as dictated by the thrust lever position and the specific
engine operating conditions. The EEC trims the metered fuel to prevent overboost when
operating at or near the thrust limits. The ELC also trims the metered fuel to prevent N1 from
exceeding the limits.
Engine and Spar Valves
The Spar Valve located in the main fuel tank and the Engine Valve located in the Fuel Control
Unit allow fuel to flow to the engine when the both valves are open when this two or one of this .
The valves open when the Engine Fire Switch is IN and the FUEL CONTROL Switch is in the
RICH or RUN position.
Both valves close when either the Fuel Control Switch is at CUTOFF or the Engine Fire Switch
is OUT.
The ENG VALVE and SPAR VALVE lights will illuminate momentarily as the valves open or
close. If the valves do not agree with the Fuel Control Switch or the respective Fire Switch
position after allowing for normal operating time, the lights remain illuminated and the EICAS
advisory message L/R FUEL SPAR VALVE or L/R ENG FUEL VALVE displays.
TASK: 5.2
Fuel transfer from one wing tank to centre tank: (The system of Boeing 777-300 ER)
The fuel system supplies fuel to the engines and the APU. The fuel is contained in a center
tank, and left and right main tanks. Surge tanks are provided in each wingtip. Fuel from the
surge tanks is drained to the center tank. A scavenge system will operate automatically to
transfer any remaining center tank fuel to the main tanks. Fuel transfer begins when the main
tanks are approximately half empty. The transfer rate is approximately 180 kg per hr, but may
be as low as 60 kg per hr.
There position is:
Fuel pump: When the main tank fuel pump switches are off, the switch PRESS lights illuminate
and the EICAS advisory message FUFL PUMP (L, R, FWD, or AFT) display. When the centre
fuel pump switches are off, the switch PRESS lights and pump pressure EICAS message are
inhibited.
Fuel pump operation: With the main fuel pumps ON, a scavenge system operates
automatically to transfer any remaining center tank fuel to the main tanks. Fuel transfer begins
when either main tank quantity is less than 52,600 pounds. Centre tank quantity must be below
35,000 pounds.
Suction feed: When main tank fuel pump is low, each engine can draw fuel from its
corresponding main tank through a suction feed line that bypasses the pumps.
Fuel cross feed: The cross feed valve isolated the left and right systems. If the valve position
does not agree with the switch position, the CROSSFEED switch VALVE light illuminates and
the EICAS advisory message FUEL CROSSFEED FWD or AFT displays.
Fuel imbalance: when the fuel quantity in left & right main tanks differs by an excessive
quantity, the EICAS alert message FUEL IMBLANCE displays.
TASK: 5.3
Fuel jettisoning system: If the aircraft is over loaded and try to land immediately the fuel
jettison system allows jettison from all fuel tanks. Fuel is jettisoned through jettison nozzle
valves inboard of each aileron. Jettison pumps in the main tanks and jettison pumps in the
centre tank pump fuel overboard through the jettison nozzle valves.
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Fuel jettison is initiated by pushing the FUEL JETTISON ARM switch to select ARMED.
The jettison system automatically sets the fuel – to – remain to the maximum landing
weight (MLW) fuel quantity.
The TO REMAIN quantity replaces fuel temperatures on the EICAS display.
Pull on and rotate the FUEL TO REMAIN selector to manually decrease or increase the
TO REMAIN quantity.
Main tank jettison begins when:
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The FUEL JETTISON NOZZLE switches are pushed ON
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The jettison nozzle valves open, and
The main tank jettison pumps operate.
If the centre tank jettison pumps are on, centre tank fuel also jettisons. Centre tank fuel will not
jettison if the centre tank jettison pumps are off.
The nozzle cannot open on the ground, regardless of switch positions.
In flight, jettison time is displayed in minutes on the fuel synoptic when the jettison arm switch is
positioned on while in the air. Jettison automatically stops when a value just above the TO
REMAIN quantity is reached. The system shuts off the main tank jettison pumps and closes
tank jettison isolation valves.
When the airplane is heavy and loaded near the forward CG, fuel is jettisoned from the centre
tank first to keep CG within limits; main tank jettison pump operation is delayed. The computed
jettison time is automatically adjusted to reflect the increased jettison time.