The document provides information on the Boeing 737 NG fuel system. It describes the three fuel tanks, their capacities and fuel quantity indicators. It outlines the fuel pumps, valves and controls. It notes limitations on fuel temperature, imbalance and loading. Procedures for refueling, defueling and cross-feeding fuel between tanks are summarized.
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B737 NG Fuel System.
1. •B 737 NG Ground School.
See the aircraft study guide at www.theorycentre.com
The information contained here is for training purposes only. It is of a general nature it is
unamended and does not relate to any individual aircraft. The FCOM must be consulted for
up to date information on any particular aircraft.
B 737 NG Ground School.
5. Introduction
The fuel system supplies fuel to the engines and the APU. Fuel is contained in
three tanks located within the wings and wing centre section.
6. Fuel Tank Capacities (Usable Fuel)
Main tanks No. 1 and No. 2 are integral with the wing structure. The centre tank lies
between the wing roots within the fuselage area and extends out into the wing
structure.
These figures represent approximate amounts of usable fuel. The appropriate weight
and balance control and loading manual gives exact figures for all conditions.
TANK Pounds/ LITERS /KILOGRAMS
Usable fuel at level attitude.
*Fuel density = 0.8029 kilograms per litre. 6.7 Pounds per US Gallon
7. Fuel Shutoff Valves
Spar fuel shutoff valves are located at the engine–mounting wing stations. The valves are
DC motor operated from the hot battery bus. The spar valve is controlled directly by the
engine start lever.
The engine fuel shutoff valves are fuel actuated, solenoid controlled valves powered from
the battery bus. During engine starting the EEC controls the Fuel metering valve (FMV)
fuel pressure from the FMV causes the HPSOV to open. The EEC receives a signal when
the start lever is in the idle position. The EEC does not close the HPSOV the signal to
close comes directly from the engine start lever being placed in the cutoff position. Or by
pulling the engine fire switch up this energises a solenoid and stops fuel pressure from
holding the HPSOV open.
Both the spar fuel shutoff valve and the engine fuel shutoff valve close whenever their
respective engine fire switch is pulled or engine start lever is placed to CUTOFF.
8. High Pressure Fuel valve mounted on the HMU.
•NO light Valve OPEN.
•DIM Blue Valve CLOSED
•BRIGHT Blue TRANSIT OR NOT IN COMMANDED
POSITION.
FUEL PRESSURE ACTUATED. DC SOLENOID CONTROL. BATTERY BUS.
ENGINE FUEL SHUTOFF VALVE
9. SPAR valve mounted on the Wing L.E. Outboard of Pylon.
•NO light Valve OPEN.
•DIM Blue Valve CLOSED
•BRIGHT Blue TRANSIT OR NOT IN COMMANDED
POSITION.
Has Ni-CAD Battery to ensure power to close.
DC MOTOR operated. Powered from the
HOT BATTERY BUS
10. Fuel Crossfeed
The engine fuel manifolds are interconnected by use of
the crossfeed valve. The valve is DC motor operated
from the battery bus.
Fuel pressure can be provided from a main tank with
operating fuel pumps to both engines by opening the fuel
crossfeed valve. Continued crossfeed use will result in a
progressive fuel imbalance.
11. CROSS FEED valve.
•NO light Valve CLOSED (NORMAL).
•DIM Blue Valve OPEN
•BRIGHT Blue TRANSIT OR NOT IN COMMANDED
POSITION.
12. Limitations
Fuel Temperature
•Maximum temperature is 49°C
•Minimum is the freeze point +3°C or
-45°C whichever is the higher
Fuel Temperature
The FUEL TEMP indicator located on the fuel control panel displays fuel temperature. A
sensor in main tank No. 1 allows monitoring of fuel temperature. The temperature indicating
system uses AC electrical power.
13. Center Tank Fuel Scavenge Jet Pump
With the main tank fuel pump No. 1 FWD Switch ON, the centre tank fuel scavenge jet
pump operates automatically to transfer any remaining centre tank fuel to main tank No. 1.
Fuel transfer begins when main tank No. 1 quantity is about one-half.
Once the fuel scavenge process begins, it continues for the remainder of the flight.
14. CENTRE TANK SCAVENGE PUMP
Pressure
from No 1
tank fwd
pump.
ON when No
1 tank about
half full.
Controlled
by a float
valve.
Centre tank fuel
To Number 1 Tank.
Transfer rate 100 to 200 Kg/Hr
15. Fuel System Limitations
Maximum tank fuel temperature: 49°C (120°F).
Minimum in flight tank : 3°C above the freezing point of
the fuel being used or -43°C, whichever is higher.
Intentional running of a centre tank fuel pump (low
pressure light illuminated) is prohibited.
Fuel Balance
Lateral imbalance between main tanks 1 and 2 must be
scheduled to be zero.
Random fuel imbalance must not exceed 453 Kgs
(1,000 Lbs)for taxi, takeoff, flight or landing.
Fuel Loading
Main tanks 1 and 2 must be full if centre tank contains
more than 453 Kgs.
Note: Centre tank fuel is always used first to minimise
wing bending loads.
16. Fuel Quantity Indication
The fuel quantity indication system calculates the usable fuel quantity in each tank.
The fuel quantity in each tank is displayed on the upper display unit and on the refuel
station panel.
Indications can be digital only or
dials with digital figures. Indications
can be in Lbs or Kg.
18. Fuel LOW Alert
Displayed (amber) –
• fuel tank quantity less than 907 kgs in related main tank
• display remains until fuel tank quantity is increased to 1134 kgs
The fuel quantity arc and digits on tank(s) with low fuel quantity turn amber.
Original figures by Boeing are in Lbs
converted to Kg’s for the remainder
of the World.
907 Kg is approximately 2,000 Lbs
Using 1 Kg = 2.2 Lbs.
19. Fuel Imbalance (IMBAL) Alert
Displayed (amber) –
• main tanks differ by more than 453 Kgs
• displayed below main tank with lower fuel quantity
• inhibited when airplane is on ground
• inhibited by fuel LOW indication when both indications exist
• displayed until imbalance is reduced to 91 Kgs
The fuel quantity arc and digits on tank with lower fuel quantity turn amber.
Note; IMBAL inhibited on ground. Limitation for take off is 453 Kgs which
must be checked manually after refuelling!
20. Fuel Configuration (CONFIG) Alert
Displayed (amber) –
• either engine running
• centre fuel tank quantity greater than 726 Kgs; and
• both centre fuel tank pump switches positioned OFF
The quantity arc and digits on the centre tank fuel
quantity indicator turn amber.
Display remains until –
• both engines not running
• centre fuel tank quantity less than 363 Kgs
• one centre fuel tank pump switch ON
The quantity arc and digits on the centre tank fuel
quantity indicator return to normal.
23. FUEL FLOW Switch (spring-loaded to RATE)
RATE – displays fuel flow to engine.
USED –
• displays fuel used since last reset
• after 10 seconds, display automatically reverts to fuel flow.
RESET –
• resets fuel used to zero
• displays fuel used for 1 second, decreases to zero, then displays fuel flow.
24. Fuel Pumps
Each fuel tank uses two AC powered fuel pumps which are cooled and lubricated by fuel
passing through the pump. Centre tank pumps produce higher pressure than main tank
pumps. This ensures that centre tank fuel is used before main tank fuel, even though all
fuel pumps are operating. Individual pressure sensors monitor the output pressure of
each pump.
There are two centre tank boost pumps in the centre tank. They are installed on the rear
spar. Access to the centre tank boost pumps is through the main wheel wells.
There are forward and aft boost pumps for the main tanks No. 1and No. 2 installed in the
centre tank. The forward boost pumps are on the front spar. The aft boost pumps are on
the rear spar. Access to the forward boost pumps is through extended krueger flaps.
Access to the aft boost pumps is through the wheel wells.
Each centre tank pump will automatically shut off, after a short delay, when that pump’s
sensor detects low output pressure. (Aircraft fit)
Note: Fuel pump LOW PRESSURE lights may flicker when tank quantity is low and the
airplane is in a climb, descent, or on the ground with a nose-down attitude.
Note: Centre tank fuel pump LOW PRESSURE lights may flicker when tank quantity is
low and the airplane is in cruise. One pump may indicate low pressure sooner than the
other due to aircraft attitude and/or slight variation between pump inlet position. Low
pressure indication may occur after centre tank quantity reads zero. Low pressure light
flickering can continue for as long as 5 minutes before the Fuel System Annunciator light
and the Master Caution lights are illuminated for the associated centre tank pump.
25. LOW PRESSURE LIGHTS- Illuminated
Pump ON and output Pressure less than 22 PSI.
•Inhibited if Pump switch OFF.
• ONE PUMP ON and LOW PRESSURE light
MASTER CAUTION AND FUEL light.
•TWO PUMPS ON and BOTH LOW PRESSURE
LIGHTS MASTER CAUTION and FUEL Light.
Centre Tank Fuel Pumps
26. • LOW PRESSURE LIGHTS- Output Pressure less
than 6 PSI. Pump On or OFF
Main Tank Fuel Pumps
27. •Filter differential Pressure above 11.5 PSI.
• BYPASS Valve opens at 15 PSI.
•Some fuel may be bypassing the filter .
28. Nitrogen Generation System (NGS) (OPTION on Some Aircraft)
The NGS converts bleed air to nitrogen-enriched air (NEA) during all phases
of flight. The NEA is delivered to the centre fuel tank to reduce flammability
of the tank. The operation of the NGS is transparent to the flight crew; it
does not require any flight crew action to operate the system, nor are there
any flight deck indications.
The NGS automatically starts operating after take-off and runs continuously
through climb, cruise, descent, landing and during taxi for a short period of
time. The NGS shuts down after a specified period of time or when bleed
pressure is no longer available. The NGS also automatically shuts down
during the following non-normal flight conditions:
• Aircraft on the ground and not in test mode
• Either engine is not running in flight
• Fire or smoke detection in the cargo or main deck areas
• Left air conditioning pack overheat
• Center tank refueling valve is open
The fuel tanks are primarily protected by precluding ignition sources; hence
dispatch with the NGS inoperative is acceptable under MEL procedures.
The NGS has an operability indicator located in the main wheel well
adjacent to the APU fire control panel.
29. Nitrogen Gas Generation System
Uses bleed air from the left pneumatic
manifold.
Air passes through an Air Separation
module.
Produces Nitrogen Enriched Air (NEA)
NEA supplied to the centre tank to
reduce O2 concentrations.
Automatic operation in air mode
Turns off a short time after landing or
when there is no bleed duct pressure.
Automatically shuts down for:
One engine shut down.
Cargo fire or smoke.
Left pack overheat.
Centre tank re-fuel valve open.
30. Nitrogen Gas Generation System
Control panel located close to APU Fire
control Right hand Main wheel well.
Check lights during pre flight inspection
Green or Blue OK.
Amber or No light system not working.
MEL dispatch requirements.
No crew action and no Flight Deck
indications.
31. Fuelling/Defueling/Ground Transfer
Rapid refuelling and defueling is accomplished at the single–point pressure refuel
station in the right wing. The refuel station is also used for the ground transfer of fuel
between tanks.
The manual defueling valve, located outboard of engine No. 2, interconnects the
engine feed system and the fuel station. It is opened for defueling and tank to tank
transfer operations.
A shutoff system is used during refuelling to automatically close the refuel valve in
each fuel tank when the tank is full.
32. As refuelling is not a normal crew duty it is covered in the FCOM section
Supplementary procedures.
Refueling
Fuel Load Distribution
Main tanks No. 1 and No. 2 should normally be serviced equally until full.
Additional fuel is loaded into the centre tank until the desired fuel load is reached.
Note: Main tanks No. 1 and No. 2 must be scheduled to be full if the centre tank
contains more than 453 Kgs of fuel. With less than 453 Kgs of centre tank fuel,
partial main tank fuel may be loaded provided the effects of balance have been
considered.
Fuel Pressure
Apply from a truck or fuel pit. A nozzle pressure of 50 psi provides approximately
1136 litres per minute.
33. Ground Fuelling Panel
Fuelling
• Fuelling station allows single-point pressure
ground fuelling only
• Auto shutoff at maximum capacity
• Max fuelling rate is 1,136 Ltr/min at 50 PSI
34. Ground Fuelling Panel
• Battery Switch Must be ON to Re-Fuel.
•28 V DC Hot Battery Bus Powers the Panel.
Valves and Gauges.
•Fueling power control switch is a
MAGNETIC switch Target is on the door!
35. Ground Fuelling Panel
• Battery Switch Must be ON to Re-Fuel.
•28 V DC Hot Battery Bus Powers the Panel.
Valves and Gauges.
Fuel Power Control Relay
BATTERY BUS
HOT BATTERY BUS
With the battery switch off the Fuel power control relay is relaxed.
36. Ground Fuelling Panel
• Battery Switch Must be ON to Re-Fuel.
•28 V DC Hot Battery Bus Powers the Panel.
Valves and Gauges.
Fuel Power Control Relay
BATTERY BUS
HOT BATTERY BUS
37. Ground Fuelling Panel
Fuel door Switch Bypass is momentary. If
door switch fails the Bypass will connect
battery power directly.
Test position checks gages. All liquid crystal
elements illuminate.
38. Ground Fuelling Panel
Blue lights indicate valve solenoid is
energised only! ( Not valve open indication.)
Fuel pressure is required to open the refuel
valves.
A shutoff system is used during refueling to
automatically close the refueling valve in each
tank when the tank is full.
MAXIMUM PRESSURE 55 PSI
39. Thumb Wheel Pre Selected Fuel quantity
More recent aircraft come with a new refuel
panel. This includes pre selectors which allow for
more automatic refuelling.
All other operations remain the same.
41. The manual defueling valve, located outboard of engine No. 2, interconnects the
engine feed system and the fuel station. It is opened for defueling and tank to tank
transfer operations.
42. With the de fuel valve open the aircraft pumps can pressurise the re fuel manifold.
By running the pumps fuel can be removed from the aircraft. By running the pumps
in one tank and opening the refuelling valve for another tank a tank to tank transfer
can be carried out. The cross feed valve is opened as needed.
43. FUEL MEASURING STICK
There are six measuring sticks in main tank 1 and main tank 2. Each fuel measuring
stick is on a fuel tank access door.
There are four fuel tank measuring sticks in the centre tank.
The inner sticks have graduation marks that show fuel height in linear units.
There are two inclinometers, one for airplane pitch, one for airplane roll, in the main
landing gear wheel well.
You use fuel height the linear unit measurement and airplane pitch and roll to
measure fuel quantity. Chapter 12 of the Aircraft Maintenance Manual has
conversion tables that change linear units, fuel height, and airplane pitch and roll
into fuel quantity.
This procedure is rather involved and will be completed by an engineer only when
the fuel quantity gauging system is inoperative.
44. A drip stick is lowered until fuel just starts to drip to get a depth reading.
45. A float stick is lowered until the float at its top is supported by fuel. This gives a fuel depth
46. Supplementary Procedures - Fuel
Refuelling
As most fuelling operations are not normally carried out by aircrew the following
are supplementary Procedures to aid when needed.
Refuelling with Battery Only
Refuelling with No AC or DC Power Source Available
Ground Transfer of Fuel
Fuel Crossfeed Valve Check
