•B 737 NG Ground School.See the aircraft study guide at www.theorycentre.comThe information contained here is for training purposes only. It is of a general nature it isunamended and does not relate to any individual aircraft. The FCOM must be consulted forup to date information on any particular aircraft.B 737 NG Ground School.
IntroductionThe fuel system supplies fuel to the engines and the APU. Fuel is contained inthree tanks located within the wings and wing centre section.
Fuel Tank Capacities (Usable Fuel)Main tanks No. 1 and No. 2 are integral with the wing structure. The centre tank liesbetween the wing roots within the fuselage area and extends out into the wingstructure.These figures represent approximate amounts of usable fuel. The appropriate weightand balance control and loading manual gives exact figures for all conditions.TANK Pounds/ LITERS /KILOGRAMSUsable fuel at level attitude.*Fuel density = 0.8029 kilograms per litre. 6.7 Pounds per US Gallon
Fuel Shutoff ValvesSpar fuel shutoff valves are located at the engine–mounting wing stations. The valves areDC motor operated from the hot battery bus. The spar valve is controlled directly by theengine start lever.The engine fuel shutoff valves are fuel actuated, solenoid controlled valves powered fromthe 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 whenthe start lever is in the idle position. The EEC does not close the HPSOV the signal toclose comes directly from the engine start lever being placed in the cutoff position. Or bypulling the engine fire switch up this energises a solenoid and stops fuel pressure fromholding the HPSOV open.Both the spar fuel shutoff valve and the engine fuel shutoff valve close whenever theirrespective engine fire switch is pulled or engine start lever is placed to CUTOFF.
High Pressure Fuel valve mounted on the HMU.•NO light Valve OPEN.•DIM Blue Valve CLOSED•BRIGHT Blue TRANSIT OR NOT IN COMMANDEDPOSITION.FUEL PRESSURE ACTUATED. DC SOLENOID CONTROL. BATTERY BUS.ENGINE FUEL SHUTOFF VALVE
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 COMMANDEDPOSITION.Has Ni-CAD Battery to ensure power to close.DC MOTOR operated. Powered from theHOT BATTERY BUS
Fuel CrossfeedThe engine fuel manifolds are interconnected by use ofthe crossfeed valve. The valve is DC motor operatedfrom the battery bus.Fuel pressure can be provided from a main tank withoperating fuel pumps to both engines by opening the fuelcrossfeed valve. Continued crossfeed use will result in aprogressive fuel imbalance.
CROSS FEED valve.•NO light Valve CLOSED (NORMAL).•DIM Blue Valve OPEN•BRIGHT Blue TRANSIT OR NOT IN COMMANDEDPOSITION.
LimitationsFuel Temperature•Maximum temperature is 49°C•Minimum is the freeze point +3°C or-45°C whichever is the higherFuel TemperatureThe FUEL TEMP indicator located on the fuel control panel displays fuel temperature. Asensor in main tank No. 1 allows monitoring of fuel temperature. The temperature indicatingsystem uses AC electrical power.
Center Tank Fuel Scavenge Jet PumpWith the main tank fuel pump No. 1 FWD Switch ON, the centre tank fuel scavenge jetpump 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.
CENTRE TANK SCAVENGE PUMPPressurefrom No 1tank fwdpump.ON when No1 tank abouthalf full.Controlledby a floatvalve.Centre tank fuelTo Number 1 Tank.Transfer rate 100 to 200 Kg/Hr
Fuel System LimitationsMaximum tank fuel temperature: 49°C (120°F).Minimum in flight tank : 3°C above the freezing point ofthe fuel being used or -43°C, whichever is higher.Intentional running of a centre tank fuel pump (lowpressure light illuminated) is prohibited.Fuel BalanceLateral imbalance between main tanks 1 and 2 must bescheduled to be zero.Random fuel imbalance must not exceed 453 Kgs(1,000 Lbs)for taxi, takeoff, flight or landing.Fuel LoadingMain tanks 1 and 2 must be full if centre tank containsmore than 453 Kgs.Note: Centre tank fuel is always used first to minimisewing bending loads.
Fuel Quantity IndicationThe 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 refuelstation panel.Indications can be digital only ordials with digital figures. Indicationscan be in Lbs or Kg.
FUEL Quantity IndicatorsDisplayed (white) – indicates usable fuel in related tank:• standby AC power is required.
Fuel LOW AlertDisplayed (amber) –• fuel tank quantity less than 907 kgs in related main tank• display remains until fuel tank quantity is increased to 1134 kgsThe fuel quantity arc and digits on tank(s) with low fuel quantity turn amber.Original figures by Boeing are in Lbsconverted to Kg’s for the remainderof the World.907 Kg is approximately 2,000 LbsUsing 1 Kg = 2.2 Lbs.
Fuel Imbalance (IMBAL) AlertDisplayed (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 KgsThe 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 whichmust be checked manually after refuelling!
Fuel Configuration (CONFIG) AlertDisplayed (amber) –• either engine running• centre fuel tank quantity greater than 726 Kgs; and• both centre fuel tank pump switches positioned OFFThe quantity arc and digits on the centre tank fuelquantity indicator turn amber.Display remains until –• both engines not running• centre fuel tank quantity less than 363 Kgs• one centre fuel tank pump switch ONThe quantity arc and digits on the centre tank fuelquantity indicator return to normal.
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.
Fuel PumpsEach fuel tank uses two AC powered fuel pumps which are cooled and lubricated by fuelpassing through the pump. Centre tank pumps produce higher pressure than main tankpumps. This ensures that centre tank fuel is used before main tank fuel, even though allfuel pumps are operating. Individual pressure sensors monitor the output pressure ofeach pump.There are two centre tank boost pumps in the centre tank. They are installed on the rearspar. 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 thecentre tank. The forward boost pumps are on the front spar. The aft boost pumps are onthe 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’ssensor detects low output pressure. (Aircraft fit)Note: Fuel pump LOW PRESSURE lights may flicker when tank quantity is low and theairplane 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 islow and the airplane is in cruise. One pump may indicate low pressure sooner than theother due to aircraft attitude and/or slight variation between pump inlet position. Lowpressure indication may occur after centre tank quantity reads zero. Low pressure lightflickering can continue for as long as 5 minutes before the Fuel System Annunciator lightand the Master Caution lights are illuminated for the associated centre tank pump.
LOW PRESSURE LIGHTS- IlluminatedPump ON and output Pressure less than 22 PSI.•Inhibited if Pump switch OFF.• ONE PUMP ON and LOW PRESSURE lightMASTER CAUTION AND FUEL light.•TWO PUMPS ON and BOTH LOW PRESSURELIGHTS MASTER CAUTION and FUEL Light.Centre Tank Fuel Pumps
• LOW PRESSURE LIGHTS- Output Pressure lessthan 6 PSI. Pump On or OFFMain Tank Fuel Pumps
•Filter differential Pressure above 11.5 PSI.• BYPASS Valve opens at 15 PSI.•Some fuel may be bypassing the filter .
Nitrogen Generation System (NGS) (OPTION on Some Aircraft)The NGS converts bleed air to nitrogen-enriched air (NEA) during all phasesof flight. The NEA is delivered to the centre fuel tank to reduce flammabilityof the tank. The operation of the NGS is transparent to the flight crew; itdoes not require any flight crew action to operate the system, nor are thereany flight deck indications.The NGS automatically starts operating after take-off and runs continuouslythrough climb, cruise, descent, landing and during taxi for a short period oftime. The NGS shuts down after a specified period of time or when bleedpressure is no longer available. The NGS also automatically shuts downduring 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 openThe fuel tanks are primarily protected by precluding ignition sources; hencedispatch with the NGS inoperative is acceptable under MEL procedures.The NGS has an operability indicator located in the main wheel welladjacent to the APU fire control panel.
Nitrogen Gas Generation SystemUses bleed air from the left pneumaticmanifold.Air passes through an Air Separationmodule.Produces Nitrogen Enriched Air (NEA)NEA supplied to the centre tank toreduce O2 concentrations.Automatic operation in air modeTurns off a short time after landing orwhen 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.
Nitrogen Gas Generation SystemControl panel located close to APU Firecontrol Right hand Main wheel well.Check lights during pre flight inspectionGreen or Blue OK.Amber or No light system not working.MEL dispatch requirements.No crew action and no Flight Deckindications.
Fuelling/Defueling/Ground TransferRapid refuelling and defueling is accomplished at the single–point pressure refuelstation in the right wing. The refuel station is also used for the ground transfer of fuelbetween tanks.The manual defueling valve, located outboard of engine No. 2, interconnects theengine feed system and the fuel station. It is opened for defueling and tank to tanktransfer operations.A shutoff system is used during refuelling to automatically close the refuel valve ineach fuel tank when the tank is full.
As refuelling is not a normal crew duty it is covered in the FCOM sectionSupplementary procedures.RefuelingFuel Load DistributionMain 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 tankcontains 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 beenconsidered.Fuel PressureApply from a truck or fuel pit. A nozzle pressure of 50 psi provides approximately1136 litres per minute.
Ground Fuelling PanelFuelling• Fuelling station allows single-point pressureground fuelling only• Auto shutoff at maximum capacity• Max fuelling rate is 1,136 Ltr/min at 50 PSI
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 aMAGNETIC switch Target is on the door!
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 RelayBATTERY BUSHOT BATTERY BUSWith the battery switch off the Fuel power control relay is relaxed.
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 RelayBATTERY BUSHOT BATTERY BUS
Ground Fuelling PanelFuel door Switch Bypass is momentary. Ifdoor switch fails the Bypass will connectbattery power directly.Test position checks gages. All liquid crystalelements illuminate.
Ground Fuelling PanelBlue lights indicate valve solenoid isenergised only! ( Not valve open indication.)Fuel pressure is required to open the refuelvalves.A shutoff system is used during refueling toautomatically close the refueling valve in eachtank when the tank is full.MAXIMUM PRESSURE 55 PSI
Thumb Wheel Pre Selected Fuel quantityMore recent aircraft come with a new refuelpanel. This includes pre selectors which allow formore automatic refuelling.All other operations remain the same.
Ground De-Fuelling PanelManual De-fuelling ValveIf the De fuel valve is open theaccess door will not close.
The manual defueling valve, located outboard of engine No. 2, interconnects theengine feed system and the fuel station. It is opened for defueling and tank to tanktransfer operations.
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 pumpsin one tank and opening the refuelling valve for another tank a tank to tank transfercan be carried out. The cross feed valve is opened as needed.
FUEL MEASURING STICKThere are six measuring sticks in main tank 1 and main tank 2. Each fuel measuringstick 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 mainlanding gear wheel well.You use fuel height the linear unit measurement and airplane pitch and roll tomeasure fuel quantity. Chapter 12 of the Aircraft Maintenance Manual hasconversion tables that change linear units, fuel height, and airplane pitch and rollinto fuel quantity.This procedure is rather involved and will be completed by an engineer only whenthe fuel quantity gauging system is inoperative.
A drip stick is lowered until fuel just starts to drip to get a depth reading.
A float stick is lowered until the float at its top is supported by fuel. This gives a fuel depth
Supplementary Procedures - FuelRefuellingAs most fuelling operations are not normally carried out by aircrew the followingare supplementary Procedures to aid when needed.Refuelling with Battery OnlyRefuelling with No AC or DC Power Source AvailableGround Transfer of FuelFuel Crossfeed Valve Check