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In-flight fuel imbalance occurs when the quantity of fuel between the fueltanks in the left and right wings is unequal. A fuel imbalance can occur for
many reasons, including acceptable variations in the performance of fuelsystem components, variations in engine fuel burn characteristics, faults ininternal fuel system components, or fuel system or structural faults thatcause fuel to leak overboard. Operators can avoid unnecessary dispatchdelays and maintenance work by understanding the causes of in-flight fuelimbalance, proper fuel management, fuel imbalance indication, and airplanedispatch procedures following the display of fuel imbalance indications.
IN-FLIGHT
RICK COLELLA
LEAD ENGINEER
FLIG HT OPERAT
E N G I N E E R I N G
BOEING COMME
AIRPLANES GRO
MIKE ZIMMER
LEAD ENGINEER
S E RVICE ENGIN
BOEING COMME
AIRPLANE S GRO
3
A E R O
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CAUSES OF IN-FLIGHTFUEL IMBALANCE
C o nd i t io ns fre q u ently identif ied asb ei ng re s p o nsible for in-flig ht fueli m ba l a nce incl ude differe nces in fuel
system compone nt perfo r ma nc e,
2wing tanks. On three-engine airplanes,
center tank fuel and any auxiliary tankfuel is used until the remaining fuel in
the center tank equals the fuel quantityin the tanks on the left and right wings.At this point, fuel is used equally fromthe wing tanks and center tank.
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PROPER FUEL MANAGEMENT
Before flight, the fuel load between
the left and right wing tanks must besymmetrical to maintain airplane lateral
b a la nc e. On two- and fo u r - e ng i ne air-planes, fuel is first distributed equally
to the wing tanks until those tanksare full (figs. 1 and 2). Any additionalfuel required for flight is loaded in the
center tank and any auxiliary fueltanks, such as the horizontal stabilizer
tank on the 747-400. On three-engine
airplanes (fig .3 ) , fuel is first distributedequally to the wing tanks and thec e nter tank until the wing tanks arefull. Any additional fuel required is then
distributed to the remaining volume inthe center tank and in any auxiliary
fuel tanks.
On two- and four-engine airplanes,center tank fuel and auxiliary tank fuelare generally used first. Once thesetanks are empty, fuel is used from the
TYPICAL TWO-ENGINEFUEL TANK CONFIGURATION1
FIGURE
1
hen fuel is being used
from an airplanes wing
tanks in flight, normal
fuel ma na geme nt pro-
ce du res require keeping equal
quantities of fuel in each wing
to maintain airplane lateral bal-
ance. An in-flight fuel imbalance
condition occurs if the fuel
quantities in the wings become
unequal. To ma i nt aine qual quan-
tities in each wing and ensure a
high level of airplane dispatch
reliability, even after display of
a fuel imbalance indication,
operators should understand
1. Proper fuel management.
2. Causes of in-flight fuelimbalance.
3. Fuel imbalance indications.
4. Fuel imbalance procedures.
5. Airplane dispatch after
display of fuel imbalance
indication.
6. Resources for addressing
in-flight fuel imbalance.
W
737-600 FUEL CAPACITY
Tanks Gallons Liters
Main 1 1,261 4,773
Main 2 1,261 4,773
Center 4,175 15,804
Total 6,697 25,350
TYPICAL FOUR-ENGINEFUEL TANK CONFIGURATION2
FIGURE
747-400 FUEL CAPACITY
Tanks Gallons Liters
Reserve 2,644 10,008
Main 1 and 4 8,964 33,932
Main 2 and 3 25,092 94,984
Center wing 17,164 64,973
Optionalstabilizer 3,300 12,492
Total 53,864 203,897
Total withoptional tank 57,164 216,389
TYPICAL THREE-ENGINEFUEL TANK CONFIGURATION3
FIGURE
MD-11 FUEL CAPACITY
Tanks Gallons Liters
Main 1 and 3 12,216 46,242
Main 2 9,622 36,423
Center wing 14,729 55,755
Stabilizer 1,959 7,416
Total 38,526 145,836
Main 3
Stabilizer
Main 2
Main 2
Main 1
Reserve 3Main 4
Main 3
Main 1
Main 2
Main 1
Optionalstabilizer
Main 2
Reserve 2
Center wing
Center
Center wing
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d i ffe rent rates of eng i ne fuel burn,i nt ernal fuel system fa i l ure s, or fuel
system or struc t ural fa u l t s. Severa lother conditions inthe following list
were identified from maintenance inves-tigations of reported fuel imbalance
events. Some imbalances were causedby a combination of these conditions:
Variations in fuel pump outputpressure between fuel tanks.
Variations in fuel requirements
between the left and right engines.
Fuel pump check valve failures.
Internal manifold and coupling
leaks.
Center wing tank jettison transfer
valve failures (747).
Hydromechanical scavenge system.
Debris and contamination inthe tanks.
Fuel quantity indication system(FQIS) anomalies.
Structural fuel leaks or transferbetween tanks.
Maintenance troubleshooting can berelatively easy on two-engine airplanes
because the fuel system can be isolatedto the left and right sides of the air-
plane. However, it can be much moredifficult to isolate the cause to any
p a r t icular compone nt on the 747,a nd it may be necessary toenter the fuel tank to determine
t he cause of the imbalanc e.
FUEL IMBALANCEINDICATIONS
On out-of-production airplanes
such as the 727 or 747-200,
the flight engineer managed
fuel use and monitored theairplane for fuel imbalance
conditions to keep imbalancesto a minimum. With the intro-
duction of the two-crewmember
flight deck, which is standard
on all current-production Boeing
airplanes, fuel system automa-
tion was incorporated to relieve
the flight crew of most fuel
management tasks. Fuel use ismonitored electronically by the
FQIS, fuel management system,
or flight deck indication system. Thesesystems monitor fuel usage and annun-
ciate a fuel imbalance condition in theflight deck when the imbalance reaches
a specific value (fig. 4). No action isrequired by the flight crew unless a
fuel imbalance indication is displayed,which the flight crew should address
on a time-available basis in accordancewith operations manual procedures(see table 1 on p. 6).
The amount of fuel imbalance allowed
b e fo re the ind ication is displayed
m i nimizes additional fuel consumptioncaused by lateral trim drag and limits
t he amo u nt of fuel balanc ing that theflight crew must accomplish. As the
fuel becomes unbalanced, lateral trim
is re q u i red to ma i ntain wing s - l e v e lf l ig ht. The lateral trim re q u i re me nt
increases airplane drag and cons e-quently increases fuel c ons um pt io n.
Waiting until the indication to balance
fuel is displayed limits the numberof t imes the fuel must be balanced
without significantly increasing fuelconsumption.
An indicated fuel imbalance does notaffect the ability of the airplane to
safely complete its scheduled flight.
The flight crew should accomplish the
fuel imbalance procedure in a timely
manner, but lateral control capabilityis not signi fic antly affected by an
i nd icated fuel imbalance. For example,approximately one-quarter to one-half
of a unit of aileron trim is required onthe 747-400 when a fuel imbalance
message is first displayed. The flight
crew has sufficient capability to controllateral imbalances much greater than
the allowable indication.
FUEL IMBALANCE PROCEDURES
If a fuel imbalance indication is dis-
played and the flight crew does not
suspect a fuel leak or confirms that afuel leak does not exist, the flight
crew should balance fuel using fuel
b a l a nc i ng pro c e du re s. If the fuel
imbalance indication occurs again in
flight, the balancing procedures shouldbe accomplished again.
After landing, an explanation for thein-flight fuel imbalance, such as a fuelleak or an in-flight engine shutdown,
may be obvious. If a component failurein the fuel system caused the fuel
i m b a l a nc e, the re q u i red ma i nt e na nc e
a c t ion may also be obvio u s. Mo s tfrequently, however, the fuel imbalance
c o nd i t ion no longer exists on the
g ro u nd, and a cause is not obvio u s.
In these cases, it is difficult to deter-
mine what maintenance action, if any,
should be taken. Some operators have
reported that very detailed trouble-
shooting, including removing
an airplane from service to
enter the fuel tank for inspec-tion, has failed to identify a
cause. Improved AircraftMaintenance Manual (AMM)and
Fault Isolation Manual (FIM)
procedures and guidelines forestablishing dispatch policies
have been developed to assist
operators in determining the
appropriate action after anindication of in-flight fuel
imbalance.
An in-flight fuel imbalance
c o ndi tion can occur on any
mo del when fuel is being
used from the wing tanks.
Most re c e nt reports of fuel
imbalance indication, however,
have been on the 747-400 and
777. These airplanes typically
FUEL IMBALANCE INDICATIONS 777 EICAS FUEL SYNOPTIC4
FIGURE
3
4
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FUEL IMBALANCE INDICATIONS1
TABLE
M o d e l
7 3 7 - 6 0 0 / - 7 0 0 /- 8 0 0 / - 9 0 0
7 4 7 - 4 0 0
7 5 7
7 6 7
7 7 7
M D - 1 1
M D - 9 0
M e s s a g e
I M B A L( D i s c rete amber light on
round-dial quantityi n d i c a t o r )
FUEL IMBAL 1-4(EICAS advisory message)
FUEL IMBAL 2-3(EICAS advisory message)
FUEL CONFIG(EICAS advisory message)
FUEL CONFIG(EICAS advisorym e s s a g e / l i g h t )
FUEL IMBALANCE(EICAS advisory message)
L AT FUEL UNBAL( A l e rt message)
L AT FUEL UNBAL( A l e rt message)
Fuel quantity condition( d i ff e re n c e )
1,000 lb (450 kg) or morebetween main tanks
3,000 lb (1,360 kg) or morebetween main tanks 1 and 4
6,000 lb (2,720 kg) or morebetween main tanks 2 and 3
Approximately 2,000 lb (900 kg)or more between main tanks
Approximately 2,000 lb (900 kg)or more between main tanks
Main tank quantities differby a pre d e t e rmined amount(see comments)
4,000 lb (1,800 kg) or morebetween tanks 1 and 3
1,500 lb (675 kg) or morebetween right and left tanks
C o m m e n t s
Illuminated amber at di ff e rence >1,000 lb(450 kg). Displayed below main tank with
lower fuel quantity. Fuel quantity arc anddigits on tank with lower fuel quantity turna m b e r. Inhibited when airplane on gro u n d .Inhibited by fuel LOW indication when bothindications exist. Displayed until imbalanced e c reases to 200 lb (90 kg).
Message no longer displayed at diff e re n c e15 sec.
A l e rt is a customer option on some a irplanes.
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fly long routes that provide more timefor a significant fuel imbalance to
develop. On the 747-400, changes tothe fuel system to accommodate the
automatic fuel management system andthe implementation of fuel imbalance
indications have resulted in more fuelimbalance reports than on other 747
models. On the 777, fluctuations inthe main tank fuel quantity indicationshave caused indicated fuel imbalance
conditions, even though the actualf u e lq ua nt i t ies re ma i ne d b a l a nc e d.
An O p erations Ma nual Bulletin andS e r v ice Bulletin have been issued fo rthis condi tion.
AIRPLANE DISPATCHAFTER DISPLAY OF FUELIMBALANCE INDICATION
D e te r m ini ng the cause of a rep o r t e d
i n -fl ight fuel imbalance is often diffi-cult because the cond i t ion ge ne ra l l y
does not exist on the gro u nd duri ngt rou b les ho o t i ng. Several components
or a combina tion of components mu s tbe conside re d. If mai ntena nce actionfails to isolate the cause of thei m b a la nc e, a de c i s ion about whether
to conti nue to dispatch the airplane,e sp e c ially if the imbalance has beenreported duri ng several flig ht s, mu stbe made. Flight crews may be reluctant
to accept an airplane that has ah i story of in-flig ht fuel imbalanc eo c c urrences if the cause has not been
ide nt i f ie d.
Bo e i ng re co m me nds that opera t orsdevelop their own polic ies re g a rdi ng
a i r pl a ne dispatch after display of afuel imbalance indication. The follow-i ng guide l i nes can assist operators
in de v e l o p i ng these polic ie s. The s eguidelines allow for some flexibility in
planning and performing the requiredmaintenance to minimize the effect on
scheduled operations.
Operators should conduct a case-by-
case review of each fuel imbalanceevent before deciding whether to con-
tinue dispatching an airplane. Somefactors to consider include the amount
of imbalance, the airplanes history offuel imbalance, and the possible cause.
5
The following list contains events thatcan cause fuel imbalance indications
and the resulting recommended actions:
External fuel leak.
The fuel leak must be corrected before
further flight.
Internal fuel leak from apressurized fuel line.
It is possible for one tank to overfilland spill fuel overboard through the
fuel vent system. This condition shouldbe corrected before further flight.
Internal fuel leak not from apressurized fuel line or a systemcomponent failure.
Appropriate maintenance and MinimumEquipment List procedures should befollowed. Continued operation of the
airplane may be acceptable with cor-rective action scheduled at the earliest
available maintenance opportunity.
Acceptable operational variationsin system components (e.g., fuelpumps, engine fuel flows, checkvalves).
Continued operation of the airplane isacceptable.
Undetermined cause.
If following appropriate maintenance
procedures has failed to identify acause, but no external fuel leaks,internal fuel leaks from a pressurized
line, or other system componentfailures are indicated, continued oper-ation of the airplane may be acceptable.However, this is acceptable only untilthe earliest available maintenanceopportunity when mo re detailedtroubleshooting can be performed.
Operators should also consider the
following when determining whetherto dispatch an airplane that has
d i s played fuel imbalance indications:
Display of a fuel imbalance indica-tion does not affect the ability ofthe airplane to safely complete a
flight. Existing procedures shouldenable the flight crew to continue
to the original destination ordivert to an alternate airport.
Display of a fuel imbalance indica-tion during a particular flight does
not mean immediate maintenancetroubleshooting is required.
Flight crews should not balance
fuel unless a fuel imbalanceindication is displayed.
If a fuel imbalance indication isdisplayed and the flight crew does
not suspect a fuel leak or confirms
that a fuel leak does not exist, theflight crew can balance fuel and
observe the fuel imbalance rates.
Data on fuel imbalance rates often
proves to be very valuable for
maintenance troubleshooting.
Maintenance personnel should trackand monitor reports of fuel imbalance
messages. Detailed troubleshooting
using the AMM and FIM should be
planned to determine and correct the
cause of the fuel imbalance. This mayrequire maintenance personnel to
enter the fuel tank.
RESOURCES FOR ADDRESSINGIN-FLIGHT FUEL IMBALANCE
All current-production Boeing airplanes
have indication systems that alert theflight crew when a fuel imbalance
reaches a specific value. Several opera-
tors have asked Boeing to clarify the
appropriate flight crew, maintenance,and dispatch procedures for trouble-shooting an in-flight fuel imbalance
indication. As a result, Boeing has pub-
lished revised AMM and FIM proceduresfor these indications. The company has
also published information on fuelimbalance indications for flight crews
and maintenance personnel.
B o e i ng has been working with oper-ators to address ma ny of the kno w n
fuel system anomalies responsible for
past fuel imbalance events. Thesesolutions include system and component
redesign, enhanced troubleshootinga p p ro a c hes in the AMM and FIM,
s e r v ice bulletins, service letters,
ma i ntenance tips, and supplementalinformation, including Flight Operations
Review letters and Operations ManualBulletins (see table 2 on p. 8).
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FUEL IMBALANCE DOCUMENTATION2
TABLE
M o d e l
7 3 7 - 6 0 0 / - 7 0 0 /- 8 0 0 / - 9 0 0
7 4 7 - 4 0 0
7 5 7
7 6 7
7 7 7
M D - 1 1
M D - 9 0
D o c u m e n t
FIM 28-21-00
AMM 28-26-00
Operations Manual 12.10FIM 28-20-01
FRM 28-22-57
FOR 747-32
7 4 7 - S L - 2 8 - 0 9 1
Operations Manual 12.30
Operations Manual NNC.12
SL 757-28-028
FIM 28-41-00
Operations Manual 12.30Operations M anual NNC.12
FIM 28-41-00
MT 767-28-014
MT 767-28-018
Operations Manual 12.30
Operations Manual NNC.12
SB 777-28-0014
SB 777-28-0015
MT 777-28-008
FIM 28-22
AMM 28-21-00AMM 28-22-15
OMB - (XX) (customized no.)
Operations Manual 12.30
Operations Manual NNC.12
FCOM VOL II FUEL-ABN-11-11/12
FIM 05-51-08
FCOM VOL II ACD FUEL-ABN-10-6
FIM 05-51-07
S u b j e c t
Fuel transfer tro u b l e s h o o t i n g
Tank-to-tank fuel transfer
Fuel alert indicationsFuel imbalance tro u b l e s h o o t i n g
Fault re p o rting codes for the flight crew toadvise maintenance personnel
Flight operations review on fuel imbalance
S e rvice letter on fuel imbalance
Fuel EICAS indications
N o n n o rmal checklist
I n a d v e rtent fuel transfer or fuel imbalancecaused by misaligned fuel valves
Fuel transfer tro u b l e s h o o t i n g
Fuel EICAS indicationsN o n n o rmal checklist
Fuel transfer/configuration tro u b l e s h o o t i n g
Maintenance tip on troubleshooting unwantedfuel transfers from main to center tank
Troubleshooting fuel vent float valve
Fuel EICAS indications
N o n n o rmal checklist
Fuel scavenge pump inlet re p l a c e m e n t
Fuel quantity processor unit softwarei n s t a l l a t i o n
Maintenance tip to troubleshoot uncommandedfuel transfers and imbalances
Fuel imbalance tro u b l e s h o o t i n g
Fuel feed manifold leak checks
FQIS fluctuations
Fuel EICAS indications
N o n n o rmal checklist
Lateral fuel imbalance
Lateral imbalance inspection
Lateral fuel imbalance
Lateral imbalance inspection
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A fuel imbalance cond i t ion can
develop in flight for several reasons,
a nd the subsequent ma i nt e na nc e
a c t io ns re q u i red to correct the
reported cond i t ion are not always
obvious. Fuel imbalance indications
have been established to annunciate
such a condition to the flight crew,
who can then balance fuel in flight if
necessary. These actions can minimize
airplane drag and fuel burn. Existing
o p e ra t io nal pro c e du res allow the
flight crew to safely complete the
a ir p l anes sche duled flig ht if fuel
imbalance occurs. Improved AMM and
FIM procedures are also available, a sare guidelines to help operators
determine the appropriate actions
following an in-flight fuel imbalance
report and establish their own
d i spatch policies.
S U M M A RY