fuel imbalance

7/25/2019 Fuel Imbalance

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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

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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


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


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.

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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



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.)



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 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



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

fuel imbalance

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