After landing, an explanation
for the in-flight fuel imbalance, such as a fuel leak or an in-flight
engine shutdown, may be obvious. If a component failure in the fuel
system caused the fuel imbalance, the required maintenance action
may also be obvious. Most frequently, however, the fuel imbalance
condition no longer exists on the ground, and a cause is not obvious.
In these cases, it is difficult to determine what maintenance action,
if any, should be taken. Some operators have reported that very
detailed troubleshooting, including removing an airplane from service
to enter the fuel tank for inspection, has failed to identify a
cause. Improved Aircraft Maintenance Manual (AMM) and Fault
Isolation Manual (FIM) procedures and guidelines for establishing
dispatch policies have been developed to assist operators in determining
the appropriate action after an indication of in-flight fuel imbalance.
An in-flight fuel imbalance
condition can occur on any model when fuel is being used from the
wing tanks. Most recent reports of fuel imbalance indication, however,
have been on the 747-400 and 777. These airplanes typically fly
long routes that provide more time for a significant fuel imbalance
to develop. On the 747-400, changes to the fuel system to accommodate
the automatic fuel management system and the implementation of fuel
imbalance indications have resulted in more fuel imbalance reports
than on other 747 models. On the 777, fluctuations in the main tank
fuel quantity indications have caused indicated fuel imbalance conditions,
even though the actual fuel quantities remained balanced. An Operations
Manual Bulletin and Service Bulletin have been issued for this condition.
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