An indicated fuel imbalance
does not affect 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 capability is
not significantly affected by an indicated fuel imbalance. For example,
approximately one-quarter to one-half of a unit of aileron trim
is required on the 747-400 when a fuel imbalance message is first
displayed. The flight crew has sufficient capability to control
lateral imbalances much greater than the allowable indication.
FUEL IMBALANCE PROCEDURES
If a fuel imbalance indication is displayed and the flight crew
does not suspect a fuel leak or confirms that a fuel leak does not
exist, the flight crew should balance fuel using fuel balancing
procedures. If the fuel imbalance indication occurs again in flight,
the balancing procedures should be accomplished again.
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.