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737-300 - 737-500 The fuel balance limit also indicates that imbalances beyond the current state may result in increased trim drag and higher fuel consumption. The Fuel Balancing supplementary procedure should be accomplished when the fuel balance limit is reached.
737-600 - 737-900 The primary purpose for fuel balance alerts are to inform the crew that imbalances beyond the current state may result in increased trim drag and higher fuel consumption. The IMBAL NNC should be accomplished when the fuel balance alert is received.
There is a common misconception among flight crews that the fuel crossfeed valve should be opened immediately after an in-flight engine shutdown to prevent fuel imbalance. This practice is contrary to Boeing recommended procedures and could aggravate a fuel imbalance. This practice is especially significant if an engine failure occurs and a fuel leak is present. Arbitrarily opening the crossfeed valve and starting fuel balancing procedures, without following the checklist, can result in pumping usable fuel overboard.
Copyright . The Boeing Company. See title page for details.
8.20 FCT 737 (TM) October 31, 2006

737 Flight Crew Training  Manual
The misconception may be further reinforced during simulator training. The fuel pumps in simulators are modeled with equal output pressure on all pumps so opening the crossfeed valve appears to maintain a fuel balance. However, the fuel pumps in the airplane have allowable variations in output pressure. If there is a sufficient difference in pump output pressures and the crossfeed valve is opened, fuel feeds to the operating engine from the fuel tank with the highest pump output pressure. This may result in fuel unexpectedly coming from the tank with the lowest quantity.
Fuel Balancing Considerations
The crew should consider the following when performing fuel balancing procedures:
.  
use of the Fuel Balancing Supplementary Procedure in conjunction with
good crew coordination reduces the possibility of crew errors

.  
routine fuel balancing when not near the imbalance limit increases the
possibility of crew errors and does not significantly improve fuel
consumption

.  
during critical phases of flight, fuel balancing should be delayed until
workload permits. This reduces the possibility crew errors and allows
crew attention to be focused on flight path control

.  
fuel imbalances that occur during approach need not be addressed if the
reason for the imbalance is obvious (e.g. engine failure or thrust
asymmetry, etc.).

 

Fuel Leak
Any time an unexpected fuel quantity indication, FMC fuel message, or imbalance condition is experienced, a fuel leak should be considered as a possible cause. Maintaining a fuel log and comparing actual fuel burn to the flight plan fuel burn can help the pilot recognize a fuel leak.
Significant fuel leaks, although fairly rare, are difficult to detect. The NNC assumes the leak is between the strut and the engine. There is no specific fuel leak annunciation on the flight deck. A leak must be detected by discrepancies in the fuel log, by visual confirmation, or by some annunciation that occurs because of a leak. Any unexpected change in fuel quantity or fuel balance should alert the crew to the possibility of a leak. If a leak is suspected, it is imperative to follow the NNC.

Low Fuel
737-300 - 737-500 A low fuel condition exists when the indicated fuel quantity in either main tank is 1,000 pounds/453 kilograms of fuel or less.
Copyright . The Boeing Company. See title page for details.
October 31, 2006 FCT 737 (TM)  8.21

737 Flight Crew Training  Manual
737-600 - 737-900
A low fuel condition exists when the fuel LOW indication is displayed.

Approach and Landing
In a low fuel condition, the clean configuration should be maintained as long as possible during the descent and approach to conserve fuel. However, initiate configuration changes early enough to provide a smooth, slow deceleration to final approach speed to prevent fuel from running forward in the tanks.
A normal landing configuration and airspeed appropriate for the wind conditions are recommended.
Runway conditions permitting, heavy braking and high levels of reverse thrust should be avoided to prevent uncovering all fuel pumps and possible engine flameout during landing roll.
Go-Around
If a go-around is necessary, apply thrust slowly and smoothly and maintain the minimum nose-up body attitude required for a safe climb gradient. Avoid rapid acceleration of the airplane. If any wing tank fuel pump low pressure light illuminates, do not turn the fuel pump switches off.

Hydraulics
Proper planning of the approach is important. Consideration should be given to the effect the inoperative system(s) has on crosswind capabilities, autoflight, stabilizer trim, control response, control feel, reverse thrust, stopping distance, go-around configuration and performance required to reach an alternate airfield.
　
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