Joined: 11 Dec 2005
|Posted: Sun Mar 02, 2008 10:58 am Post subject: Fuel Balancing in 737-300
|The primary purpose of fuel balance limitation on Boeing airplanes is for the structural life of the airframe and landing gear and not for controllability. A reduction in structural life of the airfarme or the landing gear can be caused by frequently operating with out-of-limit fuel balance conditions. Lateral control is not significantly affected when operating with fuel beyond normal balance limits. The fuel balance limit also indicates that imbalances beyond the current state may result in increased trim drag and higher fuel consumption.
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.
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 the highest pump output pressure. This may result in fuel unexpectedly coming from the tank with the lowest quantity.
Crew should consider the following when following fuel balancing procedures:
- Use of Fuel Balancing Supplementary Procedures in conjunction with good crew coordination reduces the possibilty 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 possibilty of crew errors and allows crew attention to be focussed 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.)