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November 3, 2016 by
Fuel boost pumps
Fuel boost pumps are designed as a dual pump unit that scavenges fuel by use of an inducer, and pressurizes fuel by an impeller type pump. During operation the pump units are self cooled and lubricated by fuel flowing through the assembly.
Each main tank (#1/Left and #2/Right) has a FWD and AFT fuel boost pump supplying pressurized fuel to its respective engine, or to the fuel supply manifold for APU and/or X-feeding. These 115 VAC electrical fuel boost pumps produce a flow of ± 9.000 kg/h with a pressure of more than 10 PSI. The boost pumps LOW PRESSURE light illuminates when pump pressure drops below 6 PSI. A LOW PRESSURE light on one side will not straight away trigger the MASTER CAUTION circuit as there is an electrical redundancy on that side. Of course it does illuminate on RECALL, or when both boost pumps on one side produce low or no pressure.
A discharge check valve prevents fuel flow from the engine feed manifold through the pump preventing fuel transfer into that respective tank. A bypass valve allows a secondary fuel flow path to the engine when the boost pump is inoperative, providing a suction fuel feed capability by negative fuel pressure created by operation of the engine fuel pump.
The center tank has two fuel boost pumps which are connecting to the left-, and right side of the fuel supply manifold . These 115 VAC fuel boost pumps produce a flow of ± 12.700 kg/h with a pressure of 23 psi and higher.
A LOW PRESSURE light illuminates when the center tank boost pump switch is ON, and that boost pump output pressure drops below 22 PSI. The latest modification also triggers the MASTER CAUTION light with the respective annunciator on a single center tank boost pump LOW PRESSURE indication. Most of the current used center tank boost pumps are provided with an automatic shut-off when low pressure is sensed for 15 seconds. After an auto shut-off, it is possible to reset the circuit by selecting the respective switch to OFF and back to ON. When the fuel LOW PRESSURE light illuminates, there is a 10 second delay before the MASTER CAUTION is triggered.
Important to know that the center tank boost pump is NOT equipped with a bypass valve so it is not possible to suction feed from the center tank. When both pumps fail and there is fuel in the center tank you have to consider this as unusable fuel that needs to be added up to the ZFW.
The FCOM requires that the flight deck needs to be occupied when a center tank boost pump is operating related to the worst case, that a non modified center tank boost pump is installed without the auto-shutdown feature.
Due to the position of the center boost pumps scavenge pickup, it is possible that the LOW PRESSURE light flickers in a nose up attitude when there is low quantity in the center tank after takeoff. The FCOM states that if the quantity is below 453 Kgs, to switch OFF the center boost pumps until level flight.
When the center tank is empty and the LOW PRESSURE light flickers with intermittent fuel pick-up by the pump, it may take up to 5 minutes to illuminate the associated MASTER CAUTION annunciator.
Electrical power for the boost pumps comes from:
115 VAC XFR bus 1
– 1 FWD boost pump
– 2 AFT boost pump
– Left Center boost pump
115 VAC XFR bus 2
– 1 AFT boost pump
– 2 FWD boost pump
– Right Center boost pump
The circuit breakers for the pumps are not accessible as they are located on the Power Distribution Panel (P-92) in the E & E bay. (see Electrical Supplement)
dave davidson
this is the 737 system.... the larger boeing acft use an override boost pump for the center wing tanks. these are higher volume pumps that will overtake the pressure of the main tank boost pumps. you want to burn the center tank first. almost all acft also have a scavenge pump in the center ta...View More

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