OPERATIONS MANUAL

Fuel System
Fuel Pumps:
Main boost pumps are installed in all four main tanks. Each main boost
pump is AC motor driven and provides sufficient fuel pressure to one engine
during takeoff, or to two engines in cruise. If there is no pressure in the
manifold, check valves automatically open, allowing fuel to bypass the inactive
pumps so that the engines can suction the fuel from the tanks directly.
However, suctioning alone cannot produce the same high fuel flow as the tank
pumps can, hence thrust in the takeoff range is then no longer available.
Override/jettison pumps (O/J pumps) are installed in the inboard main
tanks and in the center tank. Each O/J pump is driven by an AC motor and
provides sufficient fuel to two engines in all flight phases. To ensure the
inboard main tanks will not be completely emptied during jettison, the O/J
pump inlets are located at a slightly higher point in these tanks; that is, the
O/J pumps in an inboard main tank become inoperative at a standpipe level
of ca. 3200 kg (7000 lb). The O/J pumps in the center tank stop operating at
a standpipe level of ca. 900 kg (2000 lb).—The fuel pressure produced by the
O/J pumps is greater than that of the main boost pumps; when main boost
pumps and O/J pumps are powered at the same time, the fuel output of the
main boost pumps is hydromechanically overridden so that only the O/J
pumps provide fuel to the engines.—During jettison, the O/J pumps transfer
fuel into the jettison manifold.
Older aircraft are equipped with an electric scavenge pump powered by AC
bus 1. The electric scavenge pump starts automatically when any reserve tank
transfer valve is open and no fuel ballast is required, or when low pressure
is detected in one of the O/J pumps in the center tank. It can also be started
manually with the CWT SCAVENGE PUMP switch on the maintenance
panel. The electric scavenge pump transfers the remaining center tank fuel
into main tank 2. This may lead to a faintly asymmetrical fuel distribution.
However, aircraft fitted with an electric scavenge pump also use an older APU
fuel feed system which connects the APU to main tank 2 only. Hence a slight
imbalance in the opposite direction may already exist before the scavenging
begins; the scavenging will then rebalance it.
In newer aircraft, four hydromechanically driven jet pumps are installed
for scavenging the center tank. They pump the fuel symmetrically into main
tanks 2 and 3. The system starts automatically when the center tank quantity
decreases to ca. 1820 kg (4000 lb). Manual control is not possible. Aircraft
fitted with this hydromechanical system also have a newer APU fuel feed
system which supplies the APU symmetrically from main tanks 2 and 3.
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