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APU Fuel Feed System
The APU fuel feed system supplies fuel to the APU. This system includes an electric
boost pump located in the right main tank, a negative-G fuel supply from the left
engine's motive flow system, and two shutoff valves (APU fuel SOV and negative-G
SOV). For additional information, see Chapter 5 Auxiliary Power Unit.
Components and Operation
The components of the engine fuel feed system are as follows:
Collector Tanks
Two collector tanks, located at the lowest point of the center auxiliary tank, are
considered part of the main tank system. They receive fuel from their respective main
wing tank via gravity or scavenge ejector pump flow. Fuel from the collector tanks is
supplied to each engine-driven pump unit by main ejector pumps or electric boost
pumps.
Boost Pumps
Two DC electric boost pumps provide the engines with a standby fuel feed system.
They are available for engine starting, and as a backup fuel feed source if either main
ejector pump output pressure is too low to satisfactorily feed its respective engine. The
left boost pump is powered by the DC Battery Bus, and the right boost pump by DC
Bus 2. The boost pumps are located downstream of their respective collector tanks on
the fuel feed lines to the engine-driven pumps. During normal operation, the only pilot
action required is to arm both BOOST PUMP switches on the FUEL control panel
prior to engine start, and disarm them after shutdown.
When both BOOST PUMP switches are armed, the boost pumps are activated if low
pressure is sensed at either left or right main ejector pressure switch (this is the case
prior to engine start). If a single BOOST PUMP switch is armed, both boost pumps
will be activated if low pressure is sensed on the armed side’s main ejector pressure
switch. For example, if only the left BOOST PUMP switch is armed (pressed in), then
both pumps will activate if low pressure is sensed at the left main ejector output. The
FUEL SYSTEM
For Training Purposes Only
Sept 04
12-7
P I LOT T R A I N I N G GU I D E
boost pumps can provide fuel to both engine’s fuel feed systems through a crossover
feed line.
If a boost pump has been activated, sufficient pump output pressure is indicated by the
green ON legend in the switch/light. Insufficient output pressure is indicated by the
amber INOP legend, and the L(R) FUEL PUMP caution EICAS message.
Boost Pump Switch/Lights
Figure 12-5
Engine-Driven Pump
The engine-driven pump supplies high-pressure fuel to the engine Fuel Control Unit
(FCU) for combustion, and to the motive flow system to power the ejector pumps. The
engine-driven pump is located on the engine accessory gearbox, and operates
whenever the engine core (N2) is turning. For additional information, see Chapter 18
Power Plant.
Motive Flow
Motive flow is the continuous movement of fuel under pressure through the fuel feed
lines. Motive flow is generated by the engine-driven pump, and is sufficient to supply
the engine with its fuel intake needs through the on-side main ejector pump. If a
blockage occurs within the feed system, and motive flow fuel pressure drops (sensed
at the main ejector pump pressure switch), the electric boost pumps activate and
supply the engine with sufficient fuel for operation.
Ejector Pump Operation
Ejector pumps operate on the same principle as the venturi tube, and have no moving
parts. The high-pressure stream of fuel at the ejector’s motive flow nozzle induces a
large volume of fuel to flow into the intake of the ejector. The fuel then enters the feed
lines and flows under pressure.
P604_12_018
FUEL
BOOST PUMP
L R
ON
INOP
ON
INOP
FUEL SYSTEM
12-8 For Training Purposes Only
May 03
P I LOT T R A I N I N G GU I D E
Main Ejector and Ejector Operation
Figure 12-6
Motive Flow
From
Engine-Driven
Pump
To Engine
Fuel Feed
Line
Screen
(typ)
Inlet Check Valve – Fitted to transfer
ejector only to prevent main tank fuel
draining into aux tank when A/C is on
ground with engines not running
BOTTOM OF TANK
Vent
Motive
Flow
Collector
Tank
Screen
Supply from
Scavenge
Ejector
Supply to
Engine
Boost
Fuel Pump
Check
Valve
Main
Ejector
Main
Engine
Feed
Gravity
Feed
MAIN
EJECTOR
P604_12_021
FUEL SYSTEM
For Training Purposes Only
May 03
12-9
P I LOT T R A I N I N G GU I D E
Main Ejector Pump
There are two main ejector pumps (left and right) located on top of their respective
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