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chamber.
From the pressure chamber, fuel flows at low pressure to the
following components:
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-28-00
Page 17
October 11/01
Revision 5
(a) An engine fuel feed lines to its respective engine. Fuel flow
through the line is controlled by a normally open shutoff valve
installed between the boost pumps and engine-driven
pumps.
(b) A crossflow line interconnected to the opposite side pump
manifold. Fuel flow through the line is controlled by a
normally closed shutoff valve located in the left fuel hopper.
(c) A motive flow line connected to the fuel ejector. Fuel for the
APU is tapped from the left side motive flow line.
(d) Pump suction bypass lines which extend down to the bottom
of the hoppers. These lines are fitted with a check valve
normally held closed by boost pump pressure. If both boost
pumps should fail, the engine can draw fuel directly from the
hopper, using the suction bypass lines, bypassing the
inoperative pumps.
(2) Pump Operation:
Depressing a MAIN PUMPS switch (cockpit overhead panel, FUEL
SYSTEM section) supplies 28 VDC from the Essential DC bus to the
associated pump’s power relay. The relay then closes to supply 28
VDC operating power from the Essential DC bus to the pump’s
inverter. The inverter then supplies AC power to the pump motor.
The amber OFF legend in the switch extinguishes.
Similarly, depressing an ALT PUMPS switch supplies 28 VDC from
the Right Main DC bus to the associated pump’s power relay. The
relay then closes to supply 28 VDC operating power from the Right
Main DC bus to the pump’s inverter. The inverter then supplies AC
power to the pump motor. The amber OFF legend in the switch
extinguishes.
(3) Pump Malfunctions:
If a boost pump fails to develop 16 psi after switch selection, a pump
pressure switch energizes a fuel boost pump relay. The relay then
triggers the associated amber L-R MAIN FUEL FAIL or L-R ALT
FUEL FAIL caution message on CAS.
If fuel pressure at the engine-driven high pressure fuel pump inlet is
less than 15 psi, or both boost pumps on one side are selected OFF
(or should fail) with the crossflow (X FLOW) valve closed, a
pressure switch at the high pressure fuel pump inlet triggers the
associated red L-R FUEL PRESS LOW warning message on CAS
and on the ENGINE START system page. A respective red L / R
FUEL PRESS annunciator also illuminates on the Standby Warning
Lights Panel (SWLP), if installed. This message can occur
regardless of engine speed and output pressure of the enginedriven
low pressure fuel pump. In addition to serving as a warning to
check and set the FUEL SYSTEM panel configuration, this message
could serve as an early predictor of a fuel leak, in which case the
flight crew should be alert for erratic fuel flow indications.
(4) Autochange Circuit:
Should an operating boost pump on one side fail, an autochange
circuit is incorporated to automatically select the other pump on that
OPERATING MANUAL
2A-28-00 PRODUCTION AIRCRAFT SYSTEMS
Page 18
October 11/01
Revision 5
side. The autochange circuit is powered by the Essential DC bus
and consists of two parts: control monitor relays and transfer relays.
The first part of the autochange circuit incorporates a control monitor
relay for each boost pump. If the selected pump loses power or its
power circuit breaker opens, the control monitor relay automatically
selects the opposite boost pump on by energizing that pump’s
power relay. If a boost pump’s power circuit breaker opens, an
amber CB legend will illuminate in that pump’s control switch.
The other part of the autochange circuit incorporates a transfer relay
for each boost pump. If a boost pump is selected on but fails to
develop 16 psi output, the transfer relay automatically selects the
opposite boost pump on by energizing that pump’s power relay. The
amber OFF legend will illuminate in the failed pump’s switch.
Whenever the autochange system activates a boost pump other
than the one selected, the switch corresponding to the nowoperative
boost pump must be selected on by the flight crew. The
switch corresponding to the now-inoperative boost pump should be
selected to OFF.
Restated in summary, the autochange circuit will automatically
select the opposite boost pump on the same side if:
• A main boost pump fails, even with the alternate boost pump
not selected on
• A main boost pump power circuit breaker opens with the
pump running due to actuation of the main pump control
　
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