TM 1-1520-238-T-7
10–35
10–7.
SYSTEM DESCRIPTION (cont)
10–7
c. Fuel Crossfeed/Boost System.
(1) Purpose.
The purpose of the fuel crossfeed/boost system is to provide fuel for engine starting and
operation.
(2) System Operation (fig. 10–13).
(a) During engine start, the BOOST switch on the pilot’s FUEL panel is set to ON, or the ORIDE switch
is set to CPG and BOOST switch is set to ON on the CPG FUEL panel. When boost operation is selected,
28 VDC is supplied to the fuel boost pump shutoff valve to supply pressurized air to drive the fuel boost pump.
The fuel boost pump is driven by 19 psi of pressurized air from the PAS through the fuel boost pump shutoff valve.
The fuel boost pump pressurizes the fuel in the aft fuel cell for use during engine starting and high altitude
operations.
(b) During boost pump operation, boost pump suction draws fuel through an inlet screen on the bottom
of the 4-way check valve. Fuel is drawn into the boost pump, then pressurized and pumped through the 4-way
check valve. The output of the 4-way check valve supplies pressurized fuel to the fuel pressure switch, APU
shutoff valve, right fuel crossfeed/shutoff valve, and left fuel crossfeed/shutoff valve.
(c) The fuel pressure switch provides the pilot with an indication of boost pump operation. When the fuel
pressure reaches 8.5 to 10 psi, the switch closes, lighting the BOOST PMP ON indicator on the pilot’s
caution/warning panel.
(d) The left and right fuel crossfeed/shutoff valves (fig. 10–14) are 3-way, 4-position, electronically
controlled valves which select a fuel source for engine 1 and 2 and provide positive fuel shutoff during
emergencies. During engine start, the crossfeed/shutoff valves are electronically positioned to supply fuel to
engines 1 and 2 from the aft fuel cell.
(e) The APU fuel shutoff valve controls fuel flow to the APU fuel boost pump (fig. 10–13). When the
APU START/RUN/OFF switch on the pilot’s BTL DISCHARGE/APU panel is set to START or RUN, 28 VDC is
applied to the APU fuel shutoff valve and the APU fuel boost pump. The APU fuel boost pump is a 28 VDC motor
operated pump, which draws fuel through the APU shutoff valve from the aft fuel cell, pressurizes the fuel to 10 3
psi, and supplies pressurized fuel to the APU. When the APU FIRE PULL handle is pulled, 28 VDC is removed
from the APU fuel shutoff valve and the APU fuel boost pump, discontinuing fuel flow.
(f) During normal operation, pressurized nitrogen rich air from the NIU pressurizes the aft and forward
fuel cells to 25 3 psi. Nitrogen reduces the level of oxygen in the fuel cells so that combustion will not be
supported. Fuel is forced through the 4-way fuel supply check valve to the left and right crossfeed/shutoff valves.
(g) The left fuel crossfeed/shutoff valve selects a fuel source for engine 1 (fig. 10–14). With ENG 1
switch on the pilot’s FUEL panel set to OFF and CROSSFEED switch set to NORM, fuel is not supplied to engine
1. When ENG 1 switch is set to ON, fuel is supplied to engine 1 from the forward fuel cell. When CROSSFEED
switch is set to FWD TK, fuel is supplied to engine 1 from the forward fuel cell. When CROSSFEED switch is set
to AFT TK, fuel is supplied to engine 1 from the aft fuel cell. If the pilot or CPG ENG 1 FIRE PULL handle is
pulled, the engine 1 fuel supply is shutoff.
(h) The right fuel crossfeed/shutoff valve selects a fuel source for engine 2. With ENG 2 switch on the
pilot’s FUEL panel set to OFF and CROSSFEED switch set to NORM, fuel is not supplied to engine 2. When ENG
1 switch is set to ON, fuel is supplied to engine 2 from the aft fuel cell. When CROSSFEED switch is set to FWD
TK, fuel is supplied to engine 2 from the forward fuel cell. When CROSSFEED is set to AFT TK, fuel is supplied to
engine 2 from the aft fuel cell. If the pilot or CPG ENG 2 FIRE PULL handle is pulled, the engine 2 fuel supply is
shutoff.
