b. Standby Fuel Pumps. A submerged,
electrically-operated standby fuel pump, located within
each nacelle tank, serves as a backup unit for the
engine-driven boost pump. The standby pumps are
switched off during normal system operations. A
standby fuel pump will be operated during crossfeed
operation to pump fuel from one nacelle tank to the
opposite engine. The correct pump is automatically
selected when the CROSSFEED switch is activated.
Each standby fuel pump has an inertia switch included in
the power supply circuit. When subjected to a 5 to 6 g
shock loading, as in a crash situation, the inertia switch
will remove electrical power from the standby fuel
pumps. The standby fuel pumps are protected by two
10-ampere circuit breakers placarded STANDBY PUMP
#1 or #2, located on the overhead circuit breaker panel
(fig. 2-9), and four 5-ampere circuit breakers (two each
in parallel) on the hot battery bus.
c. Fuel Transfer Pumps. The auxiliary tank fuel
transfer system automatically transfers the fuel from the
auxiliary tank to the nacelle tank without pilot action.
Motive flow to a jet pump mounted in the auxiliary tank
sump is obtained from the engine fuel plumbing system
downstream from the engine driven boost pump and
routed through the transfer control motive flow valve.
The motive flow valve is energized to the open position,
by the control system, to transfer auxiliary fuel to the
nacelle tank to be consumed by the engine during the
initial portion of the flight. When an engine is started,
pressure at the engine driven boost pump closes a
pressure switch, which, after a 30 to 50 second time
delay to avoid depletion of fuel pressure during starting,
energizes the motive flow valve. When auxiliary fuel is
depleted, a low level float switch de-energizes the
motive flow valve after a 30 to 60 second time delay.
This time delay function prevents cycling of the motive
flow valve due to sloshing fuel. If the motive flow valve
or the associated control circuitry fails, the loss of motive
flow pressure when there is still fuel remaining in the
auxiliary fuel tank is sensed by a pressure switch which
illuminates a caution annunciator placarded #1 NO
FUEL XFR or #2 NO FUEL XFR. During engine start,
the pilot should note that the NO FUEL XFR annunciator
extinguishes 30 to 50 seconds after engine start. The
NO FUEL XFR annunciator will not illuminate if auxiliary
tanks are empty. A manual override is incorporated as a
backup for the automatic transfer system. Manual
override is initiated by placing the AUX XFER switch,
located in the fuel management panel, to the OVRD
position. This will energize the transfer control motive
flow valve. The transfer systems are protected by 5-
TRANSFER #1 or #2, located on the overhead circuit
breaker panel (fig. 2-9).
In turbulence or during maneuvers,
the NO FUEL XFR annunciators may
auxiliary fuel has completed transfer.
d. Fuel Gaging System. Fuel quantity is
measured by a capacitance type fuel gaging system.
Two fuel gages, one for the left and one for the right fuel
system, read fuel quantity in pounds. A maximum of 3%
error may be encountered in each system; however, the
system is compensated for fuel density changes due to
temperature excursions. In addition to the fuel gages,
amber #1 NAC LOW or #2 NAC LOW annunciators on
the caution/advisory annunciator panel illuminate when
there is approximately 30 minutes (approximately 58
gallons) of fuel per engine remaining (on standard day,
at sea level, maximum cruise power consumption rate).
The low fuel annunciators are level-
sensing volumetric devices and are
not compensated for fuel density,
attitude, fuel flow, etc.
The fuel gaging system is protected by individual 5-
ampere circuit breakers placarded QTY IND and FUEL
QTY, #1 and #2, located on the overhead circuit breaker
panel (fig. 2-9). A mechanical spiral float gage (fig. 2-23)
is installed in each auxiliary fuel tank to provide an
indication of fuel level when servicing the tank. The
gage is installed on the auxiliary fuel tank cover,
adjacent to the filler neck. A small sight window in the
upper wing skin permits observation of the gage.
management panel is located overhead in the cockpit
between the pilot and copilot. It contains the fuel gages,
standby fuel pump switches, crossfeed valve switch, fuel
gaging system control switch, and transfer control
(1) Fuel gaging system control switch. A
switch on the fuel management panel (fig. 2-20)
placarded FUEL QUANTITY, MAIN - AUXILIARY,
controls the fuel gaging system. When the switch is in
the MAIN position, the fuel gages read the total fuel
quantity in the left and right main fuel systems. When
the switch is in the AUXILIARY position, the fuel gages
read the fuel quantity in the left and right auxiliary tanks
(2) Standby fuel pump switches. Two
switches, placarded STANDBY PUMP - ON, located on
the fuel management panel (fig. 2-20), individually
corresponding nacelle tank. During normal aircraft
operation both switches should be OFF, so long as the
engine-driven fuel pumps are operative.