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5 is uncovered, both box-beam tanks are vented and
the sump tank interconnect valve is opened. The two
groups become a common system and will seek a
common level to equalize the static pressure head.
Fuel will flow through the open sump tank interconnect
valve only as a function of differential pressure.
With open vent valves, the fuel in both box-beam
tanks has a positive vent pressure, forcing the fuel
into the respective sump tank through interconnect A
or B.
Fuel in the sump tank is picked up by the turbinedriven
boost pump through a flexible pendulum pickup,
boosted to greater than 10 psi, and fed to the engine
through the engine feed line. Normally the right boost
pump only feeds the right engine and the left boost pump
only feeds the left engine; however, the boost pump
output lines are connected by a normally closed engine
automatic crossfeed valve. If either boost pump output
pressure falls below 9 psi, as indicated by the illumination
of the appropriate FUEL PRESS caution light, the
engine automatic crossfeed valve is commanded open.
The engine automalic crossfeed valve allows fuel from
the operating boost pump to supply pressurized fuel to
the engine on the failed side. The engine automatic
crossfeed valve is also opened when either of the Iowlevel
thermistors in cell No. 2 or 5 is uncovered; howeveri
f equalb oostp ump pressurese xist, negligible flow
will occur through the valve.
2.14.3.1 L/R FUEL PRESS Caution Lights. IlIumination
of the L or R FUEL PRESS caution light results
from a malfunction of the boost pump, failure of the
motive flow pump, exhaustion of fuel, or fuel flow interruption.
With illumination of the caution light, the
engine automatic crossfeed valve is commanded open
and the fuselage motive flow shutoff valve on the failed
side is automatically closed. Because of the reduced
pumping and transfer capacity while operating on a single
boost pump, afterburner operation is restricted to
altitudes below 15,000 feet. Fuel to both engines is supplied
from the side with the operating boost pump; therefore
a fuel quantity imbalance will result. Use of the
FEED switch to balance fuel quantity will override the
low-fuel pressure signal to the fuselage motive flow
shutoff valve, allowing normal fuel balancing procedures.
Illumination ofboth FUEL PRESS caution lights
indicates reduced (~9 psi) or loss of boosted fuel pressure
to both engines. Fuel will continue to be supplied
by suction feed; however, thrust settings should be minimized
and AB used only in emergencies. Suction feed
is drawn from an inlet at the bottom of the fuel cell that
does not incorporate a flexible pendulum pickup.
With a left or right FUEL PRESS light, flight
at zero or negative g should be avoided or
engine fuel starvation may result.
With both FUEL PRESS caution lights illuminated,
there is a potential that total loss of motive flow pressure
has occurred because of both motive flow pumps not
functioning. Total loss ofmotive flow pressurew ill preclude
tmnsfcr of any remaining wing fuel or fuel dump
and result in total segregation of the FWDMGHT and
AFT/LEFT systems since motive flow provides the
force to opent he sump tank interconnectv alve. Without
motive flow pressure, all fuselage fuel transfer is by
gravity, which makes the quantity of usable fuel a functionofaircraftattitude.
Atcruiseattitude,appmximateIy
400 pounds of usable fuel will be trapped in the afl
fuselage. After illumination of both fuel pressure caution
lights, any of the following events indicate that
some motive flow pressure is available:
1. Wing fuel transfer
2. With the FEED switch in FWD or AFT and no
transfer of external fuel
a. The feed group of the selected side remains
full.
b. Fuel migration from one side to the other.
2.14.3.2 Engine Fuel Feed During Afterburner
Operations. High Al3 fuel consumptionp lacese xtreme
demands on the engine feed system. In addition, the g
forces experienced with AB use, especially during URloaded
accelerations (bugouts) and low-g nose-high
maneuvering, tend to reduce forward fuel transfer to cell
No. 5 and the left engine sump tank (cell No.3). When
these conditions are sustained, fuel in cell No. 5 is depleted
by both high suction feed through the gravity
transfer line (C, Figure 2-32), and by reducing gravity
ORIGINAL 2-48
NAVAIR 0%F14AAD-1
Figure 2-32. Aft Fuselage Fuel Transfer
fuel transfer from cell Nos. 6 and 7. Zero- or low-g (less
than 0.5) flight tends to force the fuel remaining in cell
No. 5 toward the aft wall of the tank or, at reduced fuel
　
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