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leaking. After transitioning to WonW, balancing is reinitiated and continues until either feed tank is
below 300 lb.
In the event of a fuel transfer failure (e.g., a feed tank begins to deplete with fuel in Tank 4), feed
tank balancing begins when either feed tank drops below approximately 2,100 lb for 1 minute. This
mechanization attempts to minimize the effect of the fuel transfer failure by reducing the resulting
feed tank split.
2.2.1.7 Feed Tank Imbalance with One Engine at Idle. If one engine is intentionally reduced to
idle/low power or is commanded to IDLE by the FADEC, a higher rate of fuel depletion can be
expected from the ²good² engine's feed tank. At internal fuel weights below approximately 4,900 lb
(transfer fuel depleted), a fuel split can be expected to develop between the feed tanks (interconnect
valve is closed). If fuel burn continues to approximately 2,450 lb, the good engine feed tank depletes
and runs dry. The motive flow/boost pump output pressure on the good side drops, sets the L or R
BOOST LO caution, and opens the crossfeed valve. The good engine feeds from the opposite feed tank
through the crossfeed valve.
When driven by an idling engine, a motive flow/boost pump can support fuel flow up to 28,000 pph
through the crossfeed valve (MIL power fuel flow is approximately 12,000 pph at sea level, standard
day). If the fuel flow demand on the usable engine exceeds 28,000 pph (midrange afterburner), motive
flow/boost pump output pressure drops, setting the other BOOST LO caution, closing the crossfeed
valve, and starving the good engine. MAX power, single engine fuel flow is approximately 38,500 pph
at sea level, 0.2M, standard day (approach conditions).
Selecting afterburner on the good engine with its feed tank reading empty
results in engine flameout if fuel flow exceeds 28,000 pph.
The only way to balance a growing feed tank split is to shutdown the idling engine. This opens both
the interconnect and crossfeed valves. The risk of balancing is a loss of hydraulic and electrical
redundancy provided from the engine if left at idle.
2.2.2 Fuel Transfer System. The fuel transfer system, controlled by the SDC, is designed to keep the
feed tanks full or near full during normal engine operation. Fuel is routed from Tanks 1 and 4, the
internal wing tanks, and external fuel tanks, if installed, through three independent sets of transfer
lines. Additionally, the SDC schedules Tank 1 and 4 transfer to control fuel center of gravity (CG).
2.2.2.1 Fuel Transfer - Tanks 1 and 4. Fuel is transferred from Tanks 1 and 4 to the feed tanks by
two dual-speed electric transfer pumps, one in each tank. The low speed setting is used for normal
A1-F18EA-NFM-000
I-2-13 ORIGINAL
transfer. The high speed setting is used during high fuel flow conditions such as afterburner operation,
ARS replenishment, or fuel dump. The one exception to this is that the Tank 1 transfer pump remains
in low speed setting during afterburner operation. During normal operation, each pump pressurizes the
Tank 1 and 4 transfer line as long as its tank has transfer fuel available. The SDC shuts down the
electric transfer pumps when the respective tanks are dry (Tank 1 empty, Tank 4 approximately 300
lb).
Jet level sensors (JLS) in the feed tanks control the flow of transfer fuel from the Tank 1 and 4
transfer line. For instance, Tank 2 does not accept fuel until its fuel quantity drops to approximately
2,100 lb, uncovering the JLS and opening the transfer valve. Tank 2 accepts fuel until its fuel quantity
reaches approximately 2,450 lb, covering the JLS and closing the transfer valve. Therefore, during
normal operation, Tank 2 fuel level cycles between 2,100 and 2,450 lb as long as transfer fuel is
available (JLS cycling).
Flapper valves in Tanks 1 and 4 provide a backup gravity transfer capability in certain circumstances.
The flapper valve in Tank 4 is free flowing, gravity transferring to Tank 3 any time the Tank
4 fuel level is higher. Therefore, Tank 4 tends to keep Tank 3 full (near 2,600 lb) until the Tank 4 fuel
level drops below that of Tank 3 (wing tank fuel depleted). The flapper valve in Tank 1 is controlled
by left motive flow. The valve can be opened by the SDC following a Tank 1 transfer pump failure or
by loss of motive flow (left engine shutdown).
Since the Tank 4 transfer pump is not located on the bottom level of the tank, two motive flow
powered scavenge pumps, one routed to Tank 2 and the other to Tank 3, are installed to transfer the
last 300 lb of Tank 4. With empty transfer tanks, an excessive feed tank fuel split following symmetric
engine operation may indicate a Tank 4 scavenge pump failure. There is no SDC monitoring of the
Tank 4 scavenge pumps.
　
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