h. Fuel Purge System. Each engine is provided with a fuel purge system. The system is designed to ensure that
any residual fuel in the fuel manifolds is consumed during engine shutdown. During engine operation, compressor
discharge air is routed through a ilter and check valve, pressurizing a small air tank mounted on the engine truss. On
engine shutdown the pressure differential between the air tank and fuel manifolds causes air to be discharged from
The fuel forced into the combustion chamber is consumed, causing a momentary rise in engine TGT.
i. Fuel Vent System. Each fuel system is vented through two ram vents located in the underside of the wing
adjacent to the nacelle, and a lush vent, located on the underside of the wing adjacent to the wing tip. To prevent
icing of the vent system, one vent is recessed into the wing and the other ram vent protrudes out from the wing and
contains a heating element. The vent line at the nacelle contains an inline lame arrestor.
j. Engine Oil-to-Fuel Heat Exchanger. An engine oil-to-fuel heat exchanger located on each engine accessory
case operates continuously during engine operation to heat fuel delivered to the engine suficiently to prevent the
freezing of water which it may contain. The temperature of the delivered fuel is thermostatically regulated to remain
between 21° C and 32° C.
2-33. FUEL SYSTEM MANAGEMENT.
a. Fuel Transfer System. When the auxiliary tanks are illed, they will be used irst. During transfer of auxiliary
fuel, which is automatically controlled, the nacelle tanks are maintained full. A check valve in the gravity feed line
from the outboard wing prevents reverse fuel low. Normal gravity transfer of the main wing fuel into the nacelle
tanks will begin when auxiliary fuel is exhausted. The system will gravity feed fuel only to its respective nacelle tank,
i.e. left or right (Fig. 2-27). Fuel will not gravity feed through the crossfeed system.
b. Operation with Failed Engine-Driven Boost Pump or Standby Pump. Two pumps in each fuel system provide
inlet head pressure to the engine-driven primary high-pressure fuel pump. If crossfeed is used, a third pump, the
standby fuel pump from the opposite system, will supply the required pressure. Operation under this condition will
result in an unbalanced fuel load, as fuel from one system will be supplied to both engines while all fuel from the
system with the failed engine driven and standby boost pumps will remain unused.
2-34. FERRY FUEL SYSTEM.
Provisions are installed for connection to long-range fuel cells.