Suction Feed
When main tank fuel pump pressure is low, each engine can draw fuel from its
corresponding main tank through a suction feed line that bypasses the pumps. As the
airplane climbs, dissolved air is released from the fuel in the tank due to the decrease in air
pressure. This air may collect in the suction feed line and restrict fuel flow. At high altitude,
thrust deterioration or engine flameout may occur as a result of the fuel flow reduction.
The dissolved air in the fuel tank will eventually deplete after reaching cruise altitude. The
depletion time is dependent upon airplane altitude, fuel temperature, and type of fuel. Once
the dissolved air is depleted, the engine may be capable of suction feed operation at cruise
power. The main tank bypass valves may also be used for suction defueling.
Fuel Crossfeed
The engine fuel manifolds are interconnected by use of the crossfeed valve. The valve is
DC motor operated from the battery bus.
Fuel pressure can be provided from a main tank with operating fuel pumps to both engines
by opening the fuel crossfeed valve. Continued crossfeed use will result in a progressive
fuel imbalance.
Fuel Shutoff Valves
Spar fuel shutoff valves are located at the engine–mounting wing stations. The valves are
DC motor operated from the hot battery bus. The engine fuel shutoff valves are fuel
actuated, solenoid controlled valves powered from the battery bus. Both the spar fuel
shutoff valve and the engine fuel shutoff valve close whenever their respective engine fire
warning switch is pulled or engine start lever is placed to CUTOFF.
