Fuel Feed System: Function

Fuel Feed System: Function

1. Two methods of Pressure Controlled Fuel Feed

   • High Pressure Pumps
     • Also called override and jettison pumps
     • Deliver twice as much pressure as standard boost pumps
       • Forces discharge check valves on standard pumps to close
         • Prevents fuel flow from standard pumps
   • Sequence Valves
     • Installed at the pump outlets of standard boost pumps
       • Reduce output pressure
         • Allows some fuel back into the tank

2. Electrical Fuel Feed Management

   • Performed automatically
   • Controlled in accordance with flight phases
   • Taxiing phase, centre tank feeds engines
     • Other pumps on standby
   • Take-off phase
     • Occurs when flaps are extended for take-off
     • Engines supplied individually
       • Two boost pumps from each main tanks 2 and 3 (engines 2 and 3 respectively)
       • Double capacity centre tank boost pumps for engines 1 and 4
   • Cruise phase
     • After take-off when flaps have been retracted
     • Centre tanks and stabilizer tank are emptied first
     • Inner main tanks used next
     • All engines now supplied by double capacity override and jettison pumps in main tanks 2 and 3

3. Fuel Scavange

   • Reduces amount of non-usable fuel in the tanks
   • Most commonly used method uses jet pumps
   • Boost pumps supply jet pumps with pressure they need for operation
   • Scavenged fuel is pumped to the main inner tank
   • Another method uses electrically driven scavenge pump
     • Controlled automatically as fuel level drops below certain threshold
     • Pump fuel into a main tank

4. Fuel Recirculation

   • Ensures proper cooling of engine oil and integrated drive generator oil
   • Fuel from tank is pumped through engine oil cooler which heats the fuel and cools the engine oil
   • Fuel then passes through the high pressure gear driven engine pump to the burners on the engine
   • Some fuel is diverted and passes through a bypass valve
     • Proceeds to the Integrated Drive Generator (IDG) cooler
       • Fuel is heated further and then recirculated and mixed with new fuel ready to repeat the process
   • Works well until engine runs at lower speed and there is a high electrical load on the IDG
     • As engine runs slower, the cooling becomes critical as there may not be enough fuel for cooling
   • Solenoid activates on the fuel return valve
     • Return valve opens and returns fuel to the tank
       • Cool fuel is mixed to cool down the hot fuel

5. Fuel Boost Pumps

   • Generally impeller type
   • Impeller forces fuel from the inlet to the pump outlet by centrifugal force
   • Boost pump can be removed from canister without draining fuel
     • Special removal valve shuts off the fuel inlet as the pump is pulled out