TM 1-1510-218-10 2-58 entrance  door,  hold  a  fuel-proof  container  under  the ends, and drain about a pint of fuel out of each line. i.    Fuel  Drain  Collector  System  C D   .    Each engine is provided with a fuel drain collector system to return  fuel,  dumped  from  the  engine  during  clearing and   shutdown   operations,   back   into   its   respective nacelle  tank.    The  system  draws  power  from  the  4 feeder  bus  and  fuel  transfer  is  completely  automatic.   Fuel from the engine flow divider drains into a collector tank mounted below the aft engine accessory section.   An internal float switch actuates an electric scavenger pump, which delivers the fuel to the fuel purge line just aft of the fuel purge shutoff valve.  A check valve in the line    prevents    the    backflow    of    fuel    during    engine purging.   The circuit breaker for both pumps is located in   the   fuel   section   of   the   overhead   circuit   breaker panel, placarded  SCAVENGER  PUMP.    A  vent  line, plumbed  from  the  top  of  the  collector  tank,  is  routed through an inline flame arrester and then downward to a drain manifold on the underside of the nacelle. j.    Fuel  Purge  System    T   .      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 filter and check valve, pressurizing a small  air  tank  mounted  on  the   engine  truss  mount.   During    engine    shutdown,    the    pressure    differential between  the  air  tank  and  fuel  manifolds  causes  air  to be discharged from the air tank through a check valve and into manifolds out through the nozzles and into the combustion    chamber.        The    fuel    forced    into    the combustion chamber is consumed causing a momentary rise in engine speed. k.   Fuel   Vent   System.      Each   fuel   system   is vented    through    two    ram    vents    located    on    the underside  of  the  wing  adjacent  to  the  nacelle.    To prevent icing of the vent system, one vent is recessed into  the  wing  and  the  backup  vent  protrudes  out  from the  wing  and  contains  a  heating  element.    The  vent line at the nacelle contains an inline flame arrester. l.    Engine   Oil-to-Fuel   Heat   Exchanger.      An engine   oil-to-fuel   heat   exchanger,   located   on   each engine   accessory   case,   operates   continuously   and automatically  to  heat  the  fuel  delivered  to  the  engine sufficiently to prevent the freezing of any water, which the   fuel   might   contain.        The    temperature    of    the delivered  fuel  is  thermostatically  regulated  to  remain between +21 °C and +32 °C. 2-42.  FUEL SYSTEM MANAGEMENT. a.   Fuel Transfer System.   Fuel in the auxiliary tanks  will  be  used  first.    During  transfer  of  auxiliary fuel,   which   is   automatically   controlled,   the   nacelle tanks are maintained  full.   A swing check valve in the gravity   feed   line   from   the   outboard   wing   prevents reverse  fuel  flow.   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  side,  i.e.  left  or  right.   Refer  to  Figure  2-20.    Fuel  will  not  gravitate  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,    and    if crossfeed is used, a third pump, the standby fuel pump from   the   opposite   system,   will   supply   the   required pressure.    Refer  to  Figure  2-21.   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.  A triple failure,   which   is   highly   unlikely,   would   result   in   the engine   driven   primary  pump  operating  without  inlet head   pressure.       Should   this   situation   occur,   the affected engine can continue to operate from its own fuel supply on its engine-driven  primary  high-pressure fuel pump.