TM 1-1510-225-10 2-69 j.    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  secondary vent, located near 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 flame arrester. k.   Engine   Oil-to-Fuel   Heat   Exchanger.      An engine oil-to-fuel heat exchanger, one located on each engine  accessory  case,  operates  continuously  during engine operation to heat fuel delivered to the engine to prevent the freezing of water that the fuel may contain. 2-33.  FUEL SYSTEM MANAGEMENT. a.   Fuel  Transfer  System.    When  the  auxiliary tanks are filled, they will be used first, then the fuel in the wing tanks.  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   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  nacelle  tank,  i.e.,  left  or  right.    Fuel  will  not gravity feed through the crossfeed system. b.  Operation with Failed Engine-Driven Boost Pump   and   Standby   Pump.    Two  boost  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   boost   pumps   will remain unused. 2-34.  FERRY FUEL SYSTEM. NOTE The   ferry   fuel   system   is   installed   in   the aircraft for the specified ferry flight only. a.   Ferry  Fuel  System.    The  ferry  fuel  system consists of two 120-gallon aluminum fuel tanks, a ferry fuel  system  control  assembly,  and  a  115-cubic  foot oxygen  bottle.    The  ferry  fuel  tanks  each  contain  an electric  rotary  pump  and  a  manual  wobble  pump  for pumping  fuel  to  the  nacelle  tank.    The  fuel  tanks  are mounted on the left and right side seat tracks on each side of the cabin, the fuel control assembly is mounted across  the  aisle  directly  behind  the  pilot  and  copilot seats, and the add on oxygen bottle is mounted on the aft baggage compartment floor.  Permanently installed provisions  for  connecting  the  ferry  fuel  system  to  the nacelle   tank   are   included   in   the   fuel   system.      The provisions  consist  of  manually  operated  shutoff  valves in  each  wheel  well,  a  fuel  drain,  and  fuel  lines  which are  routed  from  each  wing  to  the  nacelle  tank.    The fuel drain, located on the underside of the wing center section  adjacent  to  the  fuselage,  should  be  drained after each use of the ferry fuel system.  The ferry tanks are  connected  to  fuel  lines  that  lead  to  the  nacelle tank. b.  Ferry Fuel Transfer. NOTE When   ferry   fuel   is   transferred   with   the aircraft  pressurized,  the  ferry  fuel  will  be transferred rapidly due to the cabin pressure differential.  The rapid transfer of fuel    occurs    when    any    ferry    fuel    tank selector   valve   and   any   aircraft   fuel   tank selector valve is open and doesn’t require operating either the electric fuel pumps or the   manual   wobble   pump.      The   one -way check  valve  in  the  surge  tank,  marked IN, equalizes  the  cabin  pressure  and  the  air pressure   over   the   fuel   in   the   ferry   fuel tanks.      Opening   the   selector   valves,   as noted  above,  opens  a  path  for  the  fuel  to the  main  aircraft  fuel  tanks,  which  have only ambient pressure over the fuel.  Cabin pressure will force the fuel out of the ferry fuel tanks into the aircraft main fuel tanks, i.e.,   from   the   pressurized   area   into   the unpressurized area. Transfer of fuel from either of the ferry fuel tanks first will not affect the aircraft center of gravity. (1)   In preparation for transfer of fuel from the ferry fuel tanks to the nacelle fuel tanks, use fuel from both nacelle fuel tanks until each is about ½ full.  Open the desired ferry fuel selector valve and open either or both fuel tank selector valve(s). (2)   Turn  on  either  or  both  ferry  fuel  pump(s) to transfer fuel to the nacelle fuel tank(s).  Monitor the quantity    of    fuel    in    the    nacelle    fuel    tanks    and discontinue  ferry  fuel  transfer  when  the  nacelle  fuel tanks    near    the    full    mark    to    prevent    venting    fuel overboard.  If either of the aircraft fuel tanks seems to be  nearing  the  full  mark  before  the  other,  discontinue fuel transfer and level the tanks in accordance with the approved Pilots' Operating Handbook before resuming fuel transfer again. (3)   Observe  the  fuel-flow  sight  gauge  in  the line  from  the  ferry  fuel  pump  to  the  fuel  tank  selector valves  for  air  bubbles.    When  air  bubbles  begin  to