TM 55-1510-222-10 f. Firewall Shutoff Valves. CAUTION Do  not  use  the  fuel  firewall  shutoff valve to shut down an engine, except in an emergency.  The engine-driven high-   pressure   fuel   pump   obtains essential  lubrication  from  fuel  flow. When   an   engine   is   operating,   this pump      may   be   severely   damaged (while cavitating) if the firewall valve is  closed  before  the  condition  lever is    moved    to    the    FUEL    CUTOFF position. The  fuel  system  incorporates  a  fuel  line  shutoff valve  mounted  on  each  engine  firewall.    The  firewall shutoff    valves    close    when    the    fire    extinguisher    T- handles  on  the  instrument  panel  are  pulled  out.  The firewall shutoff valves receive electrical power from the main buses, and also from the hot battery bus which is connected   directly   to   the   battery.      The   valves   are protected     by     5-ampere     circuit     breakers     placarded FIREWALL  VALVE  #1  or  #2  on  the  over-  head  circuit breaker  panel  (fig.  2-5),  and  FIREWALL  SHUTOFF  #1 or #2 on the hot battery bus circuit breaker board. g.  Fuel Tank Sump Drains.  A sump drain wrench is provided in the aircraft loose tools to simplify draining a small amount of fuel from the sump drain. (1) There are five sump drains and one filter drain in each wing (Table 2-3). (2) An additional drain for the extended range fuel system line extends through the bottom of the wing center  section  adjacent  to  the  fuselage.    Any  time  the extended range system is in use, a part of the preflight inspection  would  consist-of  draining  a  small  amount  of fuel   from   this   drain   to   check   for   fuel   contamination. Whenever the extended range system is removed from the   aircraft   and   the   fuel   line   is   capped   off   in   the fuselage, the remaining fuel in the line shall be drained. 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 filter   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 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 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 flush 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 flame 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 sufficiently 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 filled, they will be used first.  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 (fig. 2-18). 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. 2-38