TM 1-1520-238-10 Change 2 2-35 selected  engine  crossfeed/shutoff  valve  automatically closes. Some helicopters have been modified with green and am- ber crossfeed (X FEED) annunciator lights located on the pilot  and  CPG  caution/warning  panels  (fig  2-44.1).  The green  X  FEED  annunicator  light  will  illuminate  on  both caution/warning panels when crossfeed is selected by the pilot and both engine fuel valves are correctly positioned for the selection. The green X FEED annunicator light will also illuminate when override and crossfeed is selected by  the  CPG  and  both  valves  are  correctly  positioned. When  the  CROSSFEED  switch  on  the  pilot  fuel  control panel is placed in the FWD TK or AFT TK position the X FEED  caution/warning  lights  will  be  inhibited  for  3   1 second while the fuel valves move to their selected posi- tion. The amber X FEED annunicator light and the MAS- TER CAUTION light (fig 2-43) will illuminate on both cau- tion/warning panels when the pilot has selected crossfeed or  the  CPG  has  selected  override  and  one  or  both  fuel valves are incorrectly positioned. The amber X FEED cau- tion/warning lights will also illuminate when a fuel valve is in the CLOSED position (in response to actuation of a fire handle  or  pilot  fuel  control  panel  ENG  1  or  ENG  2  fuel switch in the OFF position. NOTE During engine start the amber X FEED cau- tion/warning  lights  will  illuminate  if  the CROSSFEED switch is not in the AFT TK position. c.   External Tank Switch (EXT TK). The pilot acti- vates  fuel  transfer  from  as  many  as  four  external  fuel tanks to the internal fuel cells by positioning the EXT TK switch  to  ON.  The  switch  opens  the  auxiliary  fuel  tank shutoff valve and the two transfer shutoff valves. Opening the auxiliary fuel tank shutoff valve allows pressurized air to force fuel from the left auxiliary fuel tank into the forward fuel cell and from the right auxiliary fuel tank into the aft fuel cell. When all auxiliary tanks are empty, the EXT EMP caution  light  on  the  pilot  caution/warning  panel  illumi- nates. NOTE · The REFUEL VALVE switch (located on the external fuel servicing panel (fig 2-45) must  be  closed  for  operation  of  the TRANS switch to be effective. The trans- fer pump will not transfer fuel if the refuel valve is open. · The  fuel  transfer  system  is  the  primary method of balancing loads. d.   Transfer  Switch  (TRANS). The  TRANS  switch controls the position of the fuel transfer air valve. The fuel transfer air valve directs pressurized air to the bi-direction- al transfer pump which allows fuel transfer between fuel cells. With the switch in the TO AFT or TO FWD position, pressurized air is directed to the air motor which turns the transfer  pump  and  transfers  fuel  forward  or  aft.  If  the transfer pump is transferring fuel and the fuel cell to which it is transferring becomes full, a fuel level control valve will shut off the fuel flow through the pump. There is no need to stop the transferring operation until the fuel cells are at the desired level. Some helicopters have been modified with green and am- ber transfer (FUEL XFR) annunciator lights located on the pilot  and  CPG  caution/warning  panels  (fig  2-44.1).  The green FUEL XFR annunicator light will illuminate on both caution/warning panels when the pilot or CPG fuel trans- fer switch is in the transfer position and transfer occurs. The amber FUEL XFR annunicator light and the MASTER CAUTION light (fig 2-43) will illuminate on both caution/ warning panels when the pilot or CPG fuel transfer switch is in the transfer position and transfer does not occur. WARNING The CROSSFEED switch shall be set to the NORM position at all times unless ex- ecuting emergency procedures for FUEL PSI ENG 1 and FUEL PSI ENG 2. Using the fuel system crossfeed may result in a dual engine flameout if one of the engine fuel valves fails to properly position. e.   BOOST Switch. The boost pump (located in the aft fuel cell) is used for starting the engines and when the FUEL PSI ENG 1 and FUEL PSI ENG 2 caution lights illu- minate. The BOOST switch electrically opens the boost pump shutoff valve which directs pressurized air to the air driven boost pump. When the switch is in the OFF posi- tion, the engines receive fuel (through a suction feed sys- tem)  via  the  engine-mounted  fuel  pumps.  When  the BOOST  switch  is  in  the  ON  position,  and  the  CROSS- FEED switch is in the AFT TK position, both engines feed from  the  aft  fuel  cell.  The  boost  pump  is  automatically started and shut down during the engine start sequence. Shutdown occurs at approximately 52% N_G. It should be noted that the pilot CROSSFEED switch must be in the AFT TK position in order for the BOOST switch to latch on.