Four   discreet   dc   voltages   for   in-flight selection  of  functioning  delay  times  are  used  in the Mk 344 Mods and Mk 376 fuzes. Arming delay times are automatically selected by the Mk 31 safety device. MK 31 SAFETY DEVICE The  Mk  31  safety  device  is  used  to  adapt  the fuze to the fuze well of the bomb, provide mechanical safing of the fuze and unlock the timer-decelerometer. The safety device contains a pop-out pin that locks the fuze in an unarmed condition. The spring-loaded pin  is  held  in  the  safe  position  by  either  a  safety cotter  pin  or  an  arming  wire.  When  the  weapon  is released from the aircraft, the arming wire is pulled from  the  pop-out  pin,  allowing  the  pin  to  pop  out, unlock the decelerometer, thus initiating the arming time.  When  using  the  Mk  344  Mod  0  or  Mod  1 fuze, the arming is completed 5.5 seconds after the pop-out pin is released. For the Mk 376 fuze, arming is  completed  10.0  seconds  after  release  from  the aircraft  if  the  free-fall  mode  of  delivery  is  used.  If  the Mk  31  safety  device  senses  weapon  deceleration (Snakeye fins open), the internal circuits of the fuze are  switched,  and  the  fuze  becomes  armed  in  2.6 seconds.  The  quicker  arming  time  is  required  to ensure  the  fuze  is  fully  armed  for  low-altitude delivery. If deceleration is not sensed by 2.6 seconds, the  fuze  arming  delay  continues  to  the  10.0-second arming  time. FUNCTIONAL  OPERATION The  following  description  applies  specifically  to  the Mk  376  fuze.  However,  the  Mk  344  fuze  operates identically to the Mk 376, except that it is operationally restricted to the unretarded (5.5-second delay) delivery mode  only. Two  arming  delays  are  used  in  the  Mk  376 fuze—2.6  seconds  for  retarded  delivery  and  10.0 seconds  for  unretarded  delivery.  The  appropriate arming   delay   is   automatically   selected   by   the fuze  according  to  the  actual  delivery  mode  of  the weapon.  That  is,  if  the  weapon  does  not  retard, whether  intentionally  or  unintentionally,  the fuze  automatically  provides  a  10.0-second  arming delay. At release, the arming wire is withdrawn and a charging voltage (+300, +195, –195, or –300 Vdc) is applied to the fuze. The pilot selects the voltage in flight by  the  fuze  function  control  set  located  in  the  cockpit. The fuze polarity and level of the fuze charging voltage is important only with respect to functioning delay. Arming is the same in any case. A regulator in the fuze converts  the  applied  voltage  to  the  required  level  and polarity. It is then applied to the energy storage unit and the  2.6-second  timer.  If  the  weapon  decelerates,  the  Mk 31  safety  device  senses  the  deceleration  and  causes  the retard  switch  to  close.  At  2.6  seconds,  the  timer completes its cycle and transfers the voltage to the rotor-actuating  bellows.  The  bellows  operate  and  turn the rotor to the armed position. If the weapon does not decelerate, the retard switch does  not  close.  The  2.6-second  timer  continues  to  run. At 3.8 seconds, the Mk 31 safety device causes the voltage  to  transfer  to  the  input  of  the  rotor-actuating bellows.  At  10.0  seconds,  the  bellows  operates  and turns the rotor to the armed position. FMU-143E/B  ELECTRIC  TAIL FUZE The FMU- 143E/B fuze (fig. 1-11) is used with the GBU-24B/B. It is initiated by the FZU-32B/B initiator which is used to generate and supply power to arm the fuze. The safe condition is verified by the presence of a safety pin or arming wire through the pop-out pin (gag rod). FMU-139  (SERIES)  ELECTRONIC BOMB FUZE The FMU-139 (series) electronic bomb fuze (fig. 1-12) is an electronic impact or impact-delay fuze. It is used in Mk 80 series general-purpose bombs, including laser guided bombs. The arming times are in-flight selectable, and the functioning delay must be set during weapon  assembly.  There  are  three  arming  times  (2.6, 5.5,  and  10.0  seconds)  and  four  functioning  delay settings   (10,   25,   and   60   milliseconds,   and instantaneous). Only 2.6/60, 2.6/25, 2.6/10, and 2.6/inst high  drag  arm/delay  switch  positions  are  authorized  for Navy/Marine Corps use. The low drag arm time switch should always be in the X position. The low drag arm time  rotary  switch  is  positioned  at  X  for  shipping, storage, and all FFCS (fuze function control set) use. The FMU-139 fuze differs from the Mk 344 and Mk 376 fuzes in that the gag rod and arming wire housing are located in the center of the faceplate (fig. 1-13). 1-16