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 At release, the arming wire is withdrawn and a charging voltage (+300, +195, –195, or –300 Vdc) is applied to the fuze.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