Physical Description

The Mk 339 Mod 1 fuze is similar to the Mod 0 fuze. Several improvements are discussed in the following paragraphs.

The primary and option functioning delays are preset during assembly at the factory for both the Mod 0 and Mod 1 fuzes. The fuzes are preset at 1.2 seconds for primary delay, and the option delay is preset at 4.0 seconds. The Mod 0 fuze (without the option time wire) is preset at 4.0 seconds on the option delay. The primary delay is not used on this fuze The primary delay is set at zero time.

You already know the primary and option delays for the Mod 0 and Mod 1 fuzes are preset at the factory. These time delays can be reset during weapon preparation to meet various tactical requirements. The primary and option time delays for the Mod 0 fuze can be adjusted from 1.2 to 50.0 seconds by adjusting the primary time setting dial and the option time setting dial, respectively. The functional delays for both the primary and option modes of the Mod 1 fuze can be adjusted from 1.2 to 100 seconds in the same manner.

You can tell if the fuze has shifted from the primary to the option mode by the functional mode indicator. You do this by checking the time setting observation window for both the Mod 0 and Mod 1 fuzes. If the arming wire has been accidently pulled during handling, the fuzes shift to the option mode. Once the option wire is pulled, the Mod 0 fuze is committed to the option mode and cannot be reset to the primary mode. However, the Mod 1 fuze can be reset to the primary mode by reinstalling the option time wire.

The fuze safe/arm indicator (callout 1 of figs. 1-2 and 1-3) provides external evidence of arming (EEA) for the Mk 339 Mod 0 and Mod 1 fuzes. The fuze safe/arm indicator is viewed through the indicator observation window in the upper nose fairing. Mod 0 fuzes (lots 1 through 14) are in a safe condition when the tip of the indicator pin does not extend into the bubble (fig. 1-4, view A). The fuze is in an armed

Figure 1-4.-Mechanical time fuze Mk 339 safe/arm indicator pin in safe and armed positions,

condition when the indicator pin extends into the bubble (fig. 1-4, view B). On newer models (lots 15 and above of Mod 0 and Mod 1 fuzes), there is a layer of green foil at the base of the indicator bubble. The fuze is in a safe condition when the green foil is intact (fig. 1-4, view C), and it is armed when the green foil is pierced by the indicator pin (fig. 1-4, view D).

Functional Description

The following paragraphs describe the sequence of events that must occur for Mk 339 fuzes to function.

MOD 0 (WITHOUT OPTION TIME WIRE).-

When the Mk 20 bomb cluster is released in flight, an impeller band assembly secures the impeller until the fuze arming wire is pulled out. As the weapon is released from the aircraft, the arming wire is pulled from the impeller band assembly, freeing the air-driven impeller. The arming wire also passes through a time starting pin, which initiates the fuze timing mechanism when the arming wire is pulled out. The airstream rotates the impeller, which physically aligns the firing train, and allows the fuze to detonate at the preset time.

MOD 1 (WITH OPTION TIME WIRE).- This fuze is physically and functionally the same as the fuze described in the preceding paragraph except that an option wire is installed If the pilot selects the primary mode of delivery when the weapon is released from the aircraft, only the arming wire is pulled out and the primary mode of the fuze is initiated. If the pilot selects the option mode of delivery, both the arming wire and the option wire are pulled out, initiating the option time mode of the fuze. If only the option time wire is pulled out on airborne release, the fuze will dud. Both the fuze arming wire and option wire must be pulled out for the fuze to function in the option mode.