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Figure 4-20.Cross-sectional view of valve during exhalation
Figure 4-23.Adjusting receiver assembly

Aircrew Survival Equipmentman 2 - Aviation theories and other practices
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Figure 4-21.—Measuring for proper size of O2 mask. 4-21; measure the distance from the tip of the bottom surface of the chin to the point of maximum depression of the nasal root (smallest part of the upper end of the nose). Once the basic size has been determined, requisition a new mask through normal supply channels. ATTACHMENT OF BAYONET AND RECEIVER MECHANISM Before you fit the mask to the helmet, you should check to see if the MBU-12/P has been configured to the correct assembly. For complete information on configuration buildup, refer to chapter 13 of NAVAIR 13-1-6.7. Once you have a complete assembly of the right size, you’re ready to fit the mask to the helmet. The mask has four mask retaining straps. You should thread the straps through the slots in the offset bayonet fittings, as shown in figure 4-22. Then insert each offset bayonet fitting into a receiver mechanism to the second locking posi- tion. When the bayonet is inserted in the receiver, the first click is caused by the entry of the bayonet into the entrance of the receiver housing, but it is not a locking position. There are normally three clicks when inserting the bayonet to the second locking position of the receiver. When the bayonet is in the second locking position, the end of the bayonet will be approximately even with the outer Figure 4-22.—Mask retaining straps. edge of the exit slot on the back of the receiver. This proper positioning of the bayonet end can be checked either visually or by passing a finger over the exit slot on the back of the receiver. Have the aircrew member place the helmet on his head and hold the oxygen mask in proper position on his face. Inspect each receiver mechanism assembly to be sure that the rotating feature of the device is locked in its central position. If the rotating feature is found not to be centered, loosen the two locking screws on the nameplate of the receiver. Adjust the receiver until the bayonet is at right angles to the receiver. Retighten the locking screws (fig. 4-23). The receiver has a rotating feature that allows a 15-degree angle of freedom so that the receiver can be adjusted slightly in either direction if this becomes necessary after attachment to the helmet. Place the receiver on the helmet so that the upper and lower straps have equal tension. The receiver should be positioned as close to the edgeroll as possible to minimize bayonet/edgeroll interference. With a grease pencil, trace around each receiver to mark its outline on the helmet (fig. 4-24). You should use this outline to align the receiver on the helmet. Now remove the helmet from the wearer and using the back plate of the receiver as a template, mark the locations for the four receiver mounting holes on the helmet (fig. 4-25). Remove the earcup assemblies and drill four mounting holes in the helmet. Do this on each side by using a No. 25 (0.1495-inch diameter) drill. It 4-28







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