q. Lining, Helmet, Shock, M45 Mask (Figure 1-18). Use of the M45 thermoplastic liner is
authorized only when you plan to wear the M45 mask. The brow pad is used in two situations:
while you are on a mission and have removed the M45 mask, and when you are going on a
mission without the M45 mask but expect to don the M45 mask during the mission. When you
use the M45 TPL, the helmet bag should contain either the M45 mask or the brow pad; you
should be wearing whichever item is not in the helmet bag. If you do not plan to use the M45
mask, do not use the M45 thermoplastic liner; use the standard thermoplastic liner instead.
r. Intercommunications Unit (Figure 1-19). The Intercommunications Unit (ICU) provides a
means of communication for dismounted aviators wearing the CB mask and for testing the
communications system on the HGU-56/P helmet. It is powered by a 9-volt battery and has a
clip and hook that can be used to carry the unit. A four-position switch provides for control of
communication. The four positions are: (1) OFF, (2) LISTEN ONLY, (3) TALK/LISTEN, (4)
The LISTEN ONLY position enables the ICU microphone to supply a communication signal to
the helmet earphones. The TALK/LISTEN position enables the ICU microphone to supply a
communication signal to the helmet earphones and enables the helmet microphone to supply a
communication signal to the ICU speaker. The HARDWIRE position is used when two aviator
helmets are connected together through a single ICU, eliminating most of the background noise.
This allows for clearer communications when aviators are not connected to the aircraft and/or
wearing the CB mask.
Figure 1-18. Lining, Helmet, Shock, M45 Mask
Figure 1-19. Intercommunications Unit