Helmet-Mounted Displays: - USAARL - The - U.S. Army

Biodynamics 209
the fitting problems, as well as improving the level of crash protection. The
TPL TM suspension method could be considered a custom fit, which
overcomes most variations in individual anthropometry, providing a greatly
enhanced level of comfort and fit.
The TPLs TM delivered with the HGU-56/P are prefitted to appropriately
sized head forms as a part of the manufacturing process. Most aviators can
remove the helmet and TPL TM straight from the box and obtain an adequate
fit with minor adjustments. If unable to obtain a comfortable fit, custom
fitting can be easily accomplished by heating the TPL TM for approximately
10 minutes in a convection oven at a temperature of 200± 5ºF. The heated
TPL TM becomes soft and pliable, retaining its new shape after it cools. The
aviator inserts the heated TPL TM into the helmet, which then is placed on
the aviator’s head for 5 minutes with downward pressure applied. [This is
accomplished by having the aviator place his hands over the helmet and
pull down towards the crown of his head.] The pressure is released and the
helmet worn for an additional 5 minutes. If an optimal fit is not achieved,
the process can be repeated, as long as the TPL TM is not overheated.
Another type of fitting system, used primarily in the U.S. Air Force and
Navy fixed-wing helmets is based on variations of a custom-fit foam
technique. One variation in the foam method involves the mixing of two
chemicals which produces a foam liner form-fit to the head; another
variation uses a wax mold which is heated and placed on the head. The
Army briefly authorized these foam systems during the period when NVGs
were first introduced, but withdrew approval due to varying foam density
and inconsistent impact protection performance. Thus, they currently are
not used in Army aviation.
Retention
Helmets are unable to provide their impact energy attenuation function
if the helmet does not remain on the head during crashes or mishaps. This
role is accomplished by the helmet retention system. Reading et al. (1984)
showed that helmet retention system failure is a significant factor in
mishaps where helmet losses occurred. Typically, modern retention
systems consist of an integrated napestrap and a chinstrap. The napestrap
runs behind the head just under the occipital region. The chinstrap runs
under the chin, being careful to avoid the areas around and about the
trachea. A properly designed retention system will prevent the helmet shell
from undergoing excessive forward or rearward rotation when the head is
exposed to crash induced acceleration(s) (Hines et al., 1990), without