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

Biodynamics 197
disability and fatality in Army rotary-wing mishaps (Shanahan, 1985).
Head impact protection is accomplished by proper helmet design, a design
which provides a protective outer shell and sufficient stopping distance
between the shell’s outer surface and the skull. The purpose of the
protective shell is to resist penetration from sharp or jagged impact surfaces
and to distribute the load over a greater contact area. The head impact
velocity in survivable helicopter crashes has been estimated at 19.6 feet per
second (5.97 meters/sec) through computer simulations and analysis of sled
test results. This number is based on the potential flail velocity of the
occupant’s upper torso (Desjardins, et al., 1989).
Human head impact tolerance is an area of continuing research. The
USAARL has recommended a test head form threshold of 150 to 175G,
depending on the impact location. See Table 7.1. A review of performance
specifications for other helmet applications (i.e., motorcycle, bicycle,
equestrian, fixed-wing aviator, etc.) reveals a range of thresholds ranging
from 200 up to 400 G. The USAARL recommended value for the
headband region (175G) is based on the concussion threshold to linear
accelerations, not on skull fracture, fatality, or rotational acceleration
thresholds. The USAARL recommended value for the earcup and crown
regions (150G) is based on the risk of basilar skull fracture concomitant
with impacts to those areas and the high frequency of occurrence in Army
helicopter crashes (Shanahan, 1985).
Table 7.1.
Impact attenuation maximum G thresholds.
Impact location Impact velocity (m/s)
Minimum Maximum
Crown
Left earcup
Right earcup
Front
Rear
Left side
Right side
4.88 4.95
5.98 6.05
5.98 6.05
5.98 6.05
5.98 6.05
5.98 6.05
5.98 6.05
Drop height
(meters)
1.22
1.83
1.83
1.83
1.83
1.83
1.83
Maximum G*
150
150
150
175
175
175
175