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

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Joseph R. Licina
provided a 93.9% acceptable fit, while the 15-sized scheme only increased
the acceptable fit to 97.6%. The IHADSS helmet is fielded in three sizes
(medium, large, and extra-large). The IHADSS does not accommodate the
small female, and even for the male population requires custom fitting,
taking 2 hours for an initial fit with subsequent fittings the norm. The
HGU-56/P, the current primary candidate for Comanche, has 4 shell sizes
(S, M, L, and XL) with 6 impact liner sizes. This system is designed and
has been fielded as a system requiring minimal fitting skill and time.
Support equipment required for the basic helmet fitting processes can
include screwdrivers, Velcro TM attachments, and/or special tools to remove
interior liners, communication assemblies, etc. Visionic alignment and
validation can expand the list of support equipment to in excess of $30,000
(in the case of the early IHADSS fitting kits).
The Army’s first experience with custom fitting HMDs was with the
IHADSS and resulted in a number of lessons learned (Rash et al.,1987).
First was the difficulty in overcoming the Army’s decision not to identify
specialized personnel to serve as dedicated fitters due to personnel
constraints. Second was the reluctance to invest in the specialized visionics
support alignment and validation equipment initially recommended by the
manufacturer. A scaled down equipment kit was purchased and found to
be inadequate. Third was programming allotted time within the
compressed class schedule for the fitting and alignment process prior to
first flight. Fourth was the initial resistance to expending resources on a
specialized padded helmet bag, which provided greater protection for the
delicate relay optics during storage and use in the field. Fifth was the
extent of modularity/breakdown of subassemblies for the purpose of
reducing replacement costs. For example, in IHADSS, one of the most
common items for replacement was visors. However, visor replacement
required replacement of the entire visor assembly, i.e., visor housing, visor
cover, and visor track and spring assembly, increasing the cost from less
than $100.00 to just under $1000.00. [Note: This issue has been resolved
by a parts breakdown and individual component procurement.]
While quality of fit is subjective by nature, Stiffler and Wiley (1992)
have attempted to loosely quantify fit using a “fit equation” which
addresses three areas of fit: comfort, optical adjustment, and stability. The
equation is expressed as:
FIT = (comfort) + (optical adjustment) + (stability) Equation 9.1
Comfort is a critical factor because discomfort, which can manifest