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

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William E. M cLean and Clarence E. Rash
monocular night time. Aircraft type was an AH-1 Cobra equipped with an
Apache FLIR and extensive data collection capability (radar altimeter).
Instrument information or flight symbology on the FLIR image for altitude
was removed. The results showed that subjects performed poorly when
asked to provide absolute altitude estimates under any condition, but were
more consistent in estimating changes in altitude. Performance with the
FLIR was consistently worse than with the other viewing conditions. The
authors attributed the more variable results with the FLIR to poorer
resolution and changing thermal conditions over the 1½ year data collection
period.
In summary, stereopsis with night imaging devices does not seem to
provide any significant additional depth perception information over the
strong monocular cues such as motion parallax for helicopter flight. The
successful use of the monocular IHADSS in the AH-64 Apache helicopter
implies that sufficient depth estimations for pilotage can be obtained with
normal flight training with monocular as well as binocular night imaging
systems.
Visual Illusions and Spatial Disorientation
Spatial disorientation (SD) is defined by Benson (1978) as "the
situation occurring when the aviator fails to sense correctly the position,
motion, or attitude of his aircraft or of himself within the fixed coordinate
system provided by the surface of the earth and the gravitational vertical."
Often included in the definition of SD is Vyrnwy-Jones' (1988) clause:
"the erroneous perception of the aviator's own position, motion, or attitude
to his aircraft, or of his aircraft relative to another aircraft." In addition,
contact with an obstacle known to be present, but erroneously judged to be
sufficiently separated from the aircraft is included as SD.
One might infer that flight with current night vision devices would
induce some SD due to their limitations of reduced FOV, decreased
resolution, reduced depth perception, and lack of color vision, as compared
to unaided vision. However, at terrain altitudes at night, the aviator has
essentially no FOV, resolution, depth perception, or color vision with the
dark adapted eye, and could not survive in modern warfare without these
night vision devices. Training and improved technology are required to
reduce the necessary risks associated with night and adverse weather flying.
In many respects, visual illusions could be considered one of the
primary causes of spatial disorientation with night vision devices (Crowley,
1991). Crowley conducted a survey soliciting information from 223