Spatial disorientation in U.S. Army rotary-wing operations.

This paper describes two surveys concerning spatial disorientation (SD) in U.S. Army rotary-wing operations that sought to assess the hazard and to identify recommendations to control it. One survey was of accident records, and the other was of aircrew experiences. Both surveys highlighted the magnitude of the problem. The accident survey showed that 30% of class A to C accidents involved SD as a significant factor, while the aircrew survey showed that 78% of aircrews have been disoriented (8% to the extent that flight safety was threatened). Both surveys showed a significant increase in SD associated with combat operations. Several differences between the two surveys were noted: 90% of the reviewed accidents were thought to involve type I (unrecognized) SD compared with only 43% of the reported incidents; both pilots in a particular aircraft were considered to have been disoriented in at least 59% of accidents compared with 23% of incidents; sudden loss of visual cues ("brownout," "whiteout," or inadvertent entry to instrument meteorological conditions) accounted for 25% of SD accidents compared with 13% of incidents; and 62% of the accidents occurred at night compared with only 36% of incidents. Neither survey showed any association between SD and fatigue or other human factors. The results of both surveys suggested that crew coordination, alerting devices (e.g., audio warnings on the radar altimeter), flight information displays, and autopilot functions would be good targets for improvement.

The hazard of spatial disorientation during helicopter flight using night vision devices.

INTRODUCTION: Night Vision Devices (NVDs) provide an enormous advantage to the operational effectiveness of military helicopter flying by permitting flight throughout the night. However, compared with daytime flight, many of the depth perception and orientational cues are severely degraded. These degraded cues predispose aviators to spatial disorientation (SD), which is a serious drawback of these devices. METHODS: As part of an overall analysis of Army helicopter accidents to assess the impact of SD on military flying, we scrutinized the class A-C mishap reports involving night-aided flight from 1987 to 1995. The accidents were classified according to the role of SD by three independent assessors, with the SD group further analyzed to determine associated factors and possible countermeasures. RESULTS: Almost 43% of all SD-related accidents in this series occurred during flight using NVDs, whereas only 13% of non-SD accidents involved NVDs. An examination of the SD accident rates per 100,000 flying hours revealed a significant difference between the rate for day flying and the rate for flight using NVDs (mean rate for daytime flight = 1.66, mean rate for NVD flight = 9.00, p < 0.001). The most important factors associated with these accidents were related to equipment limitations, distraction from the task, and training or procedural inadequacies. CONCLUSIONS: SD remains an important source of attrition of Army aircraft. The more than fivefold increase in risk associated with NVD flight is of serious concern. The associated factors and suggested countermeasures should be urgently addressed.

Flight simulator evaluation of a novel flight instrument display to minimize the risks of spatial disorientation.

BACKGROUND: Spatial disorientation (SD) in flight remains a major source of attrition. Many SD accidents would occur regardless of the instrument display in use, since the aircrew are simply not looking at the instruments. However, there are a number of accidents which might be amenable to improved instrument displays. In an attempt to improve maintenance and reattainment of correct orientation with a reduced cognitive workload, a novel instrument display has been developed. This paper describes an assessment of the display in a UH-60 helicopter flight simulator. HYPOTHESIS: This study tested the hypothesis that during instrument flight and recovery from unusual attitudes, the novel display permits a more accurate maintenance and reestablishment of flight parameters than the standard flight instruments. METHODS: There were 16 male aviators who flew a simulated instrument flight profile and recovery from unusual attitudes using both the standard flight instruments and the novel display. The two display formats were tested both with and without a secondary task. RESULTS: When compared with the standard instruments, both control of flight parameters and recovery from unusual attitudes were significantly improved when using the novel display. Analysis of the secondary task scores showed that cognitive workload was reduced when using the novel display compared with the standard instruments. CONCLUSIONS: Results from all aspects of the assessment indicated benefits of the new display. Future testing should be carried out during real flight, and the display should be further developed to be used in a head-up or helmet-mounted device.

Visual vestibular interaction in the dynamic visual acuity test during voluntary head rotation.

BACKGROUND: Although intact vestibular function is indispensable to maintaining spatial orientation, no good screening tests of vestibular function are implemented in the aviation community. High frequency voluntary head rotation was selected as a vestibular stimulus to isolate the vestibulo-ocular reflex (VOR) from visual influence. METHOD: A dynamic visual acuity test that incorporates voluntary head rotation was evaluated as a potential vestibular function screening tool: 27 normal subjects performed voluntary sinusoidal head rotation at frequencies from 0.7-4 Hz under 3 different visual conditions: visually-enhanced VOR, normal VOR, and visually suppressed VOR. Standardized Bailey-Lovie chart letters were presented on a computer monitor in front of the subject, who then was asked to read the letters while rotating his head horizontally. The electro-oculogram and dynamic visual acuity score were recorded and analyzed. RESULTS: There was no significant difference in gain or phase shift among 3 visual conditions in the frequency range 2.8-4 Hz. The dynamic visual acuity score shifted less than 0.3 log MAR at frequencies under 2.0 Hz. CONCLUSION: The dynamic visual acuity test at frequencies around 2 Hz can be recommended for evaluating vestibular function.