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Helmet mounted displays (HMDs) are being considered for a variety of uses in military aviation including training, mission rehearsal, and as a primary flight display in the cockpit. There is a need for considerable research into issues related to the use of this technology and the effects on pilot performance, particularly spatial orientation. One critical issue is the field-of-view (FOV) necessary when flying military aircraft. Since limited FOV provided by HMDs significantly decreases visual stimuli, the purpose of this research was to determine how reduced FOV affects head movements, including the Opto-kinetic Cervical Reflex (OKCR), and pilot performance.

Twelve rated U.S. military pilots completed seven simulated flight tasks in a stationary dome. Head tilt, pitch, and yaw were examined as a function of aircraft bank and FOV (40° , 60° and 100° circular). The number of control reversal errors was analyzed to investigate signs of spatial disorientation.

Figure 1 . MS-1 non-moving dome simulator at AFRL. Figure 1. Inside of MS-1 dome simulator

1. During low level VMC maneuvers (Task 1) pilots exhibited significant OKCR; however there were no significant differences among the three levels of FOV. (See Figure x)

Figure x. Head tilt as a function of aircraft bank collapsed across FOV for the low level VMC task..

2. FOV significantly affected head pitch movements under both VMC and instrument meteorological conditions (IMC).

Figure x. Head pitch as a function of aircraft bank collapsed across FOV for the low level VMC task.

Figure x. Head yaw as a function of aircraft bank collapsed across FOV for the low level VMC task..

4. Pilots demonstrated stick reversal errors when transitioning from following a lead aircraft under both VMC and IMC conditions, committing 22 reversal errors out of 72 trials (30.55%). The magnitude of the error was largest for the 40° FOV condition.

	40 Degrees (8 errors out of 24)	60 Degrees (6 errors out of 24)	100 Degrees (9 errors out of 24)
VMC	4(17.39%)	1(.04%)	5(20.83%)
IMC	4(17.39%)	5(20.83%)	4(17.39%)
Combined	Average reversal error magnitude 28.96 ^o	Average reversal error magnitude 9.30 ^o	Average reversal error magnitude 9.34 ^o

The data from this study were examined in several ways. Static analysis of average head position using ANOVA and Regression techniques provided important insight into head movement during VMC and IMC simulated flight tasks when FOV is reduced. Examination of the data with respect to time provided additional support to the analysis and conclusions.

For the three FOVs tested there were no differences in the OKCR. However, comparison to previous studies with unrestricted FOV and comparison to IMC formation tasks indicates that reduced FOV does reduce the slope of the OKCR as well as maximum head tilt. There is very likely a level at which the OKCR is suppressed and further investigation is needed.

Head yaw was not affected by FOV. Under VMC pilots yaw their heads in the same direction as the aircraft bank in order to keep a visual tally on ground references. This yaw movement may reduce the maximum head tilt. Head yaw movements are the result of a strategy for looking at information compared to head tilt, which has shown to be a reflexive movement.

Head pitch was greatly affected by FOV. Results of this study indicate significant differences in head pitch between 40 and 60° FOV conditions as well as between 40 and 100° . Pilots may be pitching their heads downward to keep the aircraft structure as a secondary visual cue. Pilots commented that they relied heavily on their instruments when flying with the 40° mask because they did not feel they had adequate situation awareness, and they felt they lost more visual cues. As a result, they tended to make more head movements transitioning between inside and outside visual cues. The differences in frequency are noticeable when viewing the videotapes. Examination of the time plots also indicates differences. Frequent transitions increase the chances of making a potentially fatal reversal error or becoming spatially disoriented. Varied and frequent inputs to the vestibular organs during head movements might result in spatial disorientation. Therefore dynamic models of head movement are needed.

The average magnitude of reversal errors is greater under the 40^o condition (X = 23^o compared to X = 9^o for 60^o and 100^o). Reducing FOV to this level may affect the ability to see secondary visual cues, however more research is necessary. The limited number of data points for reversal error data limits the power of the statistical test. However, a near level of significance at p=0.07, indicates a strong trend and coupled with pilot comments and evidence from video tapes of more frequent head movements we recommend that 40^o be avoided. There appears to be less difference between 60 and 100° . However, analysis on the frequency of head movements must be taken into consideration.

Head movements must also be considered during training or mission rehearsal. Head movements increase with decreasing FOV. It is possible that head movement strategies are critical to pilot performance. If pilots are trained on HMD systems it is possible that they may develop head movement strategies that differ from those used during actual flight. Using a large number of head movements during mission rehearsal could also lead to simulator sickness that may affect the pilots' ability to fly the mission. It is important that this be investigated. Understanding head-movement strategies may also help during pilot training. If certain head-movement strategies are better than others are, it may be possible to train pilots to model those strategies. Head movement may also provide an indication to the trainer that a pilot is having difficulty.

One advantage of HMD technology is to keep pilots looking out of the aircraft by providing symbology on a see-through HMD. FOV is just one variable in the design of a HMD. If the symbology is well designed, head pitch needed to view cockpit instruments may be reduced. Design of HMD symbology must take into consideration pilot head movements and sensory reflexes. With see-through HMDs, pilots will continue to use the horizon as a primary spatial cue. Secondary cockpit cues may be reduced or eliminated if the HMD blocks the pilot's peripheral vision. Selecting a FOV without considering other factors such as head movement, symbology movement, and frame of reference is not recommended. Loss of visual cues due to small FOV coupled with poor symbology or inappropriate frames of reference can lead to serious episodes of spatial disorientation. The use of HMDs in simulation and training that are inappropriately designed could lead to poor training and result in poor performance in real aircraft. Therefore, how pilots use visual cues as frames of reference is critical to the design of HMDs and must be investigated for training, mission rehearsal, as well as for use as a primary display during flight.

Gallimore,J.J., Patterson, F.R., Brannon, N.G., and Nalepka, J.P. (in Press). The Opto-kinetic Cervical Reflex during Formation Flight. Aviation, Space and Environmental Medicine, 70(12):1152-60.

Gallimore, J.J., Patterson, F.R., Brannon, N.G., and Nalepka, J.P. (1999). The opto-kinetic reflex during formation flight. Aviation, Space and Environmental Medicine.

This research was sponsored by the Naval Air Systems Command (PMA205) under work document number N0001998WRS14R. The views expressed on this web page are those of the authors and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, or the U.S. Government.

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