The Effects of Helmet-Mounted Display Symbology on the Opto-Kinetic Cervical Reflex and Frame of Reference

Spatial disorientation (SD) accidents are a major contributor to the Class A mishap rate in the US Air Force. A recent investigation showed that transitions between visual meteorological conditions (VMC), when pilots use real-world visual cues to fly, to instrument meteorological conditions (IMC), when pilots have to use instruments to fly, were a leading cause of SD. In VMC, the true horizon is the primary visual cue pilots use to orient themselves. In IMC, pilots must rely on a representation of the horizon as their primary visual cue to maintain spatial orientation. Research has shown that when pilots fly in VMC, they tilt their heads in the direction opposite that of aircraft roll in an effort to keep the horizon fixed in their visual field. This implies that pilots use a world frame of reference for determining orientation. However, pilots do not tilt their heads in IMC when viewing the horizon symbol on a head-down, aircraft-referenced attitude indicator. Because pilots must transition between these two frames of reference when transitioning between VMC and IMC, this may be causing SD. The helmet-mounted display (HMD) is currently being tested as a means of displaying attitude information. The HMD symbology tested portrays a conformal horizon symbol which overlays the true horizon. In VMC, pilots see the true horizon and the conformal horizon symbol simultaneously. In IMC, pilots see only the horizon symbol. It was hypothesized that pilots would tilt their heads in VMC and in IMC (due to the fact that the conformal horizon represents the true horizon). Eleven pilot-subjects completed a VMC and an IMC flight task. Results showed no practical head tilt in either task. This was attributed to the nature of the task. Task demands determine the visual information to which pilots attend. This attention narrowing may influence the strength of the OKCR.