The effect of helmet-mounted display symbology on the opto-kinetic cervical reflex, frame of reference, and pilot performance

In spite of all the latest technological advances incorporated into today's modern fighter aircraft, spatial disorientation (SD) in flight continues to be a problem. To determine their attitude, pilots need a frame of reference against which their orientation can be compared. Recently, there have been numerous studies conducted that verify and characterize a visual reflex observed in pilots called the opto-kinetic cervical reflex (OKCR), which implies that pilots use a world frame of reference to orient themselves when looking at real world visual cues. In contrast, when pilots use instruments to determine orientation, the information is portrayed in an aircraft frame of reference. Transitions between the two frames of reference appear to be related to SD incidences. New helmet-mounted display (HMD) technology is being designed for the cockpit. HMDs portray aircraft-referenced symbology superimposed on a world-referenced scene. This research was designed to investigate how pilots would use real world and symbology frames of reference under various task configurations. Because the HMD horizon symbol is conformal to the true horizon, it was hypothesized that pilots would exhibit the OKCR, when viewing the real-world horizon cue with the HMD horizon symbol as in visual meteorological conditions (VMC) and when viewing the horizon symbology alone during instrument meteorological conditions (IMC). It was also hypothesized that transitions between different visual cues would be easier for the pilot to achieve when using the HMD. Twelve pilot-subjects completed four tasks (VMC flight, IMC flight, unusual attitude recovery, and in and out of clouds) to examine frames of reference and pilot performance. Results showed that pilots did not exhibit the OKCR when using the HMD symbology during the VMC and IMC tasks, indicating that a world frame of reference was not used to perform the tasks. In tasks where both real world visuals and the HMD symbology were present, pilots were selectively attending to the HMD symbology that contained all of the information necessary to complete the task and maintain orientation. When transitioning to HMD symbology after following a lead aircraft into an unusual attitude, pilots exhibited control reversal errors, indicating again that the conformal horizon was not necessarily viewed as world-referenced. When transitioning in and out of clouds, pilots were better able to maintain commanded vertical velocity when using the HMD symbology compared to a traditional attitude indicator, which is attributed to the fact that the HMD symbology is superimposed on the real world visual scene. There are two primary implications of this research. First, it is possible to eliminate unnecessary transitions between different frames of reference by having pilots perform tasks that utilize one set of visual cues capable of providing both task and orientation information. Second, the HMD attitude symbology set used in this study was not able to keep pilots from experiencing control reversal errors, indicating that necessary transitions can still be a problem with conformal symbology. Further research is needed to determine how to facilitate transitions between different frames of reference.