The helmet-mounted display (HMD) affords continuous availability of critical flight information independent of head orientation. With appropriate information presented on a HMD, aircraft control can be maintained regardless of where the pilot is looking. This research addresses the development of an empirically based HMD symbol set. Three attitude formats and three altitude formats were evaluated within a composite fixed-wing HMD symbology layout. The attitude formats varied in basic form and symbol compression ratio. Symbol compression ratio is the ratio of the angle represented by the symbol to the symbol's subtended visual angle. High symbol compression results in symbols which represent large angles, and therefore have slow rate-of-motion relative to their uncompressed counterparts. The altitude symbologies were formed of both vertical scale and dial formats and included vertical velocity indicators. Subjects performed a flight-path maintenance task within sessions of differing "real" horizon presence and orientation. The formats were evaluated under a task which was designed to require high-accuracy flight-path maintenance. This type of task is traditionally thought to require less symbol compression. The results showed that performance was influenced by the manipulation of the attitude symbology formats. The results also suggest that symbol compression may be advantageous.
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