The objective of this investigation was to evaluate the human factors issues related to the display of Geometric Altitude on the Terrain Awareness Display (TAD) of the Enhanced Ground Proximity Warning System (EGPWS). Geometric Altitude is essentially a measure of the aircraft's True Altitude, the vertical distance above Mean Sea Level (MSL). The purpose of the display of Geometric Altitude is to indicate to the flight crew the reference altitude employed for the TAD and the Forward-Looking Terrain Avoidance Alerts. Geometric Altitude is not intended for vertical navigation. The investigation focused on the evaluation of: the independent effects of presenting a digital read-out of Geometric Altitude on the TAD; and the reference altitude employed for the depiction of terrain on the TAD (Geometric Vs Barometric Altitude). A part-task simulation facility (PC based) was employed for the evaluation. Thirty evaluation pilots were subjected to a range of conditions. The data indicate that: 1. an EGPWS that employs Geometric Altitude as the reference altitude for the terrain display and predictive alerting functions leads to an earlier and improved detection rate of an altitude deviation resulting from altimetry-related anomalies; 2. the addition of a digital read-out of Geometric Altitude on the terrain display leads to an earlier and improved detection rate of an altitude deviation resulting from altimetry-related anomalies; 3. Geometric Altitude resulted in better and more consistent pilot decision-making following the detection of an altitude anomaly — the display of Geometric Altitude does not negatively impact pilot decision making; and 4. pilot acceptability of an EGPWS employing Geometric Altitude as the reference system altitude and its display on the TAD is high. Subjects did not consider the display of Geometric Altitude to impose unrealistic mental workload demands. The data from this investigation provides evidence that display of Geometric Altitude in the flight deck would lead to improvements in operational safety.
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