To execute a successful CAT II approach and landing, the pilot uses both instrument information and visual cues from the outside scene. In the conventional cockpit these two sets of information are obtained in a sequential manner. The pilot monitors the instrument panel in a "head-down" position, and then at some point looks up and lands visually. With the head-up display (HUD), instrument information is superimposed on the outside scene, so that the pilot can see both sets of information while looking out at the external scene; thus the name, the "head-up display." Fischer made an extensive literature search on attention and cognitive switching in which the ability of pilots to extract information from superimposed visual fields was explored. The cognitive issue with the HUD was addressed by Naish in a series of laboratory, simulator, and flight tests, by Fischer (ref. 1) in the laboratory, and again by Naish in a simulator. These studies have shown that within the framework of the respective experiments, pilots were capable of perceiving and evaluating information from both sources quickly and efficiently. However, none of these studies was conclusive enough to put the cognitive issue to rest. Two *San Jose State University Foundation, San Jose, Calif. 95192. _-Ames Research Center, Moffett Field, Calif. 94035. important questions remained: (1) At each point in the approach, which information source is used for primary control of the flight path, and which for monitoring? and (2) If the display draws the primary attention of the pilot, to what extent does this impede the transmission of possibly vital information from the outside scene? The present study directly addressed these questions by exposing the pilot to various conflicts between the conformal symbology and the outside scene, and to an unexpected obstacle on the runway.