Perception of runway image shape and approach angle magnitude by pilots in simulated night landing approaches.

One cue for visual judgments of glidepath angle has been referred to as form ratio. Form ratio is defined as the ratio of vertical height of the runway to width of the far end in the runway retinal image. The ability of pilots to judge form ratios was compared with the ability to judge approach angles in the nighttime "black hole" situation in two experiments. Responses in both static and dynamic simulated approach conditions indicated a general tendency to overestimate form ratios and approach angles less than 3 degrees. Intersubject and intrasubject variability of form ratio and approach angle responses were comparable. These findings (i) do not support the utility of form ratio judgments as an aid in selecting approach angle, (ii) add to the empirical evidence of visual illusions and the danger of reliance on visual information for judgment of approach angle in the nighttime "black hole" situation where only runway light are visible, and (iii) point to variability in perception of approach angle as an important part of the problem.