( Photo No.1 : 12 NM from touchdown – Photo No.2 : 2 NM from touchdown, on PAPI glide path )
Figure 2
Effect of Terrain Down-hill Slope on Flight Path Perception
Perceived Glide Path
Actual Glide Path
Runway Environment
Runway dimensions / aspect ratio (Figure 3):
The runway aspect ratio (i.e., its length relative to its width) affects the crew visual perspective view of the runway:
A wide or short runway (low aspect ratio) creates an impression of being too low; and,
A narrow or long runway (high aspect ratio) creates an impression of being too high.
Figure 3
Center Photo : LFBO 14 R (3500 m x 45 m) / 3-degree glide slope / 200 ft RA
Runway uphill or downhill slope:
An uphill slope creates an illusion of being too high (impression of a steep glide path); and,
A downhill slope creates an illusion of being too low (impression of a shallow glide path).
Approach and runway lighting:
The approach and runway lighting (including the touchdown zone lighting) affects the dept perception as a function of:
The lighting intensity;
The daytime or night time conditions; and,
The weather conditions.
Bright runway-lights create the impression of being closer to the runway (hence on a steeper glide path);
Low intensity lights create the impression of being farther away (hence on a shallower glide path);
A non-standard spacing of runway lights also modifies the pilot’s perception of the runway distance and glide path; and,
If runway lighting is partially visible (e.g., during the downwind leg or during the base leg of a visual or circling approach), the runway may appear being farther away or at a different angle (i.e., the intercept angle is perceived as smaller than actual).
Runway Approach Aids
The following runway approach-aids and conditions may increase the crew exposure to visual illusions:
Glide slope beam being unusable beyond a specific point because of terrain or below a specific altitude/height because of approach over water;
Offset localizer course; and/or,
2-bar VASI, if used below 300 ft height above touchdown (HAT) for glide path corrections.
Weather Conditions
The following weather conditions may cause visual illusions:
Precipitation’s (e.g., rain, fog, snow):
Flying in light rain, fog , haze, mist, smoke, dust, glare or darkness usually create an illusion of being too high;
Flying in haze creates the impression that the runway is farther away, inducing a tendency to shallow the glide path and land long;
Shallow fog (i.e., fog layer not exceeding 300 ft in thickness) results in a low obscuration but also in low horizontal visibility:
When on top of a shallow fog layer, the ground (or airport and runway, if flying overhead) can be seen, but when entering the fog layer the forward and slant visibility usually are lost; 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：Flight Operations Briefing Notes Human Performance Visual Il(3)
