( 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;
　
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