Flight Operations Briefing Notes Human Performance Visual Il(4)

Entering a fog layer also creates the perception of a pitch up, thus inducing a tendency to push over and place the aircraft below the desired glide path and in a steeper-than-desired attitude;

In light rain or moderate rain, the runway may also appear fuzzy because of rain halo effect, increasing the risk of not perceiving a vertical deviation or lateral deviation during the visual segment.

The visual segment is defined as the segment flown after full transition from instruments to visual references;
Heavy rain affects depth perception and distance perception:

Rain on windshields creates a refraction and the perception of being too high, thus inducing a nose down correction that places the aircraft below  the desired flight path;

In daylight conditions, rain diminishes the apparent intensity of  the approach lighting system (ALS) resulting in the runway appearing to be farther away. As a result of this illusion, the flight crew tends to shallow  the flight path resulting in a long landing;

In night time conditions, rain increases the apparent brilliance of the ALS, making the runway appears to be closer, inducing a pitch down input and the risk of landing short of the runway threshold.

When breaking out of the overcast at both ceiling and visibility minimums (DH and RVR), the slant visibility may not allow sight of the farther bar(s) of  the VASI/PAPI, thus reducing the available visual clues for the visual segment in reduced visibility;

A snow-covered terrain together with a clouds overcast create a phenomenon called “white-out” that eliminate perception of terrain features (slope) and height above terrain.

Crosswind:

In crosswind conditions, the runway lights and environment will be angled with the aircraft heading; flight crew should maintain the drift correction and resist the tendency to align the aircraft heading with the runway centerline.

Runway surface condition (e.g., wet runway):

A wet runway does not reflect light, thus affecting depth perception by appearing to be farther away.

This visual effect usually results in a late flare and in a firm touchdown.
When landing on a wet runway, peripheral vision of runway edge lights should be used to increase the depth perception and determine the flare point.
Typical Crew Actions and Results

The following crew actions and their consequences often are cited in the analysis  of approach-and-landing incidents or accident resulting from visual illusions:
Unconscious modification of the aircraft trajectory to keep a constant perception  of visual references;

Natural tendency to descend below the glide slope or the initial glide path  (i.e., “ducking under”);

Inability to arrest the rate of descent after descending below the intended glide path  (i.e., late recognition of the flattening of runway and runway environment);