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Offset localizer course; and/or,
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2-bar VASI, if used below (typically) 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;
°. 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 flightcrew 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.
Visual Illusions Awareness
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AIRBUS INDUSTRIE
Flight Operations Support
. 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.
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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.
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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.
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