Again, it is essential to be thoroughly familiar with the approach light system to develop the proper perspective between these cues and the runway environment.
13.2 Low Stratus Clouds.
This forms more of a definite ceiling and better visibility can be expected once the ceiling is passed. The transition from instrument to visual flight is sharper with more pronounced use of visual cues after passing the ceiling. Night approaches may produce the sensation that the aircraft is high once the cloud base is passed. The pilot should continue on instruments crosschecking visual cues to confirm runway alignment. During the flare, the pilot may experience a sensation of descending below the surface of the runway. This will be specially pronounced at facilities with 300 ft wide runways. The pilot must avoid large attitude changes, which would produce over-rotation.
13.3 Precipitation.
Approach and transition to visual flight can be hazardous since moderate to heavy rain conditions may seriously affect the acquisition of visual cues and displace the pilot’s visual angle. Night approaches in these conditions can be even more critical because the pilot may be blinded by lighting, flashing strobes or runway end identifier lights.Raindrops falling on windshield and on the approaching terrain can cause visual distortions, which may lead to mismanaged approaches. Several kinds of distortion errors are possible, including those due to refraction/diffusion. Refraction here means the change of direction of light beams as they pass from one type of medium to another, as from air to water. Water slows up the passage of light and causes it to “bend” as in the familiar example of placing a stick in water.
Also familiar is the experience of seeing objects at the bottom of a pool as being at a shallower depth than their true depth, because of the refraction of light by the water. In heavy rain, even though visibility may appear fairly good to the pilot, terrain contours or lights may seem lower than their actual elevation relative to the airplane. Diffusion (“hallowing”) refers to the tendency of lights seen through moisture to spread apart and appear less intense and therefore, farther away then their actual distance. Conversely, diffusion, under other circumstances may cause approach lights to appear larger and hence nearer than they are. Incidentally, the more effective the windshield wipers are, the lesser is the distortion. So, a periodic check of wiper conditions and performance is a worthy venture. Transition to visual flight may be severely hampered by pilot’s inability to adequately maintain aircraft control in gusty or turbulent conditions.
13.4 Wind Shear.
Wind Shear is one factor that may require either adjustment to heading, sink rate and thrust, or a missed approach, even after the aircraft is properly flown to Decision Height and visual reference is established. Wind shear is the change in direction and/or velocity of the wind within vertical or horizontal planes traversed by an aircraft descending on approach. It is expressed in Knots per hundred feet altitude. If a 20-knot headwind at 100 feet dropped 10 knots at the surface it would be termed a 10-knot shear. Also, if a 10 knot cross wind component at 100 feet swung to a 10-knot headwind or tailwind at the surface, it would also be a 10-knot shear.
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