Spatial Orientation
- Spatial orientation defines our natural ability to maintain our body orientation and/or posture in relation to the surrounding environment (physical space) at rest and during motion. Genetically speaking, humans are designed to maintain spatial orientation on the ground. The flight environment is hostile and unfamiliar to the human body; it creates sensory conflicts and illusions that make spatial orientation difficult, and, in some cases, even impossible to achieve. Statistics show that between 5 to 10% of all general aviation accidents can be attributed to spatial disorientation, and 90% of these accidents are fatal.
Spatial Orientation on the Ground
 Good spatial orientation on the ground relies on the effective perception, integration, and interpretation of visual, vestibular (organs of equilibrium located in the inner ear), and proprioceptive (receptors located in the skin, muscles, tendons, and joints) sensory information. Changes in linear acceleration, angular acceleration, and gravity are detected by the vestibular system and the proprioceptive receptors, and then compared in the brain with visual information (Figure 1, at right).
Spatial Orientation In Flight
Spatial orientation in flight is sometimes difficult to achieve because the various types of sensory stimuli (visual, vestibular, and proprioceptive) vary in magnitude, direction, and frequency. Any differences or discrepancies between visual, vestibular, and proprioceptive sensory inputs result in a "sensory mismatch" that can produce illusions and lead to spatial disorientation.
Vision and Spatial Orientation
Visual references provide the most important sensory information to maintain spatial orientation on the ground and during flight, especially when the body and/or the environment are in motion. Even birds, reputable flyers, are unable to maintain spatial orientation and fly safely when deprived of vision (due to clouds or fog). Only bats have developed the ability to fly without vision but have replaced their vision with auditory echolocation. So, it should not be any surprise to us that, when we fly under conditions of limited visibility, we have problems maintaining spatial orientation.
Central Vision
Central vision, also known as foveal vision is involved with the identification of objects and the perception of colors. During instrument flight rules (IFR) flights, central vision allows pilots to acquire information from the flight instruments that is processed by the brain to provide orientational information. During visual flight rules (VFR) flights, central vision allows pilots to acquire external information (monocular and binocular) to make judgments of distance, speed, and depth.
Peripheral Vision
Peripheral vision, also known as ambient vision, is involved with the perception of movement (self and surrounding environment) and provides peripheral reference cues to maintain spatial orientation. This capability enables orientation independent from central vision and that is why we can walk while reading. With peripheral vision, motion of the surrounding environment produces a perception of self-motion even if we are standing or sitting still.
Visual References
- Visual references that provide information about distance, speed, and depth of visualized objects include:
- Comparative size of known objects at different distances.
- Comparative form or shape of known objects at different distances.
- Relative velocity of images moving across the retina. Nearby objects are perceived as moving faster than distant objects .
- Interposition of known objects. One object placed in front of another is perceived as being closer to the observer.
- Varying texture or contrast of known objects at different distances. Object detail and contrast are lost with distance.
- Differences in illumination perspective of objects due to light and shadows.
- Differences in aerial perspective of visualized objects. More distant objects are seen as bluish and blurry.
The flight attitude of an airplane is generally determined by the pilot's visual reference to the natural horizon. When the natural horizon is obscured, attitude can sometimes be maintained by visual reference to the surface below. If neither horizon nor surface visual references exist, the airplane's attitude can only be determined by artificial means such as an attitude indicator or other flight instruments. Surface references or the natural horizon may at times become obscured by smoke, fog, smog, haze, dust, ice particles, or other phenomena, although visibility may be above VFR minimums. This is especially true at airports located adjacent to large bodies of water or sparsely populated areas, where few, if any, surface references are available. Lack of horizon or surface reference is common on over-water flights, at night, or in low visibility conditions.
Visual Illusions
Visual illusions are familiar to most of us. As children, we learned that railroad tracks -contrary to what our eyes showed us- don't come to a point at the horizon. Even under conditions of good visibility, you can experience visual illusions including:
Aerial Perspective Illusions may make you change (increase or decrease) the slope of your final approach. They are caused by runways with different widths, upsloping or downsloping runways, and upsloping or downsloping final approach terrain. Pilots learn to recognize a normal final approach by developing and recalling a mental image of the expected relationship between the length and the width of an average runway (Figure 2, below—click for larger image).
A final approach over a flat terrain with an upsloping runway may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose down to decrease the altitude, which, if performed too close to the ground, may result in an accident (Figure 3, below).
A final approach over a flat terrain with a downsloping runway may produce the visual illusion of a low-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose up to increase the altitude, which may result in a low-altitude stall or a missed approach (Figure 4, below).
A final approach over an upsloping terrain with a flat runway may produce the visual illusion of a low-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose up to increase the altitude, which may result in a low-altitude stall or a missed approach (Figure 5, below).
A final approach over a downsloping terrain with a flat runway may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose down to decrease the altitude, which, if performed too close to the ground, may result in an accident (Figure 6, below).
A final approach to an unusually narrow runway or an unusually long runway may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose down to decrease the altitude, which, if performed too close to the ground may result in an accident (Figure 7, below).
A final approach to an unusually wide runway may produce the visual illusion of a low-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose up to increase the altitude, which may result in a low-altitude stall or a missed approach (Figure 8, below).
A Black-Hole Approach Illusion can happen during a final approach at night (no stars or moonlight) over water or unlighted terrain to a lighted runway beyond which the horizon is not visible. In the example (Figure 9, below), when peripheral visual cues are not available to help you orient yourself relative to the earth, you may have the illusion of being upright and may perceive the runway to be tilted left and upsloping.
However, with the horizon visible (Figure 10, below), you can easily orient yourself correctly using your central vision.
A particularly hazardous black-hole illusion involves approaching a runway under conditions with no lights before the runway and with city lights or rising terrain beyond the runway. These conditions may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respnd by lowering your approach slope (Figure 11, below).
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