Spatial disorientation

Spatial orientation in flight is difficult to achieve because numerous 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. The visual sense is considered to be the largest contributor to orientation. In 1926, Ocker was subjected to a Bárány chair equilibrium test by Dr. David A. Myers at Crissy Field; the resulting duplication of the somatogyral illusion he had experienced and a subsequent re-test, which he passed using the turn indicator, led him to develop and champion instrumented flight. With Lt. Carl Crane, Ocker published the instructional text Blind Flying in Theory and Practice in 1932. Influential advocates of instrumented flight training included Albert Hegenberger and Jimmy Doolittle. A new version of the advisory was issued in 1983 as AC 60-4A, defining spatial disorientation as "the inability to tell which way is 'up. Statistics show that between 5% and 10% of all general aviation accidents can be attributed to spatial disorientation, 90% of which are fatal. Spatial-D and G-force induced loss of consciousness (g-LOC) are two of the most common causes of death from human factors in military aviation. Training and awareness campaigns have also addressed spatial disorientation risk associated with continued visual flight into instrument meteorological conditions (IMC). In the United States, the U.S. Helicopter Safety Team (USHST) produced the 56 Seconds to Live training video and related materials about unintended IMC (UIMC), depicting a VFR helicopter pilot losing control after entering cloud; the campaign is used in safety education to emphasize early avoidance decisions and prompt transition to instrument flying when appropriate. == Physiology ==

There are four physiologic systems that interact to allow humans to orient themselves in space. Vision is the dominant sense for orientation, but the vestibular system, proprioceptive system and auditory system also play a role. Spatial orientation (the inverse being spatial disorientation, aka spatial-D) is the ability to maintain body orientation and posture in relation to the surrounding environment (physical space) at rest and during motion. Humans have evolved to maintain spatial orientation on the ground. Good spatial orientation on the ground relies on the use of visual, auditory, vestibular, and proprioceptive 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. The three-dimensional environment of flight is unfamiliar to the human body, creating sensory conflicts and illusions that make spatial orientation difficult and sometimes impossible to achieve. The result of these various visual and nonvisual illusions is spatial disorientation. Various models have been developed to yield quantitative predictions of disorientation associated with known aircraft accelerations. == The vestibular system and sensory illusions ==

The vestibular system detects linear and angular (rotational) acceleration using specialized organs in the inner ear. Linear accelerations are detected by the otolith organs, while angular accelerations are detected by the semicircular canals. Misleading sensations Without a visual reference or cues, such as a visible horizon, humans will rely on non-visual senses to establish their sense of motion and equilibrium. During the abnormal acceleratory environment of flight, the vestibular and proprioceptive systems can be misled, resulting in spatial disorientation. When an aircraft is maneuvering, inertial forces can be created by changes in vehicle speed (linear acceleration) and/or changes in direction (rotational acceleration and centrifugal force), resulting in perceptual misjudgment of the vertical, as the combined forces of gravity and inertia do not align with what the vestibular system assumes is the vertical direction of gravity (towards the center of the Earth). Under ideal conditions, visual cues will provide sufficient information to override illusory vestibular inputs, but at night or in poor weather, visual inputs can be overwhelmed by these illusory nonvisual sensations, resulting in spatial disorientation. Low visibility flight conditions include night, Similarly, it is possible to gradually climb or descend without a noticeable change in pressure against the seat. In some aircraft, it is possible to execute a loop without pulling negative g-forces so that, without visual reference, the pilot could be upside down without being aware of it. A gradual change in any direction of movement may not be strong enough to activate the vestibular system, so the pilot may not realize that the aircraft is accelerating, decelerating, or banking. , including attitude indicator (top center) and turn and slip indicator (bottom left) Gyroscopic flight instruments such as the attitude indicator (artificial horizon) and the turn and slip indicator are designed to provide information to counteract misleading sensations from the non-visual senses. Otoliths and somatogravic illusions Two otolith organs, the saccule and utricle, are located in each ear and are set at right angles to each other. The utricle detects changes in linear acceleration in the horizontal plane, while the saccule detects linear accelerations in the vertical plane; humans have evolved to assume the vertical acceleration is caused by gravity. However, the saccule and utricle can provide misleading sensory perception when gravity is not limited to the vertical plane, or when vehicle speeds and accelerations result in inertial forces comparable to the force of gravity, as the otoliths only detect acceleration, and cannot distinguish inertial forces from the force of gravity. In a 1954 study (180 – Degree Turn Experiment), the University of Illinois Institute of Aviation found that 19 out of 20 non-instrument-rated subject pilots went into a graveyard spiral soon after entering simulated instrument conditions. The 20th pilot also lost control of his aircraft, but in another maneuver. The average time between onset of instrument conditions and loss of control was 178 seconds. Spatial disorientation can also affect instrument-rated pilots in certain conditions. A powerful tumbling sensation (vertigo) can result if the pilot moves his or her head too much during instrument flight. This is called the Coriolis illusion. Because the semicircular canals are set in three different axes of rotation, if the aviator suddenly moves their head during a rotational acceleration, one canal may abruptly start to detect an angular acceleration while another ceases, resulting in a tumbling sensation. == Visual illusions ==

Even with good visibility, misleading visual inputs such as sloping cloud decks, unfamiliar runway grades, or false horizons can also form optical illusions, resulting in the pilot misjudging the vertical orientation, aircraft speed or altitude, and/or distance and depth perception; these could even combine with nonvisual illusions from the vestibular and proprioceptive systems to produce an even more powerful illusion. == Examples ==