the inability of a person to determine his true body position, motion, and altitude relative to the earth or his surroundings. Both airplane pilots and underwater divers encounter the phenomenon. Most clues with respect to orientation are derived from sensations received from the eyes, ears, muscles, and skin. The human sensory apparatus, however, is often not delicate enough to perceive slow and gradual changes in motion; also, when motion changes are abrupt, the sense organs tend to overestimate the degree of change. Spatial disorientation in aircraft can arise from flight situations or visual misinterpretation. Banks and turns often create false sensations. When turning gradually, a pilot may feel as though he were on a straight course but ascending; when a turn is corrected, the impression is that of descending. If the plane banks or ascends or descends slowly, the pilot may not perceive the change, and the plane will feel level to him. If the plane skids while turning, the sensation is one of being banked in the direction opposite from the skid. A reaction called leans is caused by level flight after a rapid roll; the inertia of the roll causes the body to lean in a direction opposite to the direction of turning even after the motion of the roll has been stopped. If the pilot rapidly looks downward while turning, the so-called Coriolis effect occurs, in which the plane feels as though it is descending. The usual reaction of the pilot is to pull back on the stick to raise the plane. In a spin, the illusion of nonmotion is created if the spin is continued long enough; when the pilot corrects the spin, he has the feeling of spinning in the opposite direction, and his natural reaction is to counter his corrective measures and go back into the original spinning pattern. This phenomenon is known as the graveyard spin. The graveyard spiral results when the sensation of turning is lost in a banked turn. Because the pilot's instruments show that he is losing altitude, he may pull back on the stick and add power, thus inducing a spiral motion. The oculogyral illusion is created by acceleration and turning: a turning target watched by a pilot while turning himself appears to move faster than it is actually going; it may appear to continue to turn even after the pilot has stopped his motion and the target has stopped. Another illusion is caused by forward acceleration: when a pilot takes off from land, the increased speed gives the impression of nosing the plane too high; to compensate the pilot may lower the nose and dive back to the ground. During a rapid deceleration the nose of the plane appears to drop; if the pilot corrects this feeling by trying to gain more altitude, the plane stalls and goes into a spin. The gravitational forces on a pilot cause the oculoagravic illusions: a target watched by a pilot appears to rise if weightlessness occurs and appears to fall when gravity is increased. Visual misinterpretations do not usually depend on acceleration factors or on the sense of equilibrium but, rather simply, on visual illusions. The autokinetic phenomenon is the apparent wandering of an object or spot of light; when following another plane at night, the pilot may have trouble distinguishing between real and apparent movements of the lead plane. If two planes are flying parallel and level but at different speeds, they give the pilots the illusion of turning. Ground lights can be mistaken for the horizon or stars; fixed beacon lights can be mistaken for another plane flying in formation. The only measures that can prevent spatial disorientation are thorough training and instrumentation.

Britannica English vocabulary.      Английский словарь Британика.