F L I G H T S A F E T Y F O U N D A T I O N HUMAN FACTORS & AVIATION MEDICINE Vol. 39 No. 1For Everyone Concerned with the Safety of Flight January/February 1992 Inflight Spatial Disorientation Pilots must learn to recognize the visual and sensory signs of approaching problems interpreting motion, position and attitude and to minimize their effects on the safety of flight. by Melchor J. Antuñano, M.D. KRUG Life Sciences/U.S. Air Force School of Aerospace Medicine and Stanley R. Mohler, M.D. Wright State University/Aerospace Medicine Program Spatial orientation refers to a person’s ability to perceive focal vision, which is involved in the recognition and motion, position and attitude in relation to the surround- identification of viewed objects (resulting from the stimu- ing environment. This capability depends upon the re- lation of the central region of the retina, the fovea, and ception, integration and interpretation of sensory inputs immediately adjacent areas). The second is peripheral from visual, vestibular (inner ear), muscular and skin vision, which provides spatial visual reference (resulting receptors. The genetic design of the spatial orientation from the stimulation of the retina beyond the foveal mechanism copes with usual human activities (sitting, area). Foveal vision provides the fine, detailed sense of standing, walking, running, and jumping, for example). the visual image. Peripheral vision is more sensitive to However, this mechanism can encounter interpretation motion and also provides overall broad field of vision problems when outside visual references are lost and an position cues. The great importance of peripheral vision illusory environment exists. These conditions include: in spatial orientation is demonstrated by the sensations accelerations (for example, spinning motions) and under of self-motion that can be experienced while watching certain other situations, including the influence of dis- wide-screen movies. The modern flight simulator takes eases and toxemias (e.g., alcohol). Under these condi- advantage of this visual characteristic. tions, spatial orientation may not be easily and accu- rately achieved. The role of the vestibular system in providing spatial orientation is extremely important when, for example, a The healthy, non-blind person in a lighted environment person is standing on a dark path under an overcast night can incorporate two modalities of vision. The first is sky with no lighting (the worst-case scenario would be standing in the pitch-black darkness of a cave). The ves- tion from the flight instruments. tibular system responds to linear (translation) and angu- lar (rotation) accelerative forces. Sensory elements of Inflight spatial disorientation describes the undesirable the inner ear vestibular system stabilize vision under condition of an individual who does not know the true lighted conditions during motion of the head, and, in attitude of the aircraft in relation to the surface of the dark areas, assist in position sensing. This system also earth. This condition involves visual and vestibular illu- provides orientational information that allows the auto- sions and results from the physiological characteristics matic execution of skilled and reflexive motor activities. and limitations of sensory mechanisms that were de- signed for maintaining spatial orientation while moving In addition to the visual and vestibular system receptors, about on the earth’s surface. there are other sensory receptors located in the muscles, tendons, ligaments and joints (proprioceptive receptors), There are several examples of sensory illusions that lead and skin (exteroceptive receptors) that assist in spatial to inflight spatial disorientation. These include both vi- orientation. These other receptors are known as mecha- sual and vestibular illusions. noreceptors because they respond to the mechanical dis- placement of tissue, and they are the sensors for touch, pressure, vibration and posture. These sensory inputs, Visual Illusions Vary concerning relationships and movements of one part of the body with respect to another, are necessary to main- Visual Illusions tain adequate orientation. In addition, the sensory infor- mation allows the determination of shape, size and tex- Illusions of altitude, distance, attitude and motion rela- ture of the structures with which the body is in direct tive to the ground occur when the available visual cues contact. are absent, unfamiliar, degraded or do not correspond with the pilot’s expectations. Among visual illusions On the ground, there is a degree of physiological com- there are aerial perspective illusions, peripheral cues, pensation that allows spatial orientation in the natural autokinesis, vection illusion and false visual cues. gravity environment even if, for example, there is an absence of vision. In this case, the vestibular system can Aerial Perspective Illusions interact with proprioceptive and exteroceptive receptors to maintain spatial orientation and postural Under normal flying conditions, pilots learn to automati- equilibrium. If there is an absence of a cally associate a typical approach path for reliable vestibular function on land, the vi- landing (for example, a three-degree slope sual system can, under suitable lighting con- Inflight sensory in relation to a horizontal runway) with a ditions, interact with proprioceptive and corresponding visual perspective of the runway exteroceptive receptors to maintain orien- spatial ahead. If the runway appears shorter than tation and posture. The compensatory ca- usual, the corresponding approach path is pacity of the spatial orientation mechanism orientation perceived as too low (less than a three-de- will be effective on land so long as there is cannot be gree slope), whereas, if the runway looks a functional integrity of at least two sen- longer than usual, the corresponding ap- sory systems. maintained after proach path is perceived as too high (more than a three-degree slope). In addition, a Maintaining spatial orientation during flight loss of outside visual approach to a sloping runway can be when the outside horizon visual reference visual horizon disorienting. When the runway is sloping is lost requires either orientation instru- down toward the approach end, the pilot ment displays or automatic stabilization references may perceive the approach as too high (be- systems. Pilot exposure to linear and angu- cause the runway appears longer than usual). lar accelerative forces during loss-of-out- without flight The opposite illusion can occur with a run- side-reference flight produces confusing instruments. way that slopes down away from the ap- vestibular and proprioceptor stimulations proach end, giving the appearance of an that result in motion illusions which impair approach that is too low. spatial orientation. Inflight sensory spatial orientation cannot be maintained after loss of outside Differences in runway width also can create visual illu- visual horizon references without flight instruments. For sions. When the runway is wider than usual, it will ap- orientation in this situation, the pilot must utilize attitude pear closer and shorter, and consequently, the aircraft information provided by the cockpit flight displays. The may be perceived as lower, and vice versa. These illu- pilot must learn to not trust intrinsic orientational sen- sions occur because the visual perspective of the runway sory perceptions during flight, but must obtain orienta- being approached does not conform to the pilot’s estab- 2 FLIGHT SAFETY FOUNDATI O N • HUMAN FACTORS & AVIATION MEDICINE • JANUARY/FEBRUARY 1992 lished mental image of a runway during typical circum- visually induced perception of self-motion (linear or an- stances. In other words, the pilot is not seeing what he gular) and is called a vection illusion. Most people have anticipated. Accidents have occurred where unsuspect- experienced this type of illusion while sitting in a stopped ing pilots have flown a visual illusion rather than reality. car as another car moves slowly by. The illusion of mo- These illusions are often more pronounced at night. tion of the subject’s car moving opposite to the other occurs. This linear vection illusion is often very compel- Visual approaches over smooth water surfaces, snow- ling, especially in trains. During takeoff or landing, a covered terrain, or flat desert terrain often are character- snowstorm or sandstorm in a given direction can create ized by few peripheral visual cues. These can result in a the illusion of the subject’s movement in the opposite misperception of height above the surface. Daytime vi- direction and impair the normal directional control of the sual approaches with reduced visibility due to fog, rain, aircraft. A pilot can experience an angular vection illu- smoke, haze or snow can make a runway appear farther sion if the anti-collision light on the aircraft is on during away as a result of restricted focal vision. Nighttime flight through clouds or fog. The pulsating light can visual approaches under reduced visibility conditions can produce a strong ambient visual stimulus indicating rota- make runway approach lights appear dimmer than they tion in the yaw or other axis. Linear and angular vection are in clear air. Dimness gives the illusion that the run- illusions are exploited by advanced flight simulators in way is farther away. order to create a more realistic sensation of flight. Peripheral Cues False Visual Cues During flight, any condition that impairs