may result from operation under visual flight rules in conditions of marginal visibility. 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. A study on the prevalence of spatial disorientation incidents concluded that "if a pilot flies long enough ... there is no chance that he/she will escape experiencing at least one episode of [spatial disorientation]. Looked at another way, pilots can be considered to be in one of two groups; those who have been disorientated, and those who will be." 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,