Human senses are not naturally geared for the in-flight environment. Pilots may experience disorientation and loss of perspective, creating illusions that range from false horizons to sensory conflict with instrument readings or the misjudging of altitude over water.
Vestibular system
The vestibular system, which is responsible for the sense of balance in humans, consists of the otolith organs and the semicircular canals. Illusions in aviation are caused when the brain cannot reconcile inputs from the vestibular system and visual system. The three semicircular canals, which recognize accelerations in pitch, yaw, and roll, are stimulated by angular accelerations; while the otolith organs, the saccule and utricle, are stimulated by linear accelerations. Stimulation of the semicircular canals occurs when the movement of the endolymph inside the canals causes movement of the crista ampullaris and the hair cells within them. Stimulation of the otolith organs occurs when gravitational forces or linear accelerations cause movement of the otolith membrane, the otoliths, or the hair cells of the macula. Somatogyral illusions occur as a result of angular accelerations stimulating the semicircular canals. Somatogravic illusions, on the other hand, occur as a result of linear accelerations stimulating the otolith organs.
Vestibular/somatogyral Illusions involving the semicircular and somatogyral canals of the vestibular system of the ear occur primarily under conditions of unreliable or unavailable external visual references and result in false sensations of rotation. These include the leans, the graveyard spin and spiral, and the Coriolis illusion.
The leans
This is the most common illusion during flight, and can be caused by a sudden return to wings-level flight following a gradual entry and prolonged application of bank that had gone unnoticed by the pilot. The reason a pilot can be unaware of such an attitude change in the first place is that human exposure to a rotational acceleration of ~1 degrees per second² or less is below the detection threshold of the semicircular canals. Rolling wings-level from such an attitude may cause an illusion that the aircraft is banking in the opposite direction. In response to such an illusion, a pilot will tend to roll back in the direction of the original bank in a corrective attempt to regain the perception of a level attitude.
Graveyard spin
The graveyard spin is an illusion that can occur with a pilot who enters into a spin, and it is characterized by the pilot becoming less aware of the sense of rotation induced by the spin as the spin continues. As the pilot becomes less aware of the spin, any correction of the spin may cause the pilot to sense that he or she is spinning in the opposite direction. As an example, if the airplane is spinning to the left but goes unnoticed for a period of time sufficient for the pilot to become desensitized to the magnitude of the spin, a small adjustment to the right rudder may leave the pilot with a sensation of spinning to the right. As a result, the pilot will apply left rudder and unknowingly re-enter the original left spin. Cross-checking the airplane's flight instruments would show that the airplane is still in a turn, which causes sensory conflict for the pilot. If the pilot does not correct the spin, the airplane will continue to lose altitude until it crashes into the ground.
Graveyard spiral
The graveyard spiral is characterized by the pilot mistakenly believing they are in wings-level flight when the aircraft is in fact engaged in a banking turn, and the pilot notices the altimeter indicating an ongoing drop in altitude. The sensory disorientation of returning from a prolonged banking turn to wings-level flight can cause the pilot to re-enter the banking turn, as in the graveyard spin illusion. While the plane continues in the turn and begins to indicate a loss of altitude, the pilot will try to correct the loss of altitude by "pulling up" on the plane's controls. Attempting to adjust the controls in this way will have the effect of tightening the radius of the turn and eventually quickening the rate of descent until the pilot is visually cued to the nature of the error or contact with the terrain occurs. Two of the most famous cases of an aircraft mishap from this form of spatial disorientation are the 1963 crash that killed singer Patsy Cline near Camden, Tennessee and the 1999 crash that killed John F. Kennedy Jr. near Martha's Vineyard.
Coriolis illusion
This involves the simultaneous stimulation of two semicircular canals and is associated with a sudden tilting (forward or backwards) of the pilot's head while the aircraft is turning. This can occur when tilting the head down (to look at an approach chart or to write on the knee pad), up (to look at an overhead instrument or switch), or sideways. This can produce an overpowering sensation that the aircraft is rolling, pitching, and yawing all at the same time, which can be compared with the sensation of rolling down a hillside. This illusion can make the pilot quickly become disoriented and lose control of the aircraft.
Vestibular/somatogravic
Somatogravic illusions are caused by linear accelerations. These illusions involving the utricle and the saccule of the vestibular system are most likely to occur under conditions with unreliable or unavailable external visual references.
Inversion An abrupt change from climb to straight-and-level flight can stimulate the otolith organs enough to create the illusion of tumbling backwards, or inversion illusion. The disoriented pilot may push the aircraft abruptly into a nose-low attitude, possibly intensifying this illusion.
Head-up The head-up illusion involves a sudden forward linear acceleration during level flight where the pilot perceives the illusion that the nose of the aircraft is pitching up. The pilot's response to this illusion would be to push the yoke or the stick forward to pitch the nose of the aircraft down. A night take-off from a well-lit airport into a totally dark sky (black hole) or a catapult take-off from an aircraft carrier can also lead to this illusion, and could result in a crash.
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