Figure: Somatosensory System, from AFM 51-37, Figure 7-3.
Thirty years ago spatial disorientation was a leading cause of military aircraft accidents and it may still be. In the civilian world of transporting people and things from Point A to Point B, it takes a much smaller toll because very few of us intentionally pull more than a G and a half or ever venture beyond a standard rate turn. But in the world of instrument flight, it is a threat. The most notable, fairly recent, mishap was that of a Boeing 737, the case of Flash Airlines 604.
What follows comes from the references shown below and my comments shown in blue.
[U.S. Army Aeromedical Training Manual, ¶9-1.] Spatial disorientation is an individual’s inability to determine his or her position, attitude, and motion relative to the surface of the earth or significant objects; for example, trees, poles, or buildings during hover. When it occurs, pilots are unable to see, believe, interpret, or prove the information derived from their flight instruments. Instead, they rely on the false information that their senses provide.
Spatial disorientation is not unusual, a properly trained pilot may have a moment of disorientation but "pulls out of it" once recognized.
[U.S. Army Aeromedical Training Manual, ¶9-2.] A sensory illusion is a false perception of reality caused by the conflict of orientation information from one or more mechanisms of equilibrium. Sensory illusions are a major cause of spatial disorientation.
We used to get this all the time while air refueling. Our eyes were on the wings of the tanker, which became our local horizon. A cloud deck below or the actual earth's horizon could disagree with the tanker's attitude and we could often find ourselves with momentary disorientation. Every now and then it got so bad we had to exchange aircraft control between pilots.
You can get it in every day flight by things like a sloping cloud deck or even a long, steady turn. The key, of course, is to check your attitude and performance instruments.
[U.S. Army Aeromedical Training Manual, ¶9-3.] Vertigo is a spinning sensation usually caused by a peripheral vestibular abnormality in the middle ear. Aircrew members often misuse the term vertigo, applying it generically to all forms of spatial disorientation or dizziness.
[U.S. Army Aeromedical Training Manual, ¶9-4.] A disoriented aviator does not perceive any indication of spatial disorientation. In other words, he does not think anything is wrong. What he sees—or thinks he sees—is corroborated by his other senses. Type I disorientation is the most dangerous type of disorientation. The pilot—unaware of a problem—fails to recognize or correct the disorientation, usually resulting in a fatal aircraft mishap:
[U.S. Army Aeromedical Training Manual, ¶9-5.] In Type II spatial disorientation, the pilot perceives a problem (resulting from spatial disorientation). The pilot, however, may fail to recognize it as spatial disorientation:
[U.S. Army Aeromedical Training Manual, ¶9-6.] In Type III spatial disorientation, the pilot experiences such an overwhelming sensation of movement that he or she cannot orient himself or herself by using visual cues or the aircraft instruments. Type III spatial disorientation is not fatal if the copilot can gain control of the aircraft.
Instrument Flying Handbook, pgs. 1-3 and 1-4
Figure: The Three Equilibrium Systems, from U.S. Army Aeromedical Training Manual, Figure 9-1.
[U.S. Army Aeromedical Training Manual, ¶9-7.] Three sensory systems—the visual, vestibular, and proprioceptive systems—are especially important in maintaining equilibrium and balance. Figure 9-1 shows these systems. Normally, the combined functioning of these senses maintains equilibrium and prevents spatial disorientation. During flight, the visual system is the most reliable. In the absence of the visual system, the vestibular and proprioceptive systems are unreliable in flight.
[U.S. Army Aeromedical Training Manual, ¶9-8.] Of the three sensory systems, the visual system is the most important in maintaining equilibrium and orientation. To some extent, the eyes can help determine the speed and direction of flight by comparing the position of the aircraft relative to some fixed point of reference. Eighty percent of our orientation information comes from the visual system.
Figure: The Vestibular System, from U.S. Army Aeromedical Training Manual, Figure 9-2.
[U.S. Army Aeromedical Training Manual, ¶9-11.] The inner ear contains the vestibular system, which contains the motion- and gravity detecting sense organs. This system is located in the temporal bone on each side of the head. Each vestibular apparatus consists of two distinct structures: the semicircular canals and the vestibule proper, which contain the otolith organs. [The figure] depicts the vestibular system. Both the semicircular canals and the otolith organs sense changes in aircraft attitude. The semicircular canals of the inner ear sense changes in angular acceleration and deceleration.
Figure: The Otolith Organs, from U.S. Army Aeromedical Training Manual, Figure 9-3.
[U.S. Army Aeromedical Training Manual, ¶9-12.] The otolith organs are small sacs located in the vestibule. Sensory hairs project from each macula into the otolithic membrane, an overlaying gelatinous membrane that contains chalk-like crystals, called otoliths. The otolith organs, shown in [the figure], respond to gravity and linear accelerations/decelerations. Changes in the position of the head, relative to the gravitational force, cause the otolithic membrane to shift position on the macula. The sensory hairs bend, signaling a change in the head position.
[U.S. Army Aeromedical Training Manual, ¶9-13.] When the head is upright, a "resting" frequency of nerve impulses is generated by the hair cells.
[U.S. Army Aeromedical Training Manual, ¶9-14.] When the head is tilted, the "resting" frequency is altered. The brain is informed of the new position.
[U.S. Army Aeromedical Training Manual, ¶9-15.] Linear accelerations/decelerations also stimulate the otolith organs. The body cannot physically distinguish between the inertial forces resulting from linear accelerations and the force of gravity. A forward acceleration results in backward displacement of the otolithic membranes. When an adequate visual reference is not available, aircrew members may experience an illusion of backward tilt.
Figure: Reaction of the Semicircular Canals to Changes in Angular Acceleration, from U.S. Army Aeromedical Training Manual, Figure 9-7.
[U.S. Army Aeromedical Training Manual, ¶9-16.] The semicircular canals of the inner ear sense changes in angular acceleration. The canals will react to any changes in roll, pitch, or yaw attitude.
[U.S. Army Aeromedical Training Manual, ¶9-17.] The semicircular canals are situated in three planes, perpendicular to each other. They are filled with a fluid called endolymph. The inertial torque resulting from angular acceleration in the plane of the canal puts this fluid into motion. The motion of the fluid bends the cupula, a gelatinous structure located in the ampulla of the canal. This, in turn, moves the hairs of the hair cells situated beneath the cupula. This movement stimulates the vestibular nerve. These nerve impulses are then transmitted to the brain, where they are interpreted as rotation of the head.
Figure: Position of Hair Cells During No Acceleration, from U.S. Army Aeromedical Training Manual, Figure 9-9.
[U.S. Army Aeromedical Training Manual, ¶9-18.] When no acceleration takes place, the hair cells are upright. The body senses that no turn has occurred.
Figure: Sensation During a Clockwise Turn, from U.S. Army Aeromedical Training Manual, Figure 9-10.
[U.S. Army Aeromedical Training Manual, ¶9-19.] When a semicircular canal is put into motion during clockwise acceleration, the fluid within the semicircular canal lags behind the accelerated canal walls. This lag creates a relative counterclockwise movement of the fluid within the canal. The canal wall and the cupula move in the opposite direction from the motion of the fluid. The brain interprets the movement of the hairs to be a turn in the same direction as the canal wall. The body correctly senses that a clockwise turn is being made.
Figure: Sensation During a Prolonged Clockwise Turn, from U.S. Army Aeromedical Training Manual, Figure 9-11.
[U.S. Army Aeromedical Training Manual, ¶9-20.] If the clockwise turn then continues at a constant rate for several seconds or longer, the motion of the fluid in the canals catches up with the canal walls. The hairs are no longer bent, and the brain receives the false impression that turning has stopped. The position of the hair cells and the resulting false sensation during a prolonged, constant clockwise turn is shown in [the figure]. A prolonged constant turn in either direction will result in the false sensation of no turn.
Figure: Sensation During Slowing or Stopping of a Clockwise Turn, from U.S. Army Aeromedical Training Manual, Figure 9-12.
[U.S. Army Aeromedical Training Manual, ¶9-21.] When the clockwise rotation of the aircraft slows or stops, the fluid in the canal moves briefly in a clockwise direction. This sends a signal to the brain that is falsely interpreted as body movement in the opposite direction. In an attempt to correct the falsely perceived counterclockwise turn, the pilot may turn the aircraft in the original clockwise direction. [The figure] shows the position of the hair cells—and the resulting false sensation when a clockwise turn is suddenly slowed or stopped.
[U.S. Army Aeromedical Training Manual, ¶9-22.] This system reacts to the sensation resulting from pressures on joints, muscles, and skin and from slight changes in the position of internal organs. It is closely associated with the vestibular system and, to a lesser degree, the visual system. Forces act upon the seated pilot in flight. With training and experience, the pilot can easily distinguish the most distinct movements of the aircraft by the pressures of the aircraft seat against the body. The recognition of these movements has led to the term "seat-of-the-pants" flying.
Aeromedical Training for Flight Personnel, Department of the Army Field Manual 3-04.301, 29 September 2000
Air Force Manual 51-37, Instrument Flying, 1 December 1976
FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.