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Spatial disorientation is a condition in which an aircraft pilot's perception of direction (proprioception) does not agree with reality. While it can be brought on by disturbances to or disease within the vestibular system, it is more typically a temporary condition resulting from flight into poor weather conditions with low or no visibility. Under these conditions the pilot may be deprived of an external visual horizon, which is critical to maintaining a correct sense of up and down while flying. A pilot who enters such conditions will quickly lose his or her spatial orientation if he or she does not have training in flying with reference to instruments. Approximately 80% of the private pilots in the United States do not have an instrument rating, and therefore are prohibited from flying in conditions where instrument skills are required. Not all pilots abide by this rule, and approximately 40% of the NTSB fatal general aviation accident reports list continuation of flight into conditions for which the pilot was not qualified as either a contributing or proximate cause.

Senses during flight

During flight, most of the senses are 'fooled' by centrifugal force, and indicate to the brain that 'down' is at the bottom of the cockpit no matter the actual attitude of the aircraft. Only the inner ear and the visual sense provide data to the contrary. The inner ear contains rotational 'accelerometers,' known as the semicircular canals, which provide information to the lower brain on rotational accelerations in the pitch, roll and yaw axes. This system is imperfect, and errors develop in the brain's estimate of rate and direction of turn in each axis. Normally these errors are corrected using information from the visual sense, in particular an external visual horizon.

Effects of disorientation

Once an aircraft enters conditions under which the pilot cannot see a distinct visual horizon, the drift in the inner ear continues uncorrected. Errors in the perceived rate of turn about any axis can build up at a rate of 0.2 to 0.3 degrees per second. If the pilot is not trained for or is not proficient in the use of gyroscopic flight instruments, these errors will build up to a point that control of the aircraft is lost, usually in a steep, diving turn known as a graveyard spiral. During the entire time, leading up to and well into the maneuver the pilot remains unaware that he is turning, believing that he is maintaining straight flight.

The graveyard spiral usually terminates when (1) the g-forces on the aircraft build up to and exceed the structural strength of the airframe, resulting in catastrophic failure, or (2) the aircraft contacts the ground. In a 1954 study, the Air Safety Foundation found that out of 20 non-instrument-rated subject pilots, 19 of the 20 entered a graveyard spiral soon after entering simulated instrument conditions. The 20th pilot also lost control of his aircraft, but in another maneuver. The average time between onset of instrument conditions and loss of control was 178 seconds.

Spatial disorientation can also affect instrument-rated pilots in certain conditions. A powerful tumbling sensation (vertigo) can be set up if the pilot moves his head too much during instrument flight. This is called the Coriolis illusion. Pilots are also susceptible to spatial disorientation during night flight over featureless terrain.

Spatial Orientation

Spatial orientation is our natural ability to maintain our body orientation and/or posture in relation to the surrounding environment (physical space) at rest and during motion. Genetically speaking, humans are designed to maintain spatial orientation on the ground. 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. Statistics show that between 5-10% of all general aviation accidents can be attributed to spatial disorientation, 90% of which are fatal.

Good spatial orientation on the ground relies on the effective perception, integration, and interpretation of visual, vestibular (organs of equilibrium located in the inner ear), and proprioceptive (receptors located in the skin, muscles, tendons, and joints) 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.

Spatial orientation in flight is difficult to achieve because numerous sensory stimuli (visual, vestibular, and proprioceptive) vary in magnitude, direction, and frequency. Any differences or discrepancies between visual, vestibular, and proprioceptive sensory inputs result in a sensory mismatch that can produce illusions and lead to spatial disorientation. Good spatial orientation relies on the effective perception, integration and interpretation of visual, vestibular (organs of equilibrium located in the inner ear) and proprioceptive (receptors located in the skin, muscles, tendons, and joints) sensory information.


inner ear with semicircular canals shown likening them to the roll, pitch and yaw axis of an aircract

Vision and orientation

Visual references provide the most important sensory information to maintain spatial orientation on the ground and during flight, especially when the body and/or the environment are in motion. Even birds, reputable flyers, are unable to maintain spatial orientation and fly safely when deprived of vision (due to clouds or fog). Only bats have developed the ability to fly without vision but have replaced their vision with auditory echolocation. So it's no surprise that when humans fly under conditions of limited visibility, they have problems maintaining spatial orientation.

The problem occurs when the outside visual input is obscured, and the seat-of-the-pants input is ambiguous. Then, you're down to just the output from the inner ear—and that's when trouble can start.

Fluid in the inner ear reacts only to rate of change, not a sustained change. For example, when you initiate a banking left turn, your inner ear will detect the roll into the turn, but if you hold the turn constant, your inner ear will compensate and rather quickly, although inaccurately, sense that it has returned to level flight.

As a result, when you finally level the wings, that new change will cause your inner ear to produce signals that make you believe you're banking to the right. This is the crux of the problem you have when flying without instruments in low visibility weather. Even the best pilots will quickly become disoriented if they attempt to fly without instruments when there are no outside visual references. That's because vision provides the predominant and coordinating sense we rely upon for stability.

Perhaps the most treacherous thing under such conditions is that the signals the inner ear produces—incorrect though they may be—feel right!

The Otolith Organs and Orientation

Two otolith organs, the saccule and utricle, are located in each ear and are set at right angles to each other. The utricle detects changes in linear acceleration in the horizontal plane, while the saccule detects gravity changes in the vertical plane. However, the inertial forces resulting from linear accelerations cannot be distinguished from the force of gravity; therefore, gravity can also produce stimulation of the utricle and saccule. A response of this type will occur during a vertical take-off in a helicopter or following the sudden opening of a parachute after a free fall.

"Seat of the pants" flying

Anyone sitting in an aircraft that is making a coordinated turn, no matter how steep, will have little or no sensation of being tilted in the air unless the horizon is visible. Similarly, it is possible to gradually climb or descend without a noticeable change in pressure against the seat. In some aircraft, it is possible to execute a loop without pulling negative "G's," so that without visual reference, you could be upside down without being aware of it. That's because a gradual change in any direction of movement may not be strong enough to activate the fluid in the semicircular canals, so you may not realize that the aircraft is accelerating, decelerating, or banking.

In the media

This phenomenon was extensively reported in the press in 1999, after John F. Kennedy, Jr.'s plane went down during a night flight over water near Martha's Vineyard. Subsequent investigation indeed pointed to spatial disorientation as a probable cause of the accident.

Intentionally-induced spatial disorientation (by use of giant mirrors) was a major plot point in the two-part TaleSpin episode "A Bad Reflection on You."

How to Prevent Spatial Disorientation

The following are basic steps that should help prevent spatial disorientation:

  • Take the opportunity to experience spatial disorientation illusions in a Barany chair, a Vertigon, a GYRO-LAB or a Virtual Reality Spatial Disorientation Demonstrator.
  • Before flying with less than 3 miles visibility, obtain training and maintain proficiency in aircraft control by reference to instruments.
  • When flying at night or in reduced visibility, use the flight instruments.
  • If intending to fly at night, maintain night-flight currency. Include cross-country and local operations at different airports.
  • If only Visual Flight Rules-qualified, do not attempt visual flight when there is a possibility of getting trapped in deteriorating weather.
  • If you experience a vestibular illusion during flight, trust your instruments and disregard your sensory perceptions.

See also

External links

Information from the following government documents is in the public domain.

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