Spatial disorientation
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Spatial disorientation is a condition in which an aircraft pilot's perception of up-and-down (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 attempted 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 spatial orientation if he 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. Unfortunately 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.
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 what 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.
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 about 0.2 to 0.3 degrees per second 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. Pilot are also susceptible to spatial disorientation during night flight over featureless terrain.
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 the likely cause.
