prolonged turn gradually, then suddenly returns to level flight, the leans may result. The gradual turn sets the fluid into the semicircular canals into motion, and rotational acceleration of two degrees per second (or less) cannot be detected. Once the aircraft suddenly returns to level flight, the continued fluid motion gives the sensation the aircraft is banking in the opposite direction of the turn that just ended; the aviator may attempt to correct the misperception of the vertical by banking into the original turn. The leans is considered the most common form of spatial disorientation. The graveyard spiral and graveyard spin are both caused by the acclimation of the semicircular canals to prolonged rotation; after a banked turn (in the case of the graveyard spiral) or spin (for the graveyard spin) of approximately 20 seconds, the fluid in the semicircular canals has been entrained into motion by friction, and the vestibular system no longer perceives a rotational acceleration. If the aviator then ends the turn or spin and returns to level flight, the continued motion of the fluid will cause a sensation the aircraft is turning or spinning in the opposite direction, and the pilot may re-enter the original turn or spin inadvertently; the aviator may not recognize the illusion before the aircraft loses too much altitude, resulting in a collision with terrain or the g-forces on the aircraft may exceed the structural strength of the airframe, resulting in catastrophic failure. One of the most infamous mishaps in aviation history involving the graveyard spiral is the crash involving John F. Kennedy Jr. in 1999. 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 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 of the turning, believing that the aircraft is maintaining straight flight. In a 1954 study (180 – Degree Turn Experiment), the University of Illinois Institute of Aviation found that 19 out of 20 non-instrument-rated subject