In general, vestibular illusions occur under conditions in which a pilot is unable to see a clear horizontal reference. Illusions are primarily caused by simultaneous sensory stimulations - The angular motion of the head and of the aircraft in two different planes can stimulate two of the three semicircular canals => The turn activates one semicircular canal and the head movement activates another. The simultaneous stimulation of two semicircular canals produces an almost unbearable sensation and illusion that the aircraft is rolling, pitching and yawing all at the same time. It can quickly disorient a pilot and cause a loss of aircraft control.
These illusions can be prevented, or at least minimised, by avoiding rapid head movements and by trusting the instruments.