Editing Spatial disorientation (section)

===Otoliths and somatogravic illusions===
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 linear accelerations in the vertical plane; humans have evolved to assume the vertical acceleration is caused by gravity. However, the saccule and utricle can provide misleading sensory perception when gravity is not limited to the vertical plane, or when vehicle speeds and accelerations result in inertial forces comparable to the force of gravity, as the otoliths only detect acceleration, and cannot distinguish inertial forces from the force of gravity.<ref name=FAA-spatial/> Some examples of this include the inertial forces experienced during a vertical take-off in a helicopter or following the sudden opening of a parachute after a free fall.{{cn|date=June 2022}}

Illusions caused by the otolith organs are called somatogravic illusions and include the Inversion, Head-Up, and Head-Down Illusions. The Inversion Illusion results from a steep ascent followed by a sudden return to level flight; the resulting relative increase in forward speed produces an illusion the aircraft is inverted.<ref name=FAA-spatial/> The Head-Up and Head-Down illusions are similar, involving sudden linear acceleration (Head-Up) or deceleration (Head-Down), leading to a misperception the nose of the aircraft is pitching up (Head-Up) or down (Head-Down); the aviator could be fooled into pitching the nose down (Head-Up) or up (Head-Down) in response, leading to a crash or a stall, respectively.<ref name=FAA-spatial/>

Typically, the Head-Up illusion occurs during take-off, as a strong linear acceleration is used to generate lift over the wing and flaps. Without a visual reference, the pilot may assume from the vestibular system the nose has pitched up and command a dive; if this occurs during take-off, the aircraft may not have sufficient altitude to recover before crashing into the ground.<ref name=ATSB-SD2007/>{{rp|7}}