Spatial Disorientation means the lack of orientation in regard to position, altitude or any movement of the aircraft. There are three systems, the body uses together to ascertain orientation and movement.
Vestibular – organs in the inner ear that sense position, by the way we are balanced.
Somatosensory – nerves in the skin, muscles, joints and hearing sense position by gravity, feeling along with sounds.
Visual – using the eyes to sense position by sight.
During visual meteorological conditions (VMC) flight, pilots use their eyes which are the major orientation source. But, when flying in instrument meteorological conditions (IMC), the visual cues are removed which gives false sensations.
Inside of both the left and right inner ear is the vestibular system, which allows pilots to sense movement in the surrounding environment. There are three semicircular canals are positioned at approximate right angles to each other. Inside are tiny hairs that deflect when acceleration in any direction, which sends a message to the brain.
When there is visual reference of the horizon and the ground the inner ear identifies the pitch, roll, and yaw movements of the aircraft. If for any reason visual reference of the horizon and ground are lost the vestibular system in the inner ear becomes unreliable.
If a pilot is not instrument rated or has many flight hours flying on instrument, it is best to avoid flight at night or in low visibility when the horizon is visible.
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