The brain tracks the body's position, movement, and orientation in space through the use of the vestibular system, vision, and cues from nerves, muscles, and tendons.
Vestibular System
The vestibular system is the body's own motion sensing system, located in the inner ear. It consists of three semicircular canals, filled with fluid and lined with tiny hairs. Whenever the head moves, it results in the fluid inside the canals to move relative to the head. The hairs sense this movement. The three canals are positioned at right angles to each other, allowing each axis of motion to be sensed. In addition to the sensing of rotation, the brain is also able to detect acceleration and deceleration with the vestibular system.
Vision
The brain uses information from the eyes to correct information received from the other two systems of motion detection.
Kinesthesia
The brain maintains awareness of the positions of the muscles and tendons throughout the body. It receives constant input from the nervous system concerning these positions, as well as pressures placed on different areas of the body. This information is interpreted by the brain to provide a feel for position, movement, and orientation.
Getting Confused
The mind has learned a lot of lessons, such as gravity always pulling downward, that it factors into its interpretation of the cues provided from its motion sensing systems. For example, the mind cannot tell the difference between G forces and gravity. As a result, aerodynamic loads can be perceived as climbs or descents. When maneuvering in an airplane, forces may be placed on a pilot that are outside the normal experience of life. As a result, the mind can become confused when trying to interpret its position, movement, and orientation.
If this confusion occurs, it is normally corrected by visual perception of the body's orientation. When visual cues are removed, however, the pilot may lose track of orientation. Sensations of motion become misleading, and the pilot may feel a very strong desire to maneuver the airplane in order to chase these false cues.
The Leans
The leans is the most commonly encountered illusion during instrument training. After a sustained turn, the mind perceived the banked attitude of the turn to be level. Upon rolling wings level, the pilot feels as if the aircraft was level and just rolled into a bank in the opposite direction of the turn. The pilot then has a strong urge to roll the aircraft back into the original turn, in order to feel like the aircraft is wings level. At this point, the pilot is seemingly confronted with two contradictory actual horizons, one perceived through the senses and one presented by the flight instruments.
Somatogravic Illusion
This illusion occurs as a result of a rapid acceleration, which produces a feeling of being nose up. The pilot wants to push the aircraft over into a nose low or even diving attitude.
Inversion Illusion
When a pilot abruptly levels off from a climb, an illusion of tumbling backwards may be perceived. This illusion compels the pilot to nose the aircraft over into a diving attitude. But, nosing the airplane over just intensifies the illusion.
Dealing with Disorientation
The pilot must overcome this desire, even if there is a very strong feeling that the airplane is doing something undesirable. Trust must be placed on the flight instruments in this situation. Properly interpret the information they provide concerning orientation in space and maneuver the airplane accordingly. Spatial disorientation normally subsides after a few minutes, once this desire to maneuver is successfully managed. Providing the brain some visual cue, such as breaking out of the clouds or spotting a lighting system or coast line, usually remedies disorientation immediately.
During flight training, pilots are trained how to rely solely on the flight instruments. This training allows the pilot to continue to fly safely should instrument conditions be encountered by accident. For example, the pilot might misjudge the distance to a cloud or fail to recognize and area of heavy rain or snow, resulting in accidental flight into instrument weather conditions.
For a flight under visual flight rules, conducted in visual flight conditions, the greatest threat of spatial disorientation results from flying over featureless terrain at night, such as a large body of water.
An instrument rating, training, and currency are good tools which aid pilots in dealing with spatial disorientation. As you gain instrument experience and experience suppressing these false sensations, trust in the flight instruments is built up. As a result of this experience of completely relying on the flight instruments, spatial disorientation fades away.
The human motion sensing system is not able to detect small changes in velocity, which can result in incorrect motion sensations.