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
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.
The brain uses information from the eyes to correct information received from
the other two systems of motion detection.
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.
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
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.
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.
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.