Acknowledgements: FAA Safety Team (FAAST)
What Is It?
Sight, supported by other senses, allows a pilot to maintain orientation while flying. However, when visibility is restricted (i.e., no visual reference to the horizon or surface detected) the body’s supporting senses can conflict with what is seen.
When this spatial disorientation occurs, sensory conflicts and optical illusions often make it difficult for a pilot to tell which way is up.
Contributing to these phenomena are the various types of sensory stimuli: visual, vestibular (organs of equilibrium located in the inner ear), and proprioceptive (receptors located in the skin, muscles, tendons and joints). Changes in linear acceleration, angular acceleration, and gravity are detected by the vestibular system and the proprioceptive receptors, and then compared in the brain with visual information.
In a flight environment, these stimuli can vary in magnitude, direction, and frequency, resulting in a “sensory mismatch” which can produce illusions and lead to spatial disorientation. Some of these illusions can lure pilots in to making poor decisions or improper control inputs.
For example, aerial perspective illusions may make you increase or decrease the slope of your final approach. They are caused by runways with different widths, up-sloping or down-sloping runways, and up-sloping or down-sloping final approach terrain.
NTSB accident data suggests that spatial disorientation may be a precursor to many general aviation accidents - particularly in night or limited visibility weather conditions. Instrument and VFR pilots are both vulnerable to spatial disorientation and optical illusions which may cause loss of aircraft control.
An auto-kinetic illusion gives you the impression that a stationary object is moving in front of the airplane’s path; it is caused by staring at a fixed single point of light (ground light or a star) in a totally dark and featureless background. This illusion can cause a misperception that such a light is on a collision course with your aircraft.
False visual reference illusions may cause you to orient your aircraft in relation to a false horizon; these illusions are caused by flying over a banked cloud, night flying over featureless terrain with ground lights that are indistinguishable from a dark sky with stars, or night flying over a featureless terrain with a clearly defined pattern of ground lights and a dark, starless sky.
How to Prevent Spatial Disorientation
You, the pilot, should understand the elements contributing to spatial disorientation so as to prevent loss of aircraft control if these conditions are inadvertently encountered.
The following steps should help prevent spatial disorientation:
· Before you fly with less than 3 miles of visibility, obtain training and maintain proficiency at flying by instruments.
· At night, or with reduced visibility, use and rely on your flight instruments. Be sure to test your flight instruments before each flight as well during your pre-flight and taxi.
· Maintain night currency if you intend to fly at night. Include cross-country and local operations at different airports.
· Study and become familiar with unique geographical conditions in areas in which you plan to operate.
· Check weather forecasts before departure, en-route, and at destination. Be alert for weather deterioration.
· Do not attempt VFR flight when there is the possibility of getting trapped in deteriorating weather.
· If you experience a visual illusion during flight (most pilots do at one me or another), have confidence in your instruments and ignore all conflicting signals your body gives you. Accidents usually happen as a result of a pilot’s indecision over relying on the instruments.
· If you are one of two pilots in an aircraft and you begin to experience a visual illusion, transfer control of the aircraft to the other pilot, since pilots seldom experience visual illusions at the same me.
· If you fly single-engine IFR frequently, consider investment in an alternate vacuum system or electric standby attitude indicator. By being knowledgeable, relying on experience, and trusting your instruments, you will be contributing to keeping the skies safe for everyone.