Vision, even under instrument conditions, is perhaps your single most vital faculty in flying. Yet, the best eyes in the world - with 20/20 sight, good depth perception, and well-developed colour vision - can play unexpected tricks on the most experienced pilot.

The eyes and brain cooperate closely to produce the sensation of sight. Illusions can arise from the eye alone, from the brain alone, or from a combination of the two. For example, when a bright light temporarily "blinds" you on a dark night your eyes may take several minutes to recover, during which time you "see" an after-image. This illusion, arising in the eye itself, is quite common and rarely causes persistent problems unless the central point of vision is affected or the light is unusually bright. The brain can create illusions by misinterpreting images which the eye reports correctly, i.e., you might misjudge the horizon because of slanted banks of clouds.

Pilots true vertigo, experienced as a feeling of dizziness and imbalance, can create or increase visual illusions. Vertigo resulting from rapid rotation of the body may be so severe that it causes quick, jerky, side-to-side movements of the eyes (a condition called "nystagmus"). This makes the surroundings appear to revolve in a direction opposite to the body's former rotation. If you have an attack of vertigo in flight, you may find yourself unable to read your instruments because they seem to be constantly moving.

During night flying, especially in extreme darkness, very little rotational movement of the body is needed to induce vertigo. Forewarned is forearmed!

However experienced you are, other types of illusions may occasionally prevent you from recognizing familiar terrain over which you are flying. Your eyes may deceive you into mistaking farm land or populated areas for landing fields - leading you to undertake a normal descent and landing approach into a hazardous area.