Definition
In aviation, visual misperceptions caused by terrain features, runway dimensions, lighting conditions, weather, or atmospheric effects that lead a pilot to misjudge altitude, distance, attitude, or approach angle, particularly during landing.
Plain English
Tricks your eyes play on you in flight. Things you see -- a sloping runway, a black hole at night, a wet surface -- can make you think you are higher, lower, closer, or further away than you really are.
Context Anchor
Encountered most often during approach and landing, especially at night, over featureless terrain, or when the runway looks different from what the pilot is used to.
Derivation
From Latin opticus (relating to sight) and illusio (a mocking or deception). The phrase points to the fact that the eye itself is being fooled -- the information reaching the brain is real, but the conclusion drawn from it is wrong.
Why Pilots Care
Unrecognized optical illusions are a leading cause of landing accidents and controlled flight into terrain.
Grounding Statement
A runway that is wider, narrower, sloped, or poorly lit can change the pilot’s visual picture even when the airplane is flying normally.
Intuition Check
Do not assume an optical illusion is a hallucination or an eyesight problem. In flying, it means a real scene is being interpreted incorrectly by the eyes and brain.
Example Sentence 1
A runway that slopes up can create an optical illusion of being too high on approach, leading the pilot to fly a lower-than-normal glidepath.
Example Sentence 2
Over dark terrain with no lights, optical illusions made the runway appear farther away than it actually was.