Definition
An incorrect approach path flown by a pilot who has been misled by a visual illusion during the visual segment of a landing. The pilot adjusts the aircraft's glidepath, airspeed, or altitude in response to what the runway environment appears to be doing, rather than what it actually is, resulting in an approach that is too high, too low, too fast, too slow, or misaligned relative to the true runway geometry.
Plain English
An approach that goes wrong because the runway or terrain tricks the pilot's eyes. The pilot flies the wrong path because what they see does not match reality.
Context Anchor
Seen in discussions of landing errors caused by optical illusions, especially when comparing the correct approach path with the path a pilot might fly after trusting misleading visual cues.
Derivation
Illusion comes from a Latin idea meaning to deceive or play tricks on someone. That fits this aviation use: the runway and surrounding scene can visually trick the pilot into thinking the airplane is higher, lower, closer, or farther than it really is.
Why Pilots Care
Failure to recognize and correct the illusion can result in a low approach, hard landing, or controlled flight into terrain.
Analogy
It is like judging a hill in the distance on a hazy day: your eyes may make it look closer or farther away than it really is, so your response can be wrong even though it feels natural.
Grounding Statement
On final approach, the runway picture can look normal while still leading the pilot toward an unsafe path.
Intuition Check
Do not read this as a planned instrument approach or a special type of procedure. Here, it means an approach path the pilot may accidentally fly because the outside view is misleading.
Example Sentence 1
A narrower-than-usual runway can create the illusion that the aircraft is higher than it really is, leading to a low approach due to illusion if the pilot is not cross-checking the VASI.
Example Sentence 2
Recognizing signs of an approach due to illusion, the crew initiated a go-around and flew the next circuit with instruments.