Visual illusions are systematic misinterpretations of visual impressions — a common accident cause on approach and at night. Trust the instruments, not the perception.
Approach illusions — runway and terrain profile
| Illusion | Description | Pilot tendency | Risk |
|---|---|---|---|
| Black-hole approach | Approach onto featureless terrain at night (over water, dark forest) | Perception "too high" → fly too low | CFIT |
| Upsloping runway | Runway slope upward | Perception "too high" → flatter/lower approach | Approach below glide path, short landing or obstacles before threshold |
| Upsloping runway — secondary impression | same configuration | Perception "approach too steep" | Pilot pulls further back, airspeed loss |
| Downsloping runway | Runway slope downward | Perception "too low" → fly too high | High touchdown point, long roll |
| Downsloping terrain before threshold | Terrain before runway falls away; runway itself may be level | Perception "correct height" despite actually insufficient descent → land long | Runway overrun, possibly runway end |
| Wider-than-usual runway | Runway wider than accustomed; especially for pilots used to narrow runways | Perception "too low" (runway fills view earlier) → early + high flare | Hard landing / stall in flare |
| Narrower-than-usual runway | Runway narrower than accustomed | Perception "too high" → low flare + flat approach | Touchdown before threshold |
Weather illusions
| Illusion | Description | Pilot tendency |
|---|---|---|
| Rain on windshield | Light refraction shifts images | Terrain appears farther → fly lower |
| Haze | Reduced contrast | Terrain appears farther → fly lower |
| Clear cold air (extra-good visibility) | Exceptionally clear sight | Terrain appears closer → fly higher |
Visual perception in motion
| Illusion | Description |
|---|---|
| Autokinesis | A single stationary point of light appears to move after 5–10 s observation (eye-muscle fatigue). Eye movement / scanning breaks the illusion |
| False horizon | Tilted cloud bank, coastal lights along sloping coastline, tilted starfield → false horizon. Use AI, don't trust the eye |
| Sectoral illumination | Individual brightly lit structures in dark surroundings distort spatial perception |
Cause of approach illusions
All these illusions rest on brain misinterpretation — the brain compares the current visual image with the habitual image (e.g. home airport with "normal" runway width). When the current image differs, the brain misinterprets one's own height instead of the runway geometry.
Recovery from an illusion
- Close eyes briefly, breathe deeply.
- Look at the instruments: AI, altimeter, heading indicator.
- Believe the instruments, not your feeling.
- On approach suspicion: go-around is always an option.
Prevention
- Stabilized approach with clear targets (speed, sink rate, altitude vs position).
- PAPI / VASI use (see Subject 010 §8.3): red/white lights show glide path.
- GPS-based approach with synthetic vision (in modern cockpits).
- AIP study pre-flight: know unfamiliar runway geometry (width, slope, surroundings) in advance.