Visual illusions
Visual illusions fool the eyes with false cues (PHAK ch.17).
Approach illusions:
| Condition | Looks like | Tempts |
|---|---|---|
| Featureless night approach (black-hole) | Too high | Flying low |
| Narrow or up-sloping runway | Too high | - |
| Wide runway | Too low | - |
| Down-sloping runway (TP 1102) | Too low | Steep approach |
| Haze (TP 1102) | Runway farther and higher | Low approach |
| Unusually clear air | Runway closer | High approach, overshoot risk |
| Fog | Pitching up | Abrupt, unwarranted steepening |
| Bright runway/approach lights over dark terrain (TP 1102) | Runway closer | Premature descent |
| Steep surrounding terrain (TP 1102) | Too low | Steep approach |
Other illusions:
- False horizon: sloping cloud or ground mistaken for real horizon.
- Bent horizon (e.g. Northern Lights) gives same false impression.
- Autokinesis: single fixed light against dark background, stared at more than a few seconds, seems to move.
- Counter by scanning, more than by adding lights or varying intensity.
- Rain/water refraction (TC AIM AIR 2.5):
- Rain on windscreen bends light: horizon looks lower by about 5°.
- Example: hilltop half a mile ahead looks roughly 260 ft lower.
- Hold extra altitude for terrain clearance in rain, en route and on final.
- Water on windscreen also degrades depth perception.
- Ground lighting (TP 1102): lights along a straight road or on a moving train can be mistaken for runway or approach lights.
Glide-path cue:
- Main cue: angle between runway aim point and horizon, equal to approach angle.
- Runway shape and terrain texture also point to horizon, only if runway and terrain are level.
- Sloping terrain or runway gives false horizon: can pull approach too steep or too shallow.
At night: other aircraft's lights can look receding when closing, or reverse. Read by intensity: brightening = approaching, dimming = moving away.
Trap: judging height or glide path from one cue (runway width, sloping horizon) while ground or weather misleads it.
References
- FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), ch.17
- TP 1102 Visual Illusions
- TC AIM AIR 2.5