By Leslie Caubble, CFI/IGI
Our eyes are an amazing organ when you think about it. They work in orchestration with our somatosensory and vestibular systems to send signals to the brain regarding movement and orientation in space. Most of the time, these systems work together flawlessly. That’s what keeps us from falling constantly while walking!
As you would suspect, if there are mixed signals sent to the brain, this can lead to false information and an illusion. If they don’t agree, then your body may feel like it’s doing something different than what your eyes are telling you. When your eyes trick your brain into believing you’re seeing something that isn’t there, that’s a visual illusion. You’ve probably at one time crossed railroad tracks that continue in a straight line. As your eyes shift further out toward the horizon, the tracks appear to narrow and eventually meet. We know this isn’t true, but our eyes send these incorrect signals to the brain.
When pilots are flying in VMC (visual meteorological conditions), vision plays a major role in sending correct signals to the brain on the pilot’s position, movement, and orientation. Take away the visual cues, such as in IMC (instrument meteorological conditions) or night, and the pilot can experience visual illusions, and even worse, spatial disorientation from the mixed signals. Here are eight visual illusions that pilots often experience:
Illusions Leading to Spatial Disorientation
False Horizon
A “false horizon” occurs when outside references give the eyes incorrect visual information to the brain, and the pilot becomes disoriented aligning the aircraft with the false horizon. This visual illusion is prevalent at night when the sky is dark, and the pilot sees patterns of stars or ground lights. An obscured natural horizon or a sloping cloud formation can also cause this visual illusion.
Autokinesis
This is another illusion that happens at night. Autokinesis occurs when there is a small, stationary light against a dark sky. If the pilot stares at it for a while, the light appears to start moving. When the pilot is fixated on the illusion of movement, they may align the aircraft with the light, and this can lead to spatial disorientation and potentially loss of control.
Illusions Leading to Landing Errors
Runway Width Illusion
Terrain can also cause certain optical illusions. These illusions happen the most often during the landing phase of flight because they pertain to the runway. A major optical illusion in this category is runway width illusion. A narrow runway can cause the eyes to perceive the aircraft being too high on approach, so the pilot flies too low of approach. If not corrected, this can lead to hitting objects on the ground due to landing short. A wider than usual runway causes the pilot to believe they are too low, therefore flying too high of an approach. The risk of this is the pilot could overshoot the runway, land too long, or have a hard landing due to leveling out too high.
Runway & Terrain Slope Illusion
Runway slope illusion occurs when the runway itself or surrounding terrain slopes upward or downward. If the runway has an upward slope, it creates an illusion that the aircraft is too high, therefore causing the pilot to fly too low of an approach. If the runway slopes downward, the pilot may fly too high of an approach because of the illusion they are too low. Terrain in the airport environment can also cause this illusion.
Featureless Terrain Illusion
The featureless terrain illusion is often referred to as the “black hole effect.” This illusion is more prevalent over open water, at night, or in low visibility. When there is an absence of any ground features, the illusion causes the pilot to think they are too high. When this happens on approach, the pilot may fly too low and collide into obstacles or terrain. This same black hole effect can also create visual illusions as related to the airport, runway, and approach lighting, especially if the airport is surrounded by dark terrain.
Ground & Aircraft Lighting
Ground lighting, such as lights along a straight road, can look like approach or runway lighting, and can cause the pilot to line up on the wrong approach path. Bright, high-intensity approach and runway lights can create the visual illusion that the runway is too close, so the pilot may push the nose down and land either short or too hard. The opposite is true for dimly lit approach and runway lights… the runway may seem further away, so the pilot flies a lower-than-normal approach. The landing light on the plane can also cause a visual illusion. When the landing light illuminates the runway, it makes the runway appear closer than it actually is, so the pilot might fly a higher-than-normal approach or flare too high over the runway.
Atmospheric Illusions
Water Refractions
When rain hits the windscreen, it creates the visual illusion of the horizon being lower than it actually is. This results in the pilot flying too low of an approach.
Haze/Fog
Water particles in the air due to haze or fog cause refractions of light which can trick the eyes and cause errors in depth perception. When approaching to land in lower visibility, the runway may appear further away, so the pilot flies a lower approach. The opposite is true on a clear, sunny day. This illusion causes the runway to appear closer than it actually is, therefore the pilot approaches too high causing an overshoot of the intended landing spot.
How to Cope with Visual Illusions
Now that we’ve covered some visual illusions that pilots often face, how do we mitigate these risks and cope with them?
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Plan ahead: If you know you’ll be flying at night, in low visibility, or in a remote area, consult the Chart Supplement for detailed information on runway slope, approach and airport lighting.
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Trust your instruments, including your pitch and power settings. Make your approach as stable as possible. Frequently reference your altimeter, and always be ready to go around.
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Use the VASI or PAPI lighting as a reference. Check NOTAMs for reliability and outages.
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Minimize distractions, especially when flying at night or in IMC.
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Always get pre-takeoff and enroute weather updates, especially if you might be flying into deteriorating conditions.
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Be proficient and practice often: partial panel flying, night flying, instrument scan, and flying into different airports, especially those around changing terrain.
Landing errors and spatial disorientation due to visual illusions can be prevented by being familiar with your surroundings, your aircraft, and the weather. Anticipate what illusions might affect you before you start your approach and brief them to yourself or with your co-pilot before the flight.
Review Chapter 8 “Medical Facts for Pilots” in the Aeronautical Information Manual and Chapter 7 “Aeromedical Factors” in the Pilot’s Handbook of Aeronautical Knowledge for further study.
Safe flying!
By Leslie Caubble, CFI/IGI
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