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Visual illusions in flight


There can be no question as to the importance of eyesight to the safety of flight. Eyesight is a critical component for maintaining attitude orientation, scanning for collision avoidance, and for accomplishing a multitude of other important flight tasks.

While the eyes nearly always see exactly what we are looking at, interpretation of what is seen is not always so clear.

Occasionally the brain gets in the way of correctly interpreting what we see, and it is at these times during flight that we might get into trouble. The term “illusion” is usually applied to those instances where, although the environment is correctly sensed, meaning that we are seeing all there is to see, perception of the visual information does not conform to patterns that the brain can correctly interpret.

Sometimes an impoverished or featureless environment contributes to this problem. As a result, our actions in such circumstances are often influenced by an incorrect interpretation. The ramifications of acting on incorrectly perceived visual information is often quite serious. There are a large number of visual illusions, and many are not discussed in aviation texts or are given merely lip service. Perhaps the most common illusions deal with landing so I’ll go over those briefly and then move on to the rest.

Landing perception illusions are primarily a function of the visible runway environment. At night, in the case that a runway has no discernable lit features around it, the runway tends to appear farther away than would one that has buildings and other lit obstructions nearby. On the other hand, a brightly lit runway can actually make a runway appear closer than it really is. The science behind this illusion is straight forward; we don’t expect things in the distance to be well-lit or bright. Compensation for this illusion is to assume that the airport is closer than it really is.

The runway, even in daylight, can often be the source of the visual illusion. A runway of significantly greater width or length than average will create the illusion of being closer and thus lead to the tendency to fly low on approach.

A narrow and/or short runway creates the opposite illusion sometimes causing the pilot to think he or she is farther away than in actuality. The dangers of flying too low on approach are obvious, but the dangers of coming in too high as a result of a narrow or short runway illusion is also dangerous. In the later case, the pilot might attempt to land even after he sees the error, landing long on an already short runway!

Other runway characteristics can lead to problems as well. Consider an up-sloping runway. The far end of the runway will be higher in the windscreen than a flat runway, thus creating the illusion that you are approaching at a steeper than normal angle. Likewise, a down-sloping runway gives a flat appearance, leading to a higher than normal approach and possibly a long landing.

Landing long on a down-sloping runway is bad news since deceleration is already not as fast due to the downhill grade.

Illusions of motion: Several illusions of motion are common in and around airports. Other aircraft moving about on the ground sometimes create the illusion that our own airplane is moving. This is primarily a problem for aircraft with restricted outside view. If you’ve been in an airliner at the terminal parked near another and just happened to glimpse it being pushed back, you may have experienced illusory motion if you thought your own plane was momentarily moving forward. Similar illusory effects can be caused by blowing snow or snow snakes (those patterns of snow blowing and drifting across the surface), ripples in water, and blowing grass. These types of illusions are most easily ignored by keeping the eyes moving and by referencing known stationary objects nearby.

Brown out/ Whiteout: While a definite hazard for helicopters, brownout or whiteout can be equally scary for a fixed-wing pilot who is not aware of conditions that lead to the problem. While jet-engine reverse thrust has been known to produce whiteout conditions on snowy runways, prop blast under ideal wind circumstances can help light snow work its way to the windshield, thus restricting forward view.

Brownout is a similar problem that occurs on dusty (dry loose dirt) or sandy surfaces. The illusion for fixed-wing aircraft can be one of false motion but the bigger problem is loss of forward view.

There is not much a pilot can do when this happens, and there are no hard rules I can provide to counter the problem. Perhaps the best solution is to avoid landing and takeoff situations where this phenomenon might occur.

False Horizon: Pilots who do any serious amount of night-time flying will experience a false horizon at some point during their careers. Especially on clear nights in relatively sparsely populated areas, the pattern of stars may appear to be replicated by the pattern of ground lights. If spatially disoriented, it’s possible to confuse the two, and the pilot that unwittingly tries to align the aircraft to the wrong “sky” will soon be in serious trouble! Nighttime terrain confusion also causes problems for pilots. Ridgelines can create the appearance of a horizon; one that is not necessarily a suitable reference for maintaining the correct attitude. IFR pilots are not exempt from experiencing false horizons either. Indeed, flying between layers, especially during daylight hours when the clouds are more visible, the pilot will often find a sloping layer acts as a rather compelling horizon. Of course it is critical to rely on the instruments in these circumstances.

Optical flow rate confusion: Forward speed can be difficult to determine, especially in the absence of adequate environmental cues. In general, perception of speed on the ground is a function of what we see passing by; called optical flow. One of the difficulties of taxiing at a speed consistent with safety is that there are few objects to pass us by until reaching the ramp area.

Assessing speed under these environmentally impoverished conditions requires looking out the side window to get a direct view of the passage of runway lights, pavement lines or markings, etc.

Likewise in flight, the higher we go the less visual information is available to use in assessing forward speed. Typically the rate at which ground disappears beneath the nose gives at least an indirect indication of forward motion. Where confusion sets in is that apparent speed can look the same if high and fast as when we are lower and slower.

This is especially dangerous when other cues leads the pilot to believe he or she is high and fast when actually low and slow. Knowledge of the absolute altitude and reliance on the altimeter to maintain ground clearance is important in this instance.

Dip illusion trailing aircraft: Although not a major problem for general aviation aircraft, a dip illusion can occur when one aircraft is following another. This illusion occurs because an aircraft that is slowly falling behind the leader can maintain the same relative visual perspective if also descending at the same time. A simple matter of trigonometry. Of course the dip illusion occurs only for the following aircraft and can be avoided by carefully maintaining altitude, as well as noting how much of the windshield the leader takes up. As the follower falls behind, the leader will become smaller.

Contrast: Sometimes an illusion can occur due to lack of contrast between a potential obstacle and its surrounding environment. Certain lighting conditions can mask terrain that would otherwise be visible, most notably when the sun is positioned behind the aircraft. Still, contrast is a matter of perception and unless the pilot has training in how to tell good contrast from bad, the best he or she can do is to be well aware of the obstacle clearance heights for the area, and that lighting from behind can dull contrast of objects in view ahead of the aircraft.

Black hole illusion: One particularly dangerous night time illusion is referred to the black hole illusion or, more familiarly the black hole approach. Quite simply, when faced with an airport that lies at the end of area devoid of features and light, usually with light around or beyond the airport, the pilot gets a sensation of being high and will fly a lower than normal glidepath to the airport.

The danger of a landing short of the runway is real, and many unfortunate pilots have succumbed to this illusion. The best solution to being duped by the black hole phenomenon is the track a glideslope or other visual glidepath guidance to the runway. And consider not arriving at an unfamiliar airport at night if some sort of glidepath guidance is not available.

Blending of earth and sky: What do starlight moonless nights over sparsely populated areas, cloudy nights over water or sparsely populated areas, mountainous terrain and flat-light conditions all have in common? The answer: blending of sky and earth illusions that quickly lead to spatial disorientation.

For example, during the daylight hours with cloud cover and an extremely still large area of water, the water can reflect the clouds making it appear virtually identical to the sky. This is not a huge problem unless the pilot inadvertently enters an unusual attitude and then is faced with the decision of which way is up! Again, follow the instruments. Unusual attitude recovery training is also a good idea.

Activation of depth perception is also influenced by large calm water surfaces. Ask any seaplane pilot about the dangers of a “glassy” water landing! This month we’ve discussed some of the most common visual illusions that a pilot might experience. Next month we’ll tackle some of the non-visual illusions or problems caused by the way our eyes work (or don’t work!) during flight.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.