Factors Causing Visual Illusion

Eyeball Error LESSONS IN FLYING

If we were meant to fly we would have evolved a vision system more like birds.  Birds see more sharply than humans, with some birds of prey even sporting a second fovea - that part of the retina where light receptors needed for vision are concentrated.  Humans have one fovea. Birds are able to adjust focus more rapidly because they have extra muscles to change the eye's focal length. They have also developed enlarged optic lobes to process visual information faster. Their enhanced visual system is critical for flight. The human visual system provides 80 percent of total spatial orientation.  However, unlike birds, our vision is not adapted to the demanding nature of flight with the high speeds and three dimensions of movement requiring a lot of visual information to help judge distance, height and changes in these parameters.

It is the limitations of human vision that make it susceptible to illusions, particularly at night, in poor light or in conditions with few external visual cues. Powerful illusions can occur when the pilot, presented with limited visual cues, "fills in" the rest of the picture, or when what the pilot sees conflicts with a strong response from the sense of balance or orientation, which has its sensors in the inner ears. Approach and landing illusions can have serious consequences.

American Airlines Crash

One of the deadliest air crashes occurred 40 years ago in November 1965 when American Airlines flight 383 crashed 3 km short of the Cincinnati airport runway threshold. The aircraft was under full power during a visual approach in deteriorating weather conditions. Of the 62 on board, 58 died.

While the official listing of probable causes said the accident was due to the failure of the crew to properly monitor the altimeters during a visual approach into deteriorating visibility conditions, a common visual illusion was also implicated. Studies of illusory effects produced by lights on the Ohio River bank, viewed in conjunction with the more distant but 400ft higher runway lights, demonstrated that pilots approaching to land on this runway could receive visual cues that produced powerful sensations of being much higher than their aircraft's actual altitude.

Factors Causing Visual Illusion

A range of factors can induce illusions on approach and landing. In the airport environment, these include ground texture and features; off-airport light patterns, such as brightly lit parking lots or streets; the "black hole" effect along the final approach flight path; and uphill-sloping or downhill sloping terrain in the airport vicinity.

In the runway environment, the factors include the runway dimensions, runway slope (uphill gradient or downhill gradient), terrain drop off, approach and runway lighting and the condition of the runway. The cloud ceiling, visibility and obstructions to vision are key weather factors that contribute to illusions.

In the approach and landing phase of flight, visual illusions are most critical when you are transitioning from IMC and instrument references to VMC and visual references.  Any illusion can lead to crew corrections that cause the aircraft to deviate from the vertical or horizontal flight path.

Runway Appearance. The visual appearance of the runway, its dimensions, slope and lighting has a large role to play in creating illusions. Generally, pilots assume the runway is flat as they make their final approach to land. At a certain point from the runway, you expect to see a particular runway shape, which helps you judge whether you are too high, too low or correctly on the glide slope. If the runway shape is not as you expect, you will be tempted to make corrections to the flight to make the runway appear as it should.

Approach on Down Sloping Runway. If you are at a particular height and distance from the runway, a down-sloping runway will present a smaller image to the eyes. This will give you the same impression as if you were making a low approach to a level runway, as in both conditions you see less of the runway- So flying an approach to a down-sloping runway will seem like you are undershooting, even if you are on the correct glide-slope. If you are unaware of the sloping characteristics of the runway, you may fly higher to make the runway appear normal. This will look to you like you are on the correct approach path, but in reality you will be flying too high an approach. This could lead to an unstable approach and tricky landing, or a landing well down the runway, which may result in an overrun.

Approach on up Sloping Runway. The opposite will occur if you fly an approach to an up sloping runway. In this situation you will see too much of the runway, which will give you the impression that you are high and overshooting. If you attempt to correct this impression, you will fly a lower than normal approach, which could result in a hard landing, or possibly even a landing short of the runway.

Effect of the Width of Runway. The width of the runway is also an important consideration. It also depends on the width of the runway that you are used to operating on. At a certain distance and height from a runway, you expect to see the runway fill a certain amount of your peripheral vision, which helps you judge height from the runway and distance to touchdown.

Wider Runway. If you fly an approach to a runway that is wider than the one you are used to, at the same height and distance from the runway you will see more of the runway and this will give you the impression that you are much closer to the runway than you actually are. This may cause you to flare the aircraft at a much greater height from the runway than you would prefer. The result of this can be a high rate of descent and subsequent heavy landing, or at the very least an unstable approach and memorable arrival.

Narrower Runway. The opposite situation can arise when you fly into a narrower runway than the one you are used to. At the same height and distance from the runway, you will see less of the runway in your peripheral vision than normal. This may give you the impression that you have still some distance to travel before getting to that point where you begin the flare and reduce the throttle. You may well arrive on the runway before you are expecting to, with an unexpected hard landing.

Effect of the Length of Runway. The length of the runway is also a contributor to approach illusions. It also depends on the length of runway you are generally used to. At the same height and distance from the runway, a long runway will present a bigger Image to the eyes than a shorter one. The impression you will get is that you are too high on approach. This may lead you to make the same corrections as during the approach to an up-sloping runway, with the same potential outcomes of an unstable approach and possible undershoot. A shorter runway than you are used to will give you the opposite impression, that you are too low on approach. This may make you take corrective action similar to that with the down-sloping runway, and end up making an unstable, high approach.

Effect of Runway Lighting. The lighting on a runway can also cause illusions. Your depth perception is influenced by the lighting intensity and ambient conditions. Bright runway lights create the impression of being closer to the runway, and so the idea that you are on a stepper glidepath. On the other hand, low intensity lights create the impression of being further way, and so on a shallower glidepath. Also non-standard spacing of runway lights can modify your perception of distance the runway. If the runway is only partially visible, say, while on base leg during a visual or circling approach, the runway may appear further away or at a different angle.

Effect of Weather. Weather conditions can affect depth and distance perception. Flying in light rain, fog, haze, mist, smoke dust, glare or darkness usually creates an illusion of being too high.

Flying through fog. If you are on top of shallow fog about 300 ft in thickness, the ground can be seen, but when you enter the fog layer you will lose forward and slant visibility. This can create the perception of a pitch up, which can cause you to respond with an inappropriate nose-down correction that steepens the approach path.

Flying through Haze/ Light Rain. Flying in haze creates the impression that the runway is further away, inducing a tendency to shallow the glide path. When you are in light or moderate rain the runway may appear indistinct because of the rain halo effect, increasing the risk of misperception of the vertical or horizontal deviation during the visual segment flown after transition from instrument to visual references.

Flying through Heavy Rain. Heavy rain can also affect depth and distance perception. Rain on the windshield creates refraction effects that can lead you to believe that the aircraft is too high, with the risk of an unwarranted nose-down correction and flight below the desired flight path.

Effect of Approach Lighting System (ALS). In daylight, rain diminishes the apparent intensity of the ALS, resulting in the runway appearing to be further away. As a result you might be tempted to shallow the flight path. Rain at night increases the apparent brilliance of the ALS making the runway appear to be closer than it really is. The risk is that you will land short of the runway threshold.

Runway surface conditions can also induce illusions. Because a wet runway reflects very little light you may think that the aircraft is further away from the runway, contributing to the risk of a late flare and hard landing.

Prevention

To reduce the dangers of visual illusions on approach and landing, accident prevention strategies and personal lines of defence should be developed. You should assess your exposure to visual illusions on your route, and understand what conditions might increase the possibility of visual illusions. During the approach briefing you should review the hazards, including

Ceiling and visibility conditions.

Weather, including wind, turbulence, rain, showers, fog or smoke.

You should know the surrounding terrain features and man-made obstacles. At night, an unlit hillside between a lit area and the runway may prevent you from correctly perceiving surrounding terrain.

Specific airport and runway hazards, such as obstructions, the black-hole effect and off-airport light patterns.

The type of approach including the approach lights.

VASI or PAPI Availability.

Multi crew operations on approach and landing should adhere to defined task sharing after the transition to visual flying. This should involve

Monitoring by the pilot flying of outside visual references while referring to instrument references to support and monitor the flight path during the visual portion of the approach.

Monitoring by the pilot not flying of head down references while the pilot flying flies and looks outside for effective cross-check and back-up.

If you are flying in to an unfamiliar airfield, consider the implications of the runway dimensions. If it is narrower than you are used to, think of the approach illusions that may affect you on final approach to land.

Conclusion

Approach illusions are relatively common in aviation. Landing an aircraft is an intensive visual task, requiring both focal vision and peripheral vision to help build an accurate model of the aircraft's flight path so that the flare and touchdown can be accurately judged.

Visual illusions are a fact of life. While there is no substitute for good airmanship, being aware of the possible illusions that may arise during a particular flight will help you increase the safety of the flight.

Source: MEATBALL Naval Aviation Journal