Virtually every pilot has experienced some form of illusion during the approach and landing phases of a flight. Few cause more than an untidy approach or a hard landing, unfortunately fewer still are recognized for what they are - ILLUSIONS. The approach and landing is the most demanding phase of a flight due to the precision required and the increased workload. The last thing that a pilot needs is some form of disorienting illusion to interfere with these final flight segments. There are a variety of illusions that can create problems during the approach and landing. Fortunately it is usually possible, through understanding and preparation, to prevent these illusions from causing problems.

While flying an approach we continuously monitor our progress on the glideslope, the flare, and the landing. Unconsciously we compare each approach to a ‘model’ built up in our mind from all previous approaches and landings. This ‘model’ lets us know where things should  be, how they should look, and how they should move relative to one another at different times during the approach and landing. Our sense of vision is of prime importance in aviation (See ‘Visual dominance can lead to illusions and disorientation’, Aviation Bulletin, March 1992) and it is usually visual cues that allow us to recognize whether our approach is above, below, or on the planned glideslope. These visual cues include the apparent shape and size of the runway, the spacing and size of runway markings, the relative size of nearby objects such as windsock, cars, and buildings, and the way objects move in relation to one another and the aircraft. It is these cues, and others, that we continuously compare with our mind’s ‘model’ to determine whether the approach is progressing as expected. When the visual cues do not fit our expectations (‘model’) we usually recognize the approach as not being ‘right’ and make adjustments accordingly. This may  include corrective alteration of the power settings, the aircraft attitude or heading, flap or gear extension, or in the extreme a missed approach.

The effect of visual perspective during approach. The middle panel shows a simplified view of the runway during an approach flown down a correct glideslope. The left and right panels show, respectively, approaches flown below and above the correct glideslope. Note the  difference in runway perspective.

During a visual approach (to an airfield not equipped with VASIS, PAPI, or similar) we maintain our glideslope primarily by the perspective of the runway and the surrounding terrain. If we fall below our intended glideslope the runway will appear fore-shortened (Figure 1 - left panel) and the surrounding terrain may appear to move by more quickly than usual. Structures such as fence lines or buildings near the airstrip will also take on a different perspective. During a night approach the runway shape will appear fore-shortened and the runway lights will appear closer together than expected.

Conversely, if we approach higher than planned, the runway will appear to be stretched out in front of us (Figure 1 - right panel) and the terrain beside and below us may appear to move past slower than expected. At night a high approach will result in the runway lights looking further apart than usual as well as the runway being stretched out. It’s not difficult to understand how differences in runway width and slope can interfere with an approach. A runway that is wider than you are used to will cause the illusion that you are lower than your anticipated glideslope (Figure 2 - left panel). This could cause the unwary to approach too high and flare too early. I remember flying into Melbourne’s Tullamarine airport, for the first time, and flaring at about 100 feet. All of my previous flying had been on thin grass strips and I was unprepared for this illusion caused by the runway width. A thinner runway than expected gives the impression of extra height. This causes the illusion that you are higher than your intended glideslope (Figure 2 - right panel). Inappropriate ‘correction’ in this situation can result in a low approach and possibly a hard landing.

The effect of runway width during approach. The middle panel shows a simplified view of a ‘normal’ runway during an approach. The left panel shows the visual perspective of a correct approach flown to a wider-than expected runway. The right panel illustrates the effect of a thinner-than expected runway. Compare these ‘perspectives’ with those of Figure 1.

Runway slope can have a similar effect in that a downsloping runway appears fore-shortened giving the illusion that your approach is low (Figure 3 - left panel) and an upsloping runway looks stretched and your approach seems too high (Figure 3 - right panel).

The effect of runway slope during approach. The middle panel shows a simplified view of a ‘normal’ horizontal runway during an approach. The left panel shows the visual perspective of a correct approach flown to an upsloping runway. The right panel illustrates the effect of a downsloping runway. Compare these ‘perspectives’ with those of Figures 1 and 2.

The absence of visual cues during night approaches makes runway illusions more likely. Colour vision is impaired at night because the colour sensors within the eye require relatively high light levels to be activated. At night our depth perception is also restricted. This is because cues such as the relative motion of the ground below, the size of buildings near the runway, or the width of nearby roads may not be visible. Night-time approach perspective depends, almost solely, on the shape formed by the runway lights. Figures 1, 2, and 3 also show the illusions that changes in runway width and slope can generate during a night visual approach. I recently flew with the W.A. Royal Flying Doctor Service from Derby to pick up a patient at Kalumburu mission in the Kimberleys*. It was a clear night with virtually no surface wind. We flew four missed approaches at Kalumburu before landing. The approaches had not ‘felt right’ to the captain. Upon landing it was discovered that the runway lights, kerosene lamps, had been placed wider than usual. This effectively produced a wider runway than expected. The experience of the pilot had probably allowed him to recognize that something was amiss and go-around rather than continue what was probably a higher than glideslope approach. Subconsciously he may have recognized a different perspective between the wider runway lighting and the nearby settlement lights. Anyone who has flown into the Royal Australian Navy’s Air Station Nowra will also appreciate some of the potential problems associated with sloping runways. Nowra’s runways don’t simply slope down at the threshold but have a pronounced dip in the middle and slope up again as you continue along their length. The slope and composition of the terrain around a runway is also a potential source of illusions during approach and landing.

The effect of sloping approach terrain. The middle panel shows a simplified view of a ‘normal’ approach over horizontal terrain. The left panel shows an approach over downsloping terrain causing an illusory perception of approaching low. The right panel shows the opposite effect as an approach is flown over upsloping terrain.

It is difficult to maintain a glideslope visually when flying a runway approach over sloping terrain. If the terrain slopes up towards the runway we suffer the illusion of being higher than glideslope (Figure 4 - left panel), because the ground is further away than we expect. Conversely an accurate approach flown over downsloping terrain will give the illusion of being lower than glideslope (Figure 4 - right panel). This is because the ground isn’t as far away as expected. A classic example of this illusion can be experienced by anyone who flies into the Denham “International” airport in Northern W.A. (Figure 5). The strip itself is on a salt-pan surrounded by dunes. The sloping dunes cause a strong illusion that your approach is lower than intended. To a lesser extent this also occurs on runway 30/12 at Perth’s Jandakot airport .

Shark Bay ‘International’ airport near Denham W.A. Sand dunes surrounding the strip cause both approaches and takeoffs to be flown over sloping terrain.

The composition of the approach terrain is also important. If you are familiar with trees being 80 - 100 feet tall on your approach path you will find an approach over small, stunted trees disconcerting. When flying a correct glideslope over smaller trees an unfamiliar pilot will perceive his approach to be too high (Figure 6).

The effect of unfamiliar terrain composition under the approach path. If used to tall trees an illusion of approaching too high may result during an approach flown over small trees.

An often quoted example of this illusion is USAF crews flying into the Aleutian Islands where the evergreen trees are much smaller than on the US mainland. In a similar manner flying over water, especially still or ‘glassy’ water, usually causes an illusion of being high. This occurs because of the lack of adequate visual cues during the approach. Lights near an airfield can cause illusory ‘false’ horizons. In the absence of a clear horizon, scattered lights on the ground may appear to be stars.

Unlit ground, merging imperceptibly with a featureless, unlit, sky may cause a dangerous ‘false horizon’ illusion. In this example the horizon appears lower than it actually is.

This may give a night flying pilot the illusory impression of a higher nose attitude. Similarly the horizon may appear lower than usual if there is unlit terrain (or water) and an overcast or unlit sky behind foreground lights (Figure 7). An example of problems caused by city lights is an American Boeing 737 incident in 1989. This aircraft mistook the lights of an industrial area for the approach lights at Kansas City International Airport. Luckily at approximately 75 feet AGL, two miles from the runway, the aircraft’s vertical stabilizer cut through a set of power lines and blacked out the industrial area. This caused the crew to initiate a go-around and subsequently make an approach using the correct lights.

The ‘black-hole’ approach illusion can also create great difficulty for the pilot. A ‘black-hole’ approach is made at night, in the absence of a discernible horizon, over unlit terrain onto a lit runway. Without peripheral visual cues such as ground lights or horizon the pilot tends to feel that his aircraft is stable and correctly positioned and that the runway moves or is poorly positioned (Figure 8). This illusion makes the ‘blackhole’ approach dangerous and difficult, and can result in a landing far short of the runway.

During a ‘black hole’ approach, in the absence of visual horizon cues, a pilot tends to assume his aircraft is stable.

The intensity of runway or approach lights may also contribute to illusory effects during night flight. Very bright lights, or lights viewed through extremely clear air, appear closer than they really are. This can cause an approach higher than glideslope and a premature flare. During poor visibility there is a temptation to make approach lights the aim point when they first come into view through the weather. Many mishaps have been caused by using approach lights as an aim point. occurs because, on glideslope, you are looking at the runway surface through a much greater thickness of fog (Figure 9).

A runway, clearly visible through fog when viewed from directly overhead, may disappear when viewed through the fog while on approach glideslope. A bank of fog is over twenty times thicker when viewed at a 3° angle.

Another important group of illusions that can occur during the approach as well as take-off are the false climb and false descent illusions that occur during dark night operations. It is this type of illusion that probably caused the loss of Beech King Air VH-LFH, her pilot, and four of her five passengers at Wondai, Queensland on 26 July 1990. These illusions will be the topic of the next article in this series. No-one is immune to visual illusions during the approach and landing phases of flight.

Almost every pilot will have experienced some of these illusions at sometime or other during their flying career. Most of us probably wonder why we botched up a particular landing without understanding the sensory illusion that caused us to fly the incorrect approach path. Those that make the effort to learn about these illusions will be better prepared for their next flight. Those who are better prepared and have a better understanding of their sensory  limitations are less likely to become victims of illusions during the approach and landing phases of flight.

Acknowledgements:

The following texts and reports are acknowledged for their contribution of information and ideas: Ernsting and King’s ‘Aviation Medicine’ (Second Edition Butterworths 1988); Gillingham’s ‘Spatial Orientation in flight’ (USAF School of Aerospace Medicine USAFSAM-TR- 85-31 1985); The USAF Strategic Air Command Instrument Flight Course article ‘Landing Illusions: When what you see is not what you get’ (Combat Crew Magazine October 1991); The Bureau of Air Safety Investigation report of the King Air accident in Queensland in 1990 (BASI Report B/901/1047). The opinions expressed are those of the author and should in no way be construed as reflecting policy of the RAAF or the CAA.