illusions during the approach and
Dr Dougal Watson
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
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
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.
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