Landing: Part II, Visual Environment
Alexander Burton, CFI
Pacific Rim Aviation Academy Inc.
Pitt Meadows Regional
Airport
“No
object is mysterious. The mystery is your eye.“
--Elizabeth Bowen, The House
in Paris, 1935-
Last
month, we took a look at the first stage of the landing process, the
approach. We discussed the idea that a good landing begins with a good
approach and that a poorly planned or executed approach will sadly reduce
the chances of performing a successful landing.
Of course,
an experienced pilot flying an aircraft with which he or she is familiar can
often salvage a poor approach and still achieve a successful landing. This
is not, however, a recommended procedure and certainly exposes the pilot and
his or her aircraft and passengers, not to mention those on the ground, to
unnecessary risk.
As the
AOPA Air Safety Foundation’s 1996 Nall Report reminds us, “Takeoff and
landing account for about three percent of the time spent in a typical
cross-country flight, but 47 percent of all accidents occur during these
phases.”
Today, I’d
like to take a small detour and talk about the visual environment and
reality we face on final approach to landing. Final approach is a
potentially exciting phase of flight. Things are developing and changing
quickly and it is important we are aware of the visual clues and potential
illusions we may have to work through to achieve our end: a successful
landing.
Our visual
information flow is critical to achieving a successful landing. There are
several factors, some environmental and some products of our
decision-making, that affect the information available to us.
Environmental factors include ambient light levels, the position of the sun
and the amount of glare we experience, rain, snow, hail, and the background
against which we must distinguish detail. Landing on a dirt strip located in
a dirt pasture presents a different set of visual images than we might see
landing on a black, paved runway with nicely painted lines set against
trimmed grass verges. An up-sloping or down-sloping runway will offer unique
visual challenges as will a runway covered with pooled water or snow.
Pilot
decision-making about visual focal points is a product of our training and
experience. Knowing where and how to look at things is an important factor
in ensuring a safe landing. A significant aspect of our decision making is
developing an understanding of how our perceptual equipments works and the
illusions to which we are subject.
A quick
review of our secondary school biology will remind us that there are two
types of light receptors in the eye: rods and cones. Cones are concentrated
near the centre of the eye, the fovea; rods are concentrated around the
periphery.
Cones are
the primary receptors for colour, visual acuity, and recognition of detail;
rods have no colour sensitivity, are poor at receiving specific detail, but
are the primary receptors for the perception of motion. Under low light
conditions, scotopic vision, rods provide the major source of visual
information to the brain. Under bright light conditions, photopic vision,
the cones predominate.
Between
scotopic illumination levels and photopic illumination levels is the mesopic
range, or mid-level lighting conditions, where both cones and rods function
well. In this light range, vision is both peripheral and central and colour
begins to become clearly apparent (1).
For sharp
vision, photopic vision levels of light are required and we must be looking
directly at an object. In low light conditions, we cannot see an object as
clearly when looking directly at it. We must look a bit to the side to
maximize our clarity. Averted vision is the best way to maximize the
efficacy of the rods (2).
Without
beating all that to death, what we know is that the centre of our vision is
our best source of information on detail and colour; the peripheral areas of
our vision are our best source of information regarding motion. Both
functions must be working in concert to develop a full spectrum of visual
perception of our environment, and the ambient light levels directly affect
the quality of our total, visual picture. In poor light conditions, we must
rely much more on our peripheral vision capabilities for necessary
information.
We also
know that, when an object is approaching us at high speed, our depth
perception, a function of our binocular—two eyes—system of visual
perception, is not nearly as efficient as when we are looking at a
stationary object. In simple terms, an object moving toward or away from us
is never in the same place once.
Under
normal circumstances, we depend on triangulation to determine distance. When
the triangulation equation is in constant and rapid change, it is much more
difficult to determine how far away an object is. The greater the magnitude
of that change, the quicker objects are approaching or receding, the poorer
our ability to accurately determine the distance.
To add
insult to injury, we are not viewing the approaching runway straight on. We
are looking both forward and downward making the perceptual problem even
more complex. As we near our intended target on the runway, our round-out
point, objects in view appear to both increase in size and move in various
directions relative to our touchdown point. This phenomenon is referred to
by those who study these things as the Expansion Theory and can be very
helpful if it is well understood (3). Our first illusion is that, visually,
we remain stationary and the world around us moves.
Points of
reference or objects that appear to be moving downward during our approach
are points we will overshoot. Points or objects appearing to move upward are
points we will undershoot, for example, the end of the runway. Points
or objects that appear to move in a horizontal pathway are ones that are
abeam of our point of intersection with the runway.
An object
or point right at our touchdown point will remain stationary in our visual
field. Everything else, as we approach, will appear to move away radially.
For example, an object that is beyond and to the left of our impact point
will appear to move up and away to the left as we approach.
At the
centre of our visual field, often referred to as the still point,
objects will remain stationary. This is the point of our intersection with
the runway and defines our point of flair.
In fact,
perspective—our perception of distance in terms of relative size—rather than
binocular depth perception takes on a much more dominant role in determining
distance and position during approach. It is not even required that a pilot
have two, functioning eyes. According to published FAA records, 4005
one-eyed people hold valid FAA pilot certificates in the United States; 75
of these pilots hold first class medicals. One of the all-time great
aviators, Wiley Post, was living proof that a one eyed pilot is quite
capable of executing successful landings.
No
difference in safety records has been noted between one eyed and two eyed
pilots.
Understanding the flow of visual information presenting itself to our little
brains during the final approach to landing can be of great help in
achieving a successful landing. The major key to allowing our brain to
process this vital information is to stabilize our approach by maintaining a
constant approach path, rate of descent, and airspeed.
A
stabilized approach allows us to control and simplify the flow of visual
information. It allows us to use our central visual acuity to find and
identify our touchdown point and to align our aircraft with the centreline
of the runway; it greatly assists us to develop a clear sense of distance
and height from our peripheral vision.
By
consciously denying the urge to fixate on our touchdown point, by
consciously keeping our gentle scan going, we also allow the capabilities of
our peripheral vision to apprehend and pass along information regarding
height, relative motion and perspective in a useful form. This becomes
particularly critical when faced with an up-sloping or down-sloping runway
that gives us the illusion of either being too high or too low on approach.
As A.
Howard Hasbrook so clearly points out, “If
a pilot's having trouble with his [or her] landings, it's a sure bet he [or
she] is not looking in the right place at the right time” (4).
End Notes
- Crawford, David, 1998, Some
Issues in Low Light Level Vision, IDA Inc., Tucson, Arizona, pg. 1
- Ibid.
- A. Howard Hasbrook, Anaatomy
of a Landing – Cue by Cue, Business and Commercial Aviation
Magazine, August 1971 issue, McGraw-Hill
4.
Ibid.
PRINCIPAL AIR
Main Terminal Chilliwack Airport
2-46244 Airport Road, Chilliwack, B.C. V2P 1A5
Phone: (604) 795-7861 Fax: (604) 795-7867
E-mail
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