020 Aircraft General Knowledge
070 Operational Procedure
What Every Pilot Shold Ask the Eye Doc
A common concern to all pilots is the retention of their unique gift of vision.
However, most pilots do not know the questions to ask the eye doctor and
typically don\'t get the special attention they deserve in the management of
their unique eye care needs. These problems are particularly magnified when the
pilot is in need of contact lenses, bifocals, and sunglasses.
More than one aviation accident has been directly attributed to contact lenses.
The pilot must understand that there are many options for contact lenses, and
they must let their attending doctor know what special visual demands they have
so that the appropriate lenses can be prescribed and carefully monitored.
Typically, there are two choices of contact lenses: rigid and soft.
Rigid lenses hold their shape and are removed at night. Rigid lenses provide the
doctor with more control in prescribing parameters; the doctor can specify the
size, shape and curvature of the lenses. Further, the doctor can measure these
parameters and confirm that the lab properly filled the prescription.
There are typically three curves specified by the doctor, and the outer two
curves act as a tear reservoir to assure that there is adequate fresh tear
exchange with each blink. This reservoir of tears is sometimes erroneously
interpreted by the patient as the edge of the lens where lights are reflected,
and is a source of glare and distraction. The pilot should caution the doctor
that there are flying scenarios, particularly night landings, where there is
potential for runway or other lighting being reflected off the tear reservoir
and temporarily visually incapacitating the pilot.
Further, with rigid lenses there is always some interference with the amount of
oxygen that the cornea must receive to maintain its clarity. This minor oxygen
deprivation results in mild swelling of the cornea and subtle clouding that
results in blurring of vision when the patient switches back to wearing
conventional glasses. Up to one hour of spectacle blur is acceptable for most
patients, but for pi- lots this may lead to disastrous results. It is important
that the pilot describe his visual demands to the doctor so that he may
carefully monitor any changes and assure that there is minimal impact to the
visual capability of the pilot in varied operational environments.
Rigid contact lenses do provide some advantages to the pilot. They are
relatively easy to handle and care for, do not contaminate, and provide an
unrestricted field of view. However, the lenses are not as comfortable as soft
lenses because there is more "lid sensation" from blinking over a rigid surface.
Soft lenses are the most comfortable of the contact lenses. They are made of a
jelly-like material that molds to the surface of the cornea and provide a
surface that mimics the cornea so there is minimal lid sensation. The lenses do
not work well with people who have astigmatism (oblong corneas) because they
mold to the oblong surface and do not correct the visual problem. Further, the
lenses are freely porous to water and oxygen, and selectively trap contaminates
that are potentially harmful to the eye. Pilots have to be particularly careful
of environmental contaminants - fuel, fumes, and grease that may inadvertently
get impregnated into the lens.
In spite of the high permeability of the lens material there is still a
disruption of oxygen supply to the cornea which results in clouded vision in
some patients. In pilots this may mean difficulty with lights and night vision.
This problem may be reduced or eliminated by careful observation by the doctor.
Some of the contamination problems are eliminated by disposable contact lenses,
which are worn for two weeks and replaced with new lenses. Whatever type of
contact lens is selected, contact lenses are serious business for pilots and
require close follow-up by an eye doctor.
Bifocals and the pilot
At age 40 there is a normal reduction in the ability to focus at objects
close-by. Pilots typically notice this as an inability to read sectional charts
and approach plates - particularly under nighttime conditions. If the pilot
describes this problem to the typical eye doctor, he is given reading glasses or
bifocals that focus at 16 inches, independent of the pilot\'s working distances.
It is, therefore, incumbent upon the pilot to provide the doctor with
measurements of all appropriate cockpit viewing distances. This would require
the pilot to get into the cockpit with a tape and carefully measure all
distances - panel instruments, yoke, trim, fuel selectors, etc. The doctor must
also know if there are overhead instruments or switches and their distances.
While the pilot is sitting in the cock-pit the height of the panel should be
noted relative to the typical seat position. The doctor needs this information
to determine how high the bifocal should be set in the glasses. If this
information is not provided, the bifocal is typically set in the frame so that
the line passes at the lower lid of the patients eye. This may be appropriate
for the general public, but may be much too high for the pilot. The resulting
compensation is to roll the head forward so that the pilot can look over the
line. Unusual head posture may result in vertigo and/or fatigue in the
performance of aviation duties.
The doctor can prescribe the appropriate bifocal power and height with the
information provided by the pilot. The doctor will prescribe a bifocal power to
focus at approximately 16 inches if the pilot does not provide the working
distance measurements. This focal distance may permit comfortable vision at arms
length, but seeing instruments on the panel may be a positioning nightmare -
requiring the pilot to move forward and roll his head back to get to the
appropriate reading distance. The disadvantages of contorting positions in the
cockpit are self-evident.
A doctor/patient partnership is necessary to optimize the prescription for the
pilot. This relationship requires that the doctor be flexible and willing to
prescribe atypical prescriptions involving oversized frames, very low bi-focal
heights, double bifocals or quad-rafocals, and reading prescriptions that
represent a compromise of powers that optimize the pilot\'s range of viewing
For example, the pilot may prefer the "invisible bifocal" for cosmetic reasons,
but the doctor must share with the pilot that there are distortion areas on each
side of the invisible bifocal that may seriously distort vision and interfere
with the pilot's scanning performance. There are reported cases where these
lenses (worn for the first time flying) disoriented the pilot so much that he
felt that he could not safely land the airplane.
The eye doctor has a wide range of options for prescription powers and must be
willing to take the time to pay attention to the special needs of patients that
are pilots and carefully assess the positive and negative aspects of the
prescription - whether it is a new pair of glasses or simply a change in
Pilots are a particularly vulnerable population to marketing ploys that
emphasize the dangers of eye damage from sunlight. In the 1980s scientists
suggested that visible blue light might cause cataracts - clouding of the lens
of the eye. It had been well-documented that ultraviolet light causes cataracts,
but the coatings applied to glasses to protect the pilot could not be seen and
were less well-accepted. However, blue light can be seen and it can be blocked
with yellow lenses. The pilot can see that the yellow lenses work by blocking
the blue, and it is much easier to market glasses that make a visible change in
it is extremely important that the pilot understand that yellow lenses may
eliminate some very important information to pilots, such as color radar or the
information on sectional charts and approach plates that is printed in subtle
shades of blue. A more appropriate solution for a sun tint is neutral gray like
Ray Ban sunglasses. For comfort, lenses can be tinted to knock out 80% of the
light and can be coated to block all of the harmful ultraviolet light, thus
providing optimal protection and comfort without color distortion.
Many times other aircraft are initially detected as a flash or glint of light,
so it is incumbent on the pilot to use glare to their advantage. Polaroid lenses
eliminate glint and glare and should not be worn by pilots except for special
use, like air-to-sea rescue where there is a requirement to see objects in the
water. Sunglasses that get darker outdoors do not typically get dark enough
behind the standard plexiglass wind screen, or stay too dark inside the cockpit
under reduced levels of illumination. It is inconvenient to switch glasses, but
clear and sunglasses are recommended. Also, lenses that are dark on top and
clear on the bottom are not recommended because on hazy days they give the
illusion of an artificial horizon at the point where the lens goes from dark to
light, and there has been one accident directly attributed to this illusion.
Here's what to look for in a pair of sunglasses:
20% to 30% light transmission (Blocks 70% to 80% of light)
optical plastic lenses (CR-39 material) - light weight
Large frame with adjustable nose pads that permit adjustment of the frame close
to the face to optimize protection.
It is the responsibility of the pilot to make the doctor aware of his special
needs, and it is necessary that the doctor listen to these special needs and
translate them into the optimum prescription incorporating the latest
technology. At times the doctor may change the prescription slightly with a
result- ant distortion that makes the patient feel like they are walking up or
down hills or may fall down or step off curbs. This illusion may occur with the
first pair of bifocals. It is very important that the doctor know that these
changes may result in serious injury or loss of life to their pilot patients.
In the final analysis, ask the right questions of your doctor and assure optimal
eye care and safety in flying performance.
Dr. William Monaco