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 distance.
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 prescription.
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 the environment.
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