John F. Kent, O. D., F.A.A.O.
There is a popular saying from World War II that states: "A pilot's eyes are his finest weapon!" Even today, in this age of radar, imaging infrared systems, head-up displays, and air traffic control, a pilot's eyes are still "his finest weapon". With the graying of the baby boom generation, more pilots today are flying with spectacles or contact lenses. A significant percentage of these pilots are over forty years of age and require bifocal spectacles to function effectively. Today's optometrists play a crucial role in ensuring that these pilots are provided with the proper visual correction they need to function efficiently.
A majority of the pilots who require spectacles for flying will wear single vision spectacles. A thorough eye examination is the first step in properly prescribing aircrew single vision spectacles. A detailed case history should be taken which includes the type of aircraft the aviator flies and the crew position. An accurate refraction is essential for top visual efficiency and the optometrist should be careful not to over-plus or over-minus the patient. If a pre-presbyopic patient is overcorrected in the minus direction, they may have trouble reading approach plates, especially at night. Over-correction in the plus direction will blur the pilot at far distance with resulting dissatisfaction. Evaluating the final prescription in a trial frame at far distance and at the cockpit working distance is a way to avoid these errors. A spare pair of clear spectacles is recommended as a backup. Antireflective lenses may be prescribed when reflections due to spectacle lenses create a problem for the pilot. The prescribing doctor should stress to the pilot that a spare pair of clear spectacles should be carried in their flight bag or, even better, their pocket, at all times when flying to serve as a backup in case something happens to the pair they are wearing. This is especially true of pilots who perform aerobatics or who are anticipating flying in turbulence where there is a good possibility the spectacles maybe dislodged.
One of the most overlooked factors in prescribing single vision aircrew spectacles is frame adjustment. The aviator should bring their flight gear such as helmet, oxygen mask, and/or headphones to the dispenser for the dispensing visit so that the primary and backup spectacles may be adjusted to properly fit when the flight gear is in place. The cockpit at 10,000 feet on a dark night is no place to be adjusting a backup pair of spectacles!
While prescribing single vision aircrew spectacles is rather straightforward, prescribing multifocal spectacles for presbyopic pilots requires a much greater clinical expertise and knowledge of the cockpit environment. The particular visual acuity demands and working distances will vary for each aircraft and flight crew position. For example, in the F-4 Phantom II, the pilot's front instrument panel is approximately 32" from the pilot's eyes while the front instrument panel for the navigator is approximately 20" from their eyes. If a pilot flies two or more different types of aircraft they may require a set of spectacles custom made for each aircraft. For example, a presbyopic pilot who requires bifocals to fly an F-16 will require a much lower bifocal height because of the F-16's 30 degree reclining seat than would an F-15 pilot who sits much more erect.
A thorough case history is essential when examining aircrew in the presbyopic age group. It is important to know the aircraft type, the size of print the aviator is required to read while flying, and the working distances at which the flyer is required to read that print. A slightly presbyopic pilot may require a bifocal prescription for a desk working distance of 14"-18" and yet not require bifocals for flying because of a more remote working distance of 24"-32". The best way to determine the true working distances for a particular pilot is for the optometrist to accompany their patient to the airport and measure the cockpit working distances in the pilot's airplane with the seat adjusted to normal flying settings. If a cockpit evaluation visit is not possible, the pilot can use the AOA Pilot Vision Evaluation Form as a guide in measuring their cockpit working distances for their optometrist.
A comprehensive examination is the next step and, as discussed earlier, it is important that the aircrew is not overcorrected in the minus or plus direction during the refraction. The AOA Pilot Vision Evaluation form or a cockpit survey will provide the information necessary for an initial bifocal add. This initial bifocal add should be inserted into a trial frame and evaluated at the cockpit working distance to determine if it provides good vision. The spectacle prescription should be evaluated with normal room illumination and also dim room illumination. The dim illumination should simulate the lighting conditions in the cockpit during night missions.
Once the bifocal add has been determined, the bifocal segment type must be chosen. The ST bifocal is usually the lens of choice. The main advantage of the ST bifocal over the progressive addition lens is the ST bifocal has an area of distance peripheral vision in the inferior temporal section of the lens. A progressive addition lens, however, will have distortion in this area of the lens. This additional peripheral vision is very helpful during aerial maneuvering and during the landing flare. There are many pilots flying successfully with progressive addition lenses, however, and the decision on which form of correction to use should be a mutual one between the pilot and his eye doctor. Certainly, if the patient is wearing a progressive lens successfully while flying, there is no need to change lens types. The eye doctor must work closely with the aviator, explaining the advantages and disadvantages of each type of correction, in order to determine the optimum prescription for each individual.
Obtaining the optimum bifocal height is one of the most crucial factors in prescribing bifocals for aircrew and one of the easiest areas to make an error. The bifocal segment height should be set so that the top of the segment falls just above the highest instrument on the aircraft's instrument panel and just below the glare shield. If a pilot wears an oxygen mask and/or helmet, they will affect the fit of the frame and the oxygen mask may cause the bifocals to position higher than normal. It is essential that the aircrew member bring in their helmet and/or oxygen mask for the dispensing visit so that a proper frame fit and bifocal height is obtained with the flight gear in place. Adjustable nose pads on the spectacle frames selected for the flying prescription will provide a significant range of adjustment to accomodate the flight gear. One way for the optometrist to eliminate some of the uncertainty in determining the proper bifocal height is to obtain several "loaner flight frames" that are of popular styles with pilots. The "flight frame" that have the power closest to the patient's prescription would be a template for determining proper bifocal height. The template frame would be fit to the aircrew member with their flight gear on. The flyer would then take the template frame to their aircraft and after obtaining their normal seat adjustment, mark on the template lens with a black marker where the highest instrument on the panel is or where the black mark falls just below the glare shield. With this method the eye doctor can accurately determine the proper bifocal height for an aviator.
The Double-D multifocal lenses are excellent prescriptions for presbyopic flight crew who must read overhead panels at relatively close distances. The Double-D segment has a vertical separation of 13mm between the top and bottom reading segments and the reading segments may be ordered with any combination of power. Because of the capability to specify a specific power for the top and bottom segments to meet the aircrew members' needs, the Double-D segment is the lens of choice for viewing overhead panels.
When aircrews are using contact lenses instead of spectacles as their flying correction, it is very important that they carry a properly adjusted spectacle correction close at hand. In case of some problem with their contact lenses, the pilot would be able to remove their contact lenses and put on their spectacles without having to bend the frame while flying in order to get the spectacle frames to fit properly.
Whenever pilots are given a new spectacle prescription, they should be informed about the possible problems they may encounter while adjusting to their new spectacles. They should be warned about the possible perceptual distortions that may make the ground appear to be slanting slightly up or down or make objects appear as if they are tilting to one side or may make objects appear larger or smaller than they really are. First time bifocal wearers should be told to try their bifocals out in a simulator or the cockpit of a parked aircraft and practice their new scanning techniques and adjust to new perceptual cues while safely on the ground. This would also be an ideal opportunity to make sure the spectacles are fitting properly. "Adjusting to New Spectacles the Easy Way" is an article on spectacle adaptation that may be reprinted and given to pilots at the time of spectacle dispensing. The article describes spectacle adaptation and suggests several methods that pilots may use to safely adjust to new spectacles.
A pilot's vision is truly his finest asset and it is the duty of optometrists to ensure flyers have the best possible vision at all times. For many flyers that means providing them with single vision or bifocal spectacles. This article was written to provide a guide and reference source to assist eye doctors in providing the optimum spectacle prescription to all their aviator patients who require spectacles while flying.
An excellent reference for further study of aviation and vision is:
The Optometrist's and Ophthalmologists Guide to Pilot's Vision, DeHaan, Warren V., 1st Edition, The American Trend Publishing Company, 1982.