Aeromedical Forum

What better flying weather than a crisp clear February day in Minnesota. You’d think you were flying a helicopter with the short takeoff runs. The air is smooth and the visibility unlimited in bright sunshine. The reflection from the snow is dazzling. I sure hope you didn’t forget those sunglasses! Sunglasses are a very important and often underappreciated piece of a pilot’s equipment. They are critical to optimize visual performance in the cockpit. If you have a good pair, they will reduce eye fatigue, reduce the negative effects of harsh sunlight, speed up the process of dark adaptation and protect eye tissues from the harmful effects of solar radiation.

Most eyeglass lenses are made of one of three materials: optical quality “crown” glass, monomer plastic (CR-39®), and polycarbonate plastic. Crown glass absorbs some UV light (the harmful stuff that can predispose you to cataracts) but special coatings to further reduce UV absorption should be added for aviation use. It’s more scratch resistant than monomer plastic or polycarbonate but much heavier. That could be a consideration if you have a big correction and have to wear “Coke bottles.” Crown glass is also less impact resistant. Don’t wear them when you’re playing handball! CR-39 plastic lenses are more light weight and impact resistant than crown glass, but they are more prone to scratching. So, if you’re like me and tend to toss around your glasses, keep a carrying case handy. Polycarbonate plastic lenses are lighter than CR-39® and the most impact-resistant lenses available. Problem is, they are more difficult to fabricate and so the cost is sometimes greater than crown glass or monomer plastic. The other drawback is their greater tendency to optical aberrations or distortions. That can be a concern when you’re trying to “see and avoid” enroute in a busy traffic pattern. Bottom line, your best use for polycarbonate lenses is for athletic events or for kids who like to get into snowball fights. You might want to steer away from them in the cockpit.

In addition to lens materials, you should also consider coatings. Crown glass and most plastic lenses require a specific coating to block residual UV radiation. That’s especially important in the cockpit where you may encounter increased UV exposure. Make sure you handle coated lenses with care. Excessive heat can cause delamination or crazing. You should also be mindful of the tint you select. The three most common tints are gray, gray-green, and brown. The FAA recommends a gray, neutral density filter because it distorts color the least. For flying, sunglass lenses should screen out only 70 - 85% of visible light and not appreciably distort color. Tints that block more than 85% of visible light are not recommended for flying. They can reduce visual acuity, making it hard to see your instruments and charts inside the cockpit.

One thing to definitely avoid in the cockpit is polarized lenses. While they may be great for snowmobiling or to spot that walleye out on the lake, they can mask the sparkle of light that reflects off shiny surfaces such as another aircraft’s wing or windscreen. This can reduce the time a pilot has to react in a “see-and-avoid” traffic situation. Another big problem with polarized lenses is their reducing or eliminating the visibility of instruments that incorporate an anti-glare filter. That’s just about all glass cockpits!

Another thing I’m a bit charry about is photochromic lenses. Glass photochromic lenses (PhotoGray® and PhotoBrown®), and plastic ones (Transitions®), automatically darken when exposed to UV and lighten in dim light. They usually take about a minute to darken and several minutes to lighten. That’s too long when you’re going in and out of clouds or stratus layers. Their ability to darken drops off markedly with temperatures above 70°f or with reduced UV exposure as can occur in some cockpits. Finally, in their faded (low light) state photochromic glass lenses may not be clear enough to be useful when flying in cloud cover or at night.

Keep these things in mind when choosing some really cool aviator’s glasses. I’m not an eye doctor and the technology of ophthalmic lenses is continually evolving. So, it would be a good idea to check with your ophthalmologist, optometrist or oculist to find out the current available choices.

Fly wisely. See you next month.

As always, comments, questions and suggestions are welcome: jdlakinmd@gmail.com.

Also, we’ve moved our office to Airlake Airport’s FBO (KLVN)! Call (952) 469-4414 for a flight physical appointment.