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To the letter

Keen-eyed Readers Comment on Monocular Vision

Dear Editor,

I refer to the article entitled "Through the Mountains" on page 6 of Aviation Safety Letter 4/97. I was a monocular-rated private pilot for approximately 20 years. Two years ago, I had laser surgery on my offending right eye. Now, albeit with the aid of glasses, I can qualify for a slick new commercial licence.

When flying or driving, I see virtually no difference from my vision before the surgery. When I do see a remarkable difference is during a rare visit to the grocery store. While walking down the aisles with the huge banners strung about, I finally do see in 3-D, or while in a forest with branches all around or during my first visit to the Skydome after surgery, again, I see in 3-D.

Where I feel our fellow aviator went wrong was with his map-reading ability. Map reading is an art. Not many people possess this seemingly simple skill. Of course, it does help to have a decent map! Usually pilots, as they get older, slowly start to lose their precious vision, whereas, because of modern technology, I have rapidly improved my vision and can now compare the two kinds of vision. Thanks for a really good Safety Letter.

Bob Wilson
Pickering, Ontario


Dear Editor,

I would like to comment on the "Through the Mountains" article in ASL 4/97. It is very far off the mark and certainly misleading. The accident involved a monocular pilot, and the article appears to suggest that the pilot’s visual handicap contributed to the accident.

In fact, it must be understood that human eyes are only binocular for a focal length of approximately 40 to 50 ft. This is because of the proximity of the eyes to each other, a distance averaging only 2.5 to 3 in. Beyond 40 ft., a monocular and a binocular pilot see the same monocular image because the angle of convergence is more acute the farther you are from the source being viewed. In effect, there is no binocular depth perception beyond 40 ft., even with two perfect eyes. Both pilots would see and use the same perceptive visual clues, but not binocular ones. Furthermore, the ability to perceive visual prompts when flying into the sunlight has nothing to do with whether the pilot does so with one or two eyes. The same goes for map reading. The problem was pilot error, for sure, but not a visual handicap.

The question of landing procedures for monocular pilots is another myth requiring debunking. The visual clues used by monocular pilots are different only close to the ground and, in fact, monocular pilots appear to have the advantage as their clues are less sensitive to the visual illusions so well documented with normal-sighted pilots. My own experience is that, in a learning environment, monocular pilots have a much better spatial awareness and consequently learn to land the aircraft in less time than the average binocular pilot does.

The only time truly monocular pilots (as opposed to those rated monocular but having normal peripheral vision) are impaired is in the area of peripheral awareness. However, pilots suffering such an impairment are very aware of their condition and compensate accordingly, even when not flying. In fact, a study of aircraft incidents will reveal that there is little difference on safety issues between monocular and binocular pilots and, corrected for percent head of population, monocular pilots would appear to have the better record.

It is clear that the accident had no relation to the pilot's monocular rating and it is regrettable that such should have been introduced into an otherwise excellent report. I hope that you may find it appropriate to offer editorial comment to correct the misconception.

Thomas R. Sommerville
Guelph, Ontario

Editor’s comment: The Transportation Safety Board’s final report on this accident (A94W0157) concluded that, although the pilot was monocular, this was not considered to be a factor in the occurrence. In addition, our Civil Aviation Medicine staff agree with Mr. Sommerville’s comments. Thanks to our readers for these eye-opening remarks.

Originally Published: Aviation Safety Letter 02/1998
Original Article: To the letter - Keen-eyed Readers Comment on Monocular Vision

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