Monovision and Aviation
BY JOSEPH T. BARR, OD, MS, EDITOR
I commend Drs. Nakagawara and Veronneau for publishing their article, "Monovision Contact Lens Use in the Aviation Environment: A Report of a Contact Lens-related Aircraft Accident," (Optometry 2000; 71(6):390-395). Although much of the information in this article was previously published, it was good to see it all in one place. It reports on the October 19, 1996, Delta Airlines Flight 554 accident at La Guardia, New York. The MD-88 "was substantially damaged in an undershoot approach." The impact sheared off the main landing gear, and the plane slid 2,700 feet down the runway, according to the article abstract. Three passengers reported minor injuries as a result of the crash.
Monovision is not permitted for commercial pilots. However, if pilots are blind in one eye and otherwise pass the vision standards, they may fly after six months of adaptation. The pilot of Delta Airlines Flight 554 reported using monovision 75 percent of the time. He was very experienced and had flown the same approach a number of times. His visual acuity was OD 20/20 distance, 20/60 near and OS 20/80 distance, 20/20 near. An evaluation after the incident reported "substantially reduced" depth perception, but did not explain which method was used. The article reports "heavy rain and fog or mist" during the landing. The runway lights were spaced differently than on most runways. The Instrument Landing System localizer location, the approach over water in those conditions and the runway light spacing "may have combined to create an illusion that caused the pilot to believe he was higher and farther away from the runway."
The authors, who work for Federal Aviation Administration, wisely point out that stereopsis is reduced with increased distance. (At that speed and in those conditions, stereopsis wouldn't be great anyway, in my opinion.) They also suggest that stereopsis is important for flying in formation and refueling tasks. Basically, they report that the conditions of the landing were poor and monocular clues to depth may have been reduced, but reduced stereopsis could not be conclusively proven as the cause. Yet, the National Transportation Safety Board (NTSB) concluded that monovision, and reduced stereopsis from the monovision, was the probable cause of the accident.
I am not saying that reduced stereopsis was not the primary problem in the accident. It's reasonable to believe, but hard to prove, that the blur in one eye was the problem. (Remember, one-eyed pilots can fly, according to FAA standards.) But I am wondering how monovision is considered the problem. Indeed, these authors successfully argue against it as the cause. I also wonder why the article mentions three times that the plane slid 2,700 feet down the runway other than that it surely sensationalized the issue.